diff --git a/src/Cargo.lock b/src/Cargo.lock
index 0c7fa600..5a5915cd 100644
--- a/src/Cargo.lock
+++ b/src/Cargo.lock
@@ -1519,6 +1519,14 @@ dependencies = [
 name = "obikalgorithm"
 version = "0.1.0"
 
+[[package]]
+name = "obikdump"
+version = "0.1.0"
+dependencies = [
+ "obikfilter",
+ "obikindex",
+]
+
 [[package]]
 name = "obikentropy"
 version = "0.1.0"
@@ -1526,6 +1534,27 @@ dependencies = [
  "obikseq",
 ]
 
+[[package]]
+name = "obikfilter"
+version = "0.1.0"
+dependencies = [
+ "obicompactvec",
+ "obikindex",
+ "obikseq",
+ "obiskio",
+ "obitaxonomy",
+]
+
+[[package]]
+name = "obikidxcache"
+version = "0.1.0"
+dependencies = [
+ "ndarray",
+ "obicompactvec",
+ "rayon",
+ "tempfile",
+]
+
 [[package]]
 name = "obikindex"
 version = "0.1.0"
@@ -1533,9 +1562,7 @@ dependencies = [
  "anyhow",
  "bitvec",
  "cacheline-ef",
- "crossbeam-channel",
  "epserde",
- "hwlocality",
  "indicatif",
  "memmap2",
  "ndarray",
@@ -1597,11 +1624,13 @@ dependencies = [
  "obidebruinj",
  "obifastwrite",
  "obikalgorithm",
+ "obikfilter",
  "obikindex",
  "obikindexer",
  "obikphylo",
  "obikrope",
  "obikseq",
+ "obikstats",
  "obipipeline",
  "obiread",
  "obiskbuilder",
@@ -1618,6 +1647,37 @@ dependencies = [
  "tracing-subscriber",
 ]
 
+[[package]]
+name = "obikmer2"
+version = "1.2.2"
+dependencies = [
+ "clap",
+ "obikalgorithm",
+ "obikindex",
+ "obikindexer",
+ "obikseq",
+ "obipipeline",
+ "obiread",
+ "obisys",
+ "tracing",
+ "tracing-subscriber",
+]
+
+[[package]]
+name = "obikmerge"
+version = "0.1.0"
+dependencies = [
+ "obicompactvec",
+ "obikfilter",
+ "obikindex",
+ "obikindexer",
+ "obikseq",
+ "obipipeline",
+ "obiskio",
+ "obisys",
+ "tracing",
+]
+
 [[package]]
 name = "obikphylo"
 version = "0.1.0"
@@ -1626,6 +1686,7 @@ dependencies = [
  "ndarray",
  "obicompactvec",
  "obikalgorithm",
+ "obikidxcache",
  "obikindex",
  "obikindexer",
  "obikseq",
@@ -1641,6 +1702,29 @@ dependencies = [
  "tracing-subscriber",
 ]
 
+[[package]]
+name = "obikquery"
+version = "0.1.0"
+dependencies = [
+ "obikindex",
+]
+
+[[package]]
+name = "obikrebuild"
+version = "0.1.0"
+dependencies = [
+ "obicompactvec",
+ "obidebruinj",
+ "obikfilter",
+ "obikindex",
+ "obikindexer",
+ "obikmerge",
+ "obikseq",
+ "obiskio",
+ "obisys",
+ "tracing",
+]
+
 [[package]]
 name = "obikrope"
 version = "0.1.0"
@@ -1649,6 +1733,16 @@ dependencies = [
  "criterion2",
 ]
 
+[[package]]
+name = "obikselect"
+version = "0.1.0"
+dependencies = [
+ "obicompactvec",
+ "obikindex",
+ "obisys",
+ "tracing",
+]
+
 [[package]]
 name = "obikseq"
 version = "0.1.0"
@@ -1660,6 +1754,15 @@ dependencies = [
  "xxhash-rust",
 ]
 
+[[package]]
+name = "obikstats"
+version = "0.1.0"
+dependencies = [
+ "obicompactvec",
+ "obikindex",
+ "rayon",
+]
+
 [[package]]
 name = "obipipeline"
 version = "0.1.0"
@@ -1712,8 +1815,11 @@ dependencies = [
 name = "obisys"
 version = "0.1.0"
 dependencies = [
+ "crossbeam-channel",
+ "hwlocality",
  "indicatif",
  "libc",
+ "rayon",
  "sysinfo",
  "tracing",
 ]
diff --git a/src/Cargo.toml b/src/Cargo.toml
index 296753ad..cf2b32bc 100644
--- a/src/Cargo.toml
+++ b/src/Cargo.toml
@@ -1,5 +1,5 @@
 [workspace]
 resolver = "3"
-members = ["obikseq", "obiread", "obiskbuilder", "obifastwrite", "obikmer","obikrope","obipipeline", "obiskio","obidebruinj", "obicompactvec", "obisys", "obikindex", "obikindexer", "obitaxonomy", "obikentropy", "obikphylo", "obikalgorithm"]
+members = ["obikseq", "obiread", "obiskbuilder", "obifastwrite", "obikmer","obikmer2","obikrope","obipipeline", "obiskio","obidebruinj", "obicompactvec", "obisys", "obikindex", "obikindexer", "obikquery", "obikdump", "obikfilter", "obikselect", "obikrebuild", "obikmerge", "obikstats", "obikidxcache", "obitaxonomy", "obikentropy", "obikphylo", "obikalgorithm"]
 [profile.release]
 debug = 1
diff --git a/src/obikdump/Cargo.toml b/src/obikdump/Cargo.toml
new file mode 100644
index 00000000..b802741d
--- /dev/null
+++ b/src/obikdump/Cargo.toml
@@ -0,0 +1,8 @@
+[package]
+name = "obikdump"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+obikindex  = { path = "../obikindex" }
+obikfilter = { path = "../obikfilter" }
diff --git a/src/obikindex/src/index/dump.rs b/src/obikdump/src/dump.rs
similarity index 97%
rename from src/obikindex/src/index/dump.rs
rename to src/obikdump/src/dump.rs
index 45ff962c..560ef95f 100644
--- a/src/obikindex/src/index/dump.rs
+++ b/src/obikdump/src/dump.rs
@@ -3,9 +3,9 @@ use std::sync::atomic::{AtomicUsize, Ordering};
 
 use rayon::prelude::*;
 
-use crate::index::error::{OKIError, OKIResult};
-use crate::index::kmer_index::KmerIndex;
-use crate::index::KmerFilter;
+use obikindex::{OKIError, OKIResult};
+use obikindex::KmerIndex;
+use obikfilter::KmerFilter;
 
 impl KmerIndex {
     /// Write a CSV table of all indexed kmers to `out`.
diff --git a/src/obikdump/src/lib.rs b/src/obikdump/src/lib.rs
new file mode 100644
index 00000000..f953444f
--- /dev/null
+++ b/src/obikdump/src/lib.rs
@@ -0,0 +1,8 @@
+//! Raw content export of an `obikindex::KmerIndex`: staging ground for
+//! code currently living in `obikindex::index`'s dump path (`dump.rs`,
+//! `dump_layer.rs`), to be migrated here to lighten that crate. Kept as
+//! a separate crate — not a module of `obikindex` — so the dependency
+//! runs one way only (dump code depends on the data model, never the
+//! reverse), same pattern as `obikindexer`/`obikquery`.
+
+mod dump;
diff --git a/src/obikfilter/Cargo.toml b/src/obikfilter/Cargo.toml
new file mode 100644
index 00000000..f1bde2f8
--- /dev/null
+++ b/src/obikfilter/Cargo.toml
@@ -0,0 +1,11 @@
+[package]
+name = "obikfilter"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+obikindex     = { path = "../obikindex" }
+obicompactvec = { path = "../obicompactvec" }
+obikseq       = { path = "../obikseq" }
+obiskio       = { path = "../obiskio" }
+obitaxonomy   = { path = "../obitaxonomy" }
diff --git a/src/obikindex/src/index/dump_layer.rs b/src/obikfilter/src/dump_layer.rs
similarity index 90%
rename from src/obikindex/src/index/dump_layer.rs
rename to src/obikfilter/src/dump_layer.rs
index 69fca0d5..47507806 100644
--- a/src/obikindex/src/index/dump_layer.rs
+++ b/src/obikfilter/src/dump_layer.rs
@@ -1,20 +1,11 @@
+use obikindex::layer::MphfLayer;
 use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix};
 use obikseq::CanonicalKmer;
-use crate::layer::{MphfLayer, OLMError};
-use obiskio::{SKError, SKResult, UnitigFileReader};
+use obiskio::UnitigFileReader;
+use obikindex::{OKIError, OKIResult};
 
-use crate::index::filter::{KmerFilter, passes_all};
-use crate::index::kmer_index::KmerIndex;
-
-fn olm_to_sk(e: OLMError) -> SKError {
-    match e {
-        OLMError::Io(e) => SKError::Io(e),
-        other => SKError::InvalidData {
-            context: "dump",
-            detail: other.to_string(),
-        },
-    }
-}
+use crate::filter::{KmerFilter, passes_all};
+use obikindex::KmerIndex;
 
 impl KmerIndex {
     /// Iterate all indexed kmers in partition `part`, calling `cb(kmer, row)` for each
@@ -35,7 +26,7 @@ impl KmerIndex {
         n_genomes: usize,
         filters: &[Box<dyn KmerFilter>],
         mut cb: impl FnMut(CanonicalKmer, Box<[u32]>) -> bool,
-    ) -> SKResult<bool> {
+    ) -> OKIResult<bool> {
         let index_dir = self.index_dir(part);
         if !index_dir.exists() {
             return Ok(true);
@@ -43,19 +34,19 @@ impl KmerIndex {
 
         let mut l = 0;
         loop {
-            let layer_dir = self.layer_dir(part, l);
+            let layer_dir = self.layer_dir(part, l)?;
             if !layer_dir.exists() {
                 break;
             }
             l += 1;
-            let mphf = MphfLayer::open(&layer_dir).map_err(olm_to_sk)?;
+            let mphf = MphfLayer::open(&layer_dir)?;
             let reader = UnitigFileReader::open_sequential(&layer_dir.join("unitigs.bin"))?;
 
             let counts_dir = layer_dir.join("counts");
             let presence_dir = layer_dir.join("presence");
 
             let cont = if use_counts && counts_dir.exists() {
-                let mat = PersistentCompactIntMatrix::open(&layer_dir).map_err(SKError::Io)?;
+                let mat = PersistentCompactIntMatrix::open(&layer_dir).map_err(OKIError::Io)?;
                 let mut cont = true;
                 for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
                     if let Some(slot) = mphf.find(kmer) {
@@ -70,7 +61,7 @@ impl KmerIndex {
                 }
                 cont
             } else if !use_counts && presence_dir.exists() {
-                let mat = PersistentBitMatrix::open(&layer_dir).map_err(SKError::Io)?;
+                let mat = PersistentBitMatrix::open(&layer_dir).map_err(OKIError::Io)?;
                 let mut cont = true;
                 for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
                     if let Some(slot) = mphf.find(kmer) {
@@ -123,7 +114,7 @@ impl KmerIndex {
         n_genomes: usize,
         filters: &[Box<dyn KmerFilter>],
         mut cb: impl FnMut(usize, usize, CanonicalKmer, Box<[u32]>) -> bool,
-    ) -> SKResult<bool> {
+    ) -> OKIResult<bool> {
         let index_dir = self.index_dir(part);
         if !index_dir.exists() {
             return Ok(true);
@@ -135,14 +126,14 @@ impl KmerIndex {
             if !layer_dir.exists() {
                 break;
             }
-            let mphf = MphfLayer::open(&layer_dir).map_err(olm_to_sk)?;
+            let mphf = MphfLayer::open(&layer_dir)?;
             let reader = UnitigFileReader::open_sequential(&layer_dir.join("unitigs.bin"))?;
 
             let counts_dir = layer_dir.join("counts");
             let presence_dir = layer_dir.join("presence");
 
             let cont = if use_counts && counts_dir.exists() {
-                let mat = PersistentCompactIntMatrix::open(&layer_dir).map_err(SKError::Io)?;
+                let mat = PersistentCompactIntMatrix::open(&layer_dir).map_err(OKIError::Io)?;
                 let mut cont = true;
                 for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
                     if let Some(slot) = mphf.find(kmer) {
@@ -157,7 +148,7 @@ impl KmerIndex {
                 }
                 cont
             } else if !use_counts && presence_dir.exists() {
-                let mat = PersistentBitMatrix::open(&layer_dir).map_err(SKError::Io)?;
+                let mat = PersistentBitMatrix::open(&layer_dir).map_err(OKIError::Io)?;
                 let mut cont = true;
                 for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
                     if let Some(slot) = mphf.find(kmer) {
diff --git a/src/obikindex/src/index/filter.rs b/src/obikfilter/src/filter.rs
similarity index 100%
rename from src/obikindex/src/index/filter.rs
rename to src/obikfilter/src/filter.rs
diff --git a/src/obikfilter/src/lib.rs b/src/obikfilter/src/lib.rs
new file mode 100644
index 00000000..26a95c23
--- /dev/null
+++ b/src/obikfilter/src/lib.rs
@@ -0,0 +1,22 @@
+//! Per-kmer filtering: `KmerFilter` judges one k-mer (its canonical
+//! sequence + its per-genome row) at a time, deciding whether it passes.
+//! Orthogonal to genome-column selection/aggregation (`obikselect`), which
+//! operates on already-retained k-mers.
+//!
+//! [`filter`] (the `KmerFilter` trait + its implementations) depends only
+//! on `obicompactvec`/`obikseq`, not on `obikindex`. [`dump_layer`] is the
+//! extension over `obikindex::KmerIndex` that actually iterates a
+//! partition's k-mers through those filters (`iter_partition_kmers`,
+//! `iter_partition_kmers_located`) — every k-mer, filtered or not, goes
+//! through this same path (`passes_all` on an empty filter list is always
+//! `true`), so this crate depends one-way on `obikindex`, not the reverse.
+
+mod filter;
+mod dump_layer;
+mod predicate;
+
+pub use filter::{
+    GroupQuorumFilter, KmerFilter, MaxGenomeCount, MaxGenomeFraction, MaxTotalCount,
+    MinComplexity, MinGenomeCount, MinGenomeFraction, MinTotalCount, passes_all,
+};
+pub use predicate::{GroupFilterParams, MetaPred};
diff --git a/src/obikindex/src/index/predicate.rs b/src/obikfilter/src/predicate.rs
similarity index 99%
rename from src/obikindex/src/index/predicate.rs
rename to src/obikfilter/src/predicate.rs
index 7791748c..db1be2d8 100644
--- a/src/obikindex/src/index/predicate.rs
+++ b/src/obikfilter/src/predicate.rs
@@ -1,9 +1,9 @@
 use std::collections::HashMap;
 
-use crate::index::GroupQuorumFilter;
+use crate::filter::GroupQuorumFilter;
 use obitaxonomy::{TaxPath, TaxPattern};
 
-use crate::index::meta::{GenomeInfo, IndexMeta};
+use obikindex::{GenomeInfo, IndexMeta};
 
 // ── Operator ──────────────────────────────────────────────────────────────────
 
diff --git a/src/obikidxcache/Cargo.toml b/src/obikidxcache/Cargo.toml
new file mode 100644
index 00000000..884e8c88
--- /dev/null
+++ b/src/obikidxcache/Cargo.toml
@@ -0,0 +1,12 @@
+[package]
+name = "obikidxcache"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+obicompactvec = { path = "../obicompactvec" }
+ndarray       = "0.17"
+rayon         = "1"
+
+[dev-dependencies]
+tempfile = "3"
diff --git a/src/obikindex/src/layer/layered_store.rs b/src/obikidxcache/src/layered_store.rs
similarity index 100%
rename from src/obikindex/src/layer/layered_store.rs
rename to src/obikidxcache/src/layered_store.rs
diff --git a/src/obikidxcache/src/lib.rs b/src/obikidxcache/src/lib.rs
new file mode 100644
index 00000000..15919976
--- /dev/null
+++ b/src/obikidxcache/src/lib.rs
@@ -0,0 +1,11 @@
+//! Caching/aggregation over already-opened `obikindex` layer/partition
+//! objects — a runtime cache shape, not part of the stateless
+//! `Index { Partition { Layer } }` directory-structure model itself.
+//! [`LayeredStore`] wraps one entry per layer (or per partition, via
+//! `LayeredStore<LayeredStore<S>>`) and propagates `ColumnWeights`/
+//! `CountPartials`/`BitPartials` aggregation across them. No dependency on
+//! `obikindex` — generic over any `S` implementing those traits.
+
+mod layered_store;
+
+pub use layered_store::LayeredStore;
diff --git a/src/obikindex/src/layer/tests/layered_store.rs b/src/obikidxcache/src/tests/layered_store.rs
similarity index 100%
rename from src/obikindex/src/layer/tests/layered_store.rs
rename to src/obikidxcache/src/tests/layered_store.rs
diff --git a/src/obikindex/Cargo.toml b/src/obikindex/Cargo.toml
index ce698577..e58cdf8e 100644
--- a/src/obikindex/Cargo.toml
+++ b/src/obikindex/Cargo.toml
@@ -19,13 +19,11 @@ niffler          = "3.0.0"
 memmap2          = "0.9.10"
 ndarray          = "0.17"
 rayon             = "1"
-crossbeam-channel = "0.5"
 serde            = { version = "1", features = ["derive"] }
 serde_json       = "1"
 indicatif        = "0.18"
 tracing          = "0.1.44"
 bitvec           = "1"
-hwlocality       = { version = "1.0.0-alpha.11", features = ["vendored"], optional = true }
 
 [dev-dependencies]
 obiread  = { path = "../obiread" }
@@ -33,7 +31,3 @@ obikseq  = { path = "../obikseq", features = ["test-utils"] }
 tempfile = "3"
 tracing-subscriber = { version = "0.3", features = ["fmt", "env-filter"] }
 anyhow   = "1"
-
-[features]
-default = ["numa"]
-numa    = ["hwlocality"]
diff --git a/src/obikindex/src/index/builder.rs b/src/obikindex/src/index/builder.rs
index 28f7f2a1..958a94ca 100644
--- a/src/obikindex/src/index/builder.rs
+++ b/src/obikindex/src/index/builder.rs
@@ -44,7 +44,11 @@ pub trait IndexBuilder: Sized {
     /// For construction paths that build partitions from scratch (`select`,
     /// `rebuild`). `merge` bootstraps by copying a source index instead, so
     /// it does not use this.
-    fn create_skeleton<P: AsRef<Path>>(output: P, config: IndexConfig, genomes: Vec<GenomeInfo>) -> OKIResult<Self>;
+    fn create_skeleton<P: AsRef<Path>>(
+        output: P,
+        config: IndexConfig,
+        genomes: Vec<GenomeInfo>,
+    ) -> OKIResult<Self>;
 
     /// Mark `output` as fully indexed, pack its column matrices, and reopen it.
     ///
@@ -61,7 +65,7 @@ pub trait IndexBuilder: Sized {
 impl IndexBuilder for KmerIndex {
     fn clear_output_for_create<P: AsRef<Path>>(output: P, force: bool) -> OKIResult<()> {
         let output = output.as_ref();
-        if Self::exists(output) {
+        if Self::is_an_index(output) {
             if force {
                 fs::remove_dir_all(output).map_err(OKIError::Io)?;
             } else {
@@ -74,11 +78,18 @@ impl IndexBuilder for KmerIndex {
         Ok(())
     }
 
-    fn create_skeleton<P: AsRef<Path>>(output: P, config: IndexConfig, genomes: Vec<GenomeInfo>) -> OKIResult<Self> {
+    fn create_skeleton<P: AsRef<Path>>(
+        output: P,
+        config: IndexConfig,
+        genomes: Vec<GenomeInfo>,
+    ) -> OKIResult<Self> {
         let output = output.as_ref();
         fs::create_dir_all(output).map_err(OKIError::Io)?;
         let meta = IndexMeta::create_at(output, config, genomes).map_err(OKIError::Io)?;
-        Ok(KmerIndex { root_path: output.to_owned(), meta: Arc::new(meta) })
+        Ok(KmerIndex {
+            root_path: output.to_owned(),
+            meta: Arc::new(meta),
+        })
     }
 
     fn finalize_indexed<P: AsRef<Path>>(output: P, rep: &mut Reporter) -> OKIResult<Self> {
diff --git a/src/obikindex/src/index/common.rs b/src/obikindex/src/index/common.rs
index cfa20609..f22fdc12 100644
--- a/src/obikindex/src/index/common.rs
+++ b/src/obikindex/src/index/common.rs
@@ -1,65 +1,25 @@
-use std::path::Path;
-
+use crate::index::error::{OKIError, OKIResult};
 use obicompactvec::{PersistentBitVecBuilder, PersistentCompactIntVecBuilder};
-use crate::layer::meta::PartitionMeta;
-use crate::layer::{layer_dir, IndexMode, OLMError};
-use obiskio::{SKError, SKResult};
-
-// ── olm_to_sk ────────────────────────────────────────────────────────────────
-
-pub fn olm_to_sk(e: OLMError, context: &'static str) -> SKError {
-    match e {
-        OLMError::Io(e) => SKError::Io(e),
-        other => SKError::InvalidData {
-            context,
-            detail: other.to_string(),
-        },
-    }
-}
-
-// ── load_meta ────────────────────────────────────────────────────────────────
-
-/// Load PartitionMeta, or recover it by probing layer directories.
-/// Indexes built before meta.json was introduced lack the file.
-pub(crate) fn load_meta(dir: &Path, context: &'static str) -> SKResult<PartitionMeta> {
-    match PartitionMeta::load(dir) {
-        Ok(m) => Ok(m),
-        Err(e) if matches!(e, OLMError::Io(ref io_e) if io_e.kind() == std::io::ErrorKind::NotFound) =>
-        {
-            let mut n = 0usize;
-            while layer_dir(dir, n).exists() {
-                n += 1;
-            }
-            let m = PartitionMeta {
-                n_layers: n,
-                mode: IndexMode::default(),
-            };
-            m.save(dir).map_err(|e| olm_to_sk(e, context))?;
-            Ok(m)
-        }
-        Err(e) => Err(olm_to_sk(e, context)),
-    }
-}
 
 // ── ColBuilder ────────────────────────────────────────────────────────────────
 
-pub(crate) enum ColBuilder {
+pub enum ColBuilder {
     Bit(PersistentBitVecBuilder),
     Int(PersistentCompactIntVecBuilder),
 }
 
 impl ColBuilder {
-    pub(crate) fn set_val(&mut self, slot: usize, value: u32) {
+    pub fn set_val(&mut self, slot: usize, value: u32) {
         match self {
             ColBuilder::Bit(b) => b.set(slot, value > 0),
             ColBuilder::Int(b) => b.set(slot, value),
         }
     }
 
-    pub(crate) fn close(self) -> SKResult<()> {
+    pub fn close(self) -> OKIResult<()> {
         match self {
-            ColBuilder::Bit(b) => b.close().map_err(SKError::Io),
-            ColBuilder::Int(b) => b.close().map_err(SKError::Io),
+            ColBuilder::Bit(b) => b.close().map_err(OKIError::Io),
+            ColBuilder::Int(b) => b.close().map_err(OKIError::Io),
         }
     }
 }
diff --git a/src/obikindex/src/index/error.rs b/src/obikindex/src/index/error.rs
index f9ddf7e4..6f95f6ee 100644
--- a/src/obikindex/src/index/error.rs
+++ b/src/obikindex/src/index/error.rs
@@ -8,6 +8,7 @@ pub enum OKIError {
     Io(io::Error),
     Json(serde_json::Error),
     Partition(SKError),
+    Layer(SKError),
     /// Source index is not in `Indexed` state.
     NotIndexed(std::path::PathBuf),
     /// Source indexes have incompatible configurations (k, m, n_bits).
@@ -20,6 +21,9 @@ pub enum OKIError {
     InvalidInput(String),
     /// Sources mix exact and approximate evidence, or use incompatible approx parameters.
     IncompatibleEvidence(String),
+    /// Malformed data encountered while reading/writing a layer file, tagged
+    /// with the operation that hit it.
+    InvalidData { context: &'static str, detail: String },
 }
 
 pub type OKIResult<T> = Result<T, OKIError>;
@@ -27,15 +31,24 @@ pub type OKIResult<T> = Result<T, OKIError>;
 impl fmt::Display for OKIError {
     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
         match self {
-            OKIError::Io(e)            => write!(f, "I/O error: {e}"),
-            OKIError::Json(e)          => write!(f, "JSON error: {e}"),
-            OKIError::Partition(e)     => write!(f, "partition error: {e}"),
-            OKIError::NotIndexed(p)    => write!(f, "index not fully built: {}", p.display()),
+            OKIError::Io(e) => write!(f, "I/O error: {e}"),
+            OKIError::Json(e) => write!(f, "JSON error: {e}"),
+            OKIError::Partition(e) => write!(f, "partition error: {e}"),
+            OKIError::Layer(e) => write!(f, "layer error: {e}"),
+            OKIError::NotIndexed(p) => write!(f, "index not fully built: {}", p.display()),
             OKIError::IncompatibleConfig => write!(f, "incompatible index configurations"),
-            OKIError::MismatchedMode   => write!(f, "count mode requires all sources to have with_counts=true"),
-            OKIError::DuplicateGenomeLabel(l) => write!(f, "duplicate genome label across sources: {l}"),
-            OKIError::InvalidInput(m)          => write!(f, "invalid input: {m}"),
-            OKIError::IncompatibleEvidence(m)  => write!(f, "incompatible evidence: {m}"),
+            OKIError::MismatchedMode => write!(
+                f,
+                "count mode requires all sources to have with_counts=true"
+            ),
+            OKIError::DuplicateGenomeLabel(l) => {
+                write!(f, "duplicate genome label across sources: {l}")
+            }
+            OKIError::InvalidInput(m) => write!(f, "invalid input: {m}"),
+            OKIError::IncompatibleEvidence(m) => write!(f, "incompatible evidence: {m}"),
+            OKIError::InvalidData { context, detail } => {
+                write!(f, "invalid data ({context}): {detail}")
+            }
         }
     }
 }
@@ -43,22 +56,28 @@ impl fmt::Display for OKIError {
 impl std::error::Error for OKIError {
     fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
         match self {
-            OKIError::Io(e)        => Some(e),
-            OKIError::Json(e)      => Some(e),
+            OKIError::Io(e) => Some(e),
+            OKIError::Json(e) => Some(e),
             OKIError::Partition(e) => Some(e),
-            _                      => None, // IncompatibleConfig, MismatchedMode, DuplicateGenomeLabel
+            _ => None, // IncompatibleConfig, MismatchedMode, DuplicateGenomeLabel
         }
     }
 }
 
 impl From<io::Error> for OKIError {
-    fn from(e: io::Error) -> Self { OKIError::Io(e) }
+    fn from(e: io::Error) -> Self {
+        OKIError::Io(e)
+    }
 }
 
 impl From<serde_json::Error> for OKIError {
-    fn from(e: serde_json::Error) -> Self { OKIError::Json(e) }
+    fn from(e: serde_json::Error) -> Self {
+        OKIError::Json(e)
+    }
 }
 
 impl From<SKError> for OKIError {
-    fn from(e: SKError) -> Self { OKIError::Partition(e) }
+    fn from(e: SKError) -> Self {
+        OKIError::Partition(e)
+    }
 }
diff --git a/src/obikindex/src/index/kmer_index.rs b/src/obikindex/src/index/kmer_index.rs
index 5f4efbd7..3aa0846b 100644
--- a/src/obikindex/src/index/kmer_index.rs
+++ b/src/obikindex/src/index/kmer_index.rs
@@ -2,13 +2,13 @@ use std::fs;
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
 
-use crate::layer::meta::PartitionMeta;
+use crate::layer::KmerLayer;
+use crate::partition::KmerPartition;
 use obisys::progress_bar;
 use rayon::prelude::*;
 
 use obikseq::{set_k, set_m};
 
-use crate::index::common::load_meta;
 use crate::index::error::{OKIError, OKIResult};
 use crate::index::meta::{GenomeInfo, IndexConfig, IndexMeta};
 
@@ -28,12 +28,15 @@ impl KmerIndex {
         genome_info: Option<GenomeInfo>,
     ) -> OKIResult<Self> {
         let root_path = path.as_ref().to_owned();
-        fs::create_dir_all(&root_path).map_err(OKIError::Io)?;
+        fs::create_dir(&root_path).map_err(OKIError::Io)?;
         set_k(config.kmer_size);
         set_m(config.minimizer_size);
         let genomes = genome_info.into_iter().collect();
         let meta = IndexMeta::create_at(&root_path, config, genomes).map_err(OKIError::Io)?;
-        Ok(Self { root_path, meta: Arc::new(meta) })
+        Ok(Self {
+            root_path,
+            meta: Arc::new(meta),
+        })
     }
 
     pub fn open<P: AsRef<Path>>(path: P) -> OKIResult<Self> {
@@ -41,16 +44,30 @@ impl KmerIndex {
         let meta = IndexMeta::open_at(&root_path).map_err(OKIError::Io)?;
         set_k(meta.config.kmer_size);
         set_m(meta.config.minimizer_size);
-        Ok(Self { root_path, meta: Arc::new(meta) })
+        Ok(Self {
+            root_path,
+            meta: Arc::new(meta),
+        })
     }
 
-    /// Return `true` if `path` contains an `index.meta` file.
-    pub fn exists<P: AsRef<Path>>(path: P) -> bool {
+    /// Return `true` if `path` points to a valid obikmer index directory.
+    ///
+    /// A directory is considered an index only when it contains an `index.meta`
+    /// file. The absence of that file means the directory is not an index, even
+    /// if it otherwise exists.
+    ///
+    /// This is a static method (associated function), not an instance method.
+    /// Call it as `KmerIndex::is_an_index(path)`.
+    ///
+    /// `path` is generic over any type implementing [`AsRef<Path>`]. This allows
+    /// callers to pass `&str`, `String`, `PathBuf`, `&Path`, etc. without an
+    /// explicit conversion; the value is coerced via `.as_ref()` inside.
+    pub fn is_an_index<P: AsRef<Path>>(path: P) -> bool {
         IndexMeta::exists(path.as_ref())
     }
 
     /// The index's root directory.
-    pub fn root_path(&self) -> &Path {
+    pub fn dir(&self) -> &Path {
         &self.root_path
     }
     pub fn meta(&self) -> Arc<IndexMeta> {
@@ -102,42 +119,48 @@ impl KmerIndex {
     /// same directory only indirectly, through this method — `obikindexer`
     /// depends on `KmerIndex`, not the other way around — see
     /// `DevDocMD/implementation/partition_layer_cache.md`.
-    pub fn partition_dir(&self, i: usize) -> PathBuf {
-        crate::partition::partition_dir(&self.root_path, i)
+    pub(crate) fn partition_dir(&self, i: usize) -> OKIResult<PathBuf> {
+        Ok(self.partition(i)?.dir().to_path_buf())
+    }
+
+    pub fn partition(&self, i: usize) -> OKIResult<KmerPartition> {
+        let n = self.n_partitions();
+        if i >= n {
+            return Err(OKIError::InvalidData {
+                context: "New KmerPartition",
+                detail: format!("Partition {i} requested, Index has only {n} partitions"),
+            });
+        }
+        Ok(KmerPartition::new(self, i))
+    }
+
+    pub(crate) fn layer_dir(&self, i: usize, j: usize) -> OKIResult<PathBuf> {
+        Ok(self.layer(i, j)?.dir().to_path_buf())
+    }
+
+    pub fn layer(&self, i: usize, j: usize) -> OKIResult<KmerLayer> {
+        self.partition(i)?.layer(j)
     }
 
     /// Path of partition `i`'s layered-index directory (`<partition>/index`).
-    pub fn index_dir(&self, i: usize) -> PathBuf {
+    pub(crate) fn index_dir(&self, i: usize) -> PathBuf {
         crate::partition::index_dir(&self.root_path, i)
     }
 
-    /// Path of layer `l` within partition `i`'s layered index.
-    pub fn layer_dir(&self, i: usize, l: usize) -> PathBuf {
-        crate::layer::layer_dir(&self.index_dir(i), l)
+    /// Number of layers in partition `i` — a disk scan, not a metadata read
+    /// (see [`KmerPartition::n_layers`](crate::partition::KmerPartition::n_layers)).
+    pub fn n_layers(&self, i: usize) -> OKIResult<usize> {
+        Ok(self.partition(i)?.n_layers())
     }
 
-    /// Partition `i`'s metadata (layer count, evidence mode). Returns
-    /// `obiskio::SKResult`, not `OKIResult` — matches the error convention
-    /// of the partition/layer-construction code below (moved here from
-    /// the former `obikpartitionner` crate, which predates `OKIError`);
-    /// `?` still converts it to `OKIResult` at any call site that needs
-    /// one (`OKIError: From<SKError>`).
-    pub fn partition_meta(&self, i: usize) -> obiskio::SKResult<PartitionMeta> {
-        load_meta(&self.index_dir(i), "partition_meta")
-    }
-
-    /// Number of layers in partition `i` — see [`partition_meta`](Self::partition_meta).
-    pub fn n_layers(&self, i: usize) -> obiskio::SKResult<usize> {
-        Ok(self.partition_meta(i)?.n_layers)
-    }
-
-    /// Evidence mode partition `i` was actually built with — see
-    /// [`partition_meta`](Self::partition_meta). Distinct from
-    /// [`evidence_mode`](Self::evidence_mode): that one is the index-level
-    /// config, this one is per-partition ground truth (the two agree in
-    /// practice, but this is what `Layer::open` needs).
-    pub fn partition_mode(&self, i: usize) -> obiskio::SKResult<crate::layer::IndexMode> {
-        Ok(self.partition_meta(i)?.mode)
+    /// Evidence mode partition `i` was actually built with — detected from
+    /// layer 0's evidence files on disk (see
+    /// [`KmerPartition::mode`](crate::partition::KmerPartition::mode)).
+    /// Distinct from [`evidence_mode`](Self::evidence_mode): that one is the
+    /// index-level config, this one is per-partition ground truth (the two
+    /// agree in practice, but this is what `Layer::open` needs).
+    pub fn partition_mode(&self, i: usize) -> OKIResult<crate::layer::IndexMode> {
+        self.partition(i)?.mode()
     }
 
     /// Number of layers per partition.
@@ -150,8 +173,8 @@ impl KmerIndex {
     }
 
     /// Path to the unitigs file for partition `part`, layer `layer`.
-    pub fn layer_unitigs_path(&self, part: usize, layer: usize) -> PathBuf {
-        self.layer_dir(part, layer).join("unitigs.bin")
+    pub fn layer_unitigs_path(&self, part: usize, layer: usize) -> OKIResult<PathBuf> {
+        Ok(self.layer_dir(part, layer)?.join("unitigs.bin"))
     }
 
     /// Pack all partition matrices into single-file format (presence → .pbmx, counts → .pcmx).
@@ -164,13 +187,14 @@ impl KmerIndex {
     /// on-disk format (see `DevDocMD/architecture/siblings.md`) — count
     /// matrices are unaffected, sparse count matrices aren't implemented
     /// (see the sparse-matrix design plan's "Explicitly deferred").
-    pub fn pack_matrices(&self, sparse: bool) -> OKIResult<()> {
+    /// TODO: To be moved somewhere else
+    pub(crate) fn pack_matrices(&self, sparse: bool) -> OKIResult<()> {
         use obicompactvec::{pack_bit_matrix, pack_compact_int_matrix, pack_sparse_bit_matrix};
 
         let n = self.n_partitions();
         let order: Vec<usize> = (0..n).collect();
         let pb = progress_bar("pack", n as u64, "partitions");
-        crate::index::numa::PartitionRunner::new().run(
+        obisys::PartitionRunner::new().run(
             &order,
             |i| -> OKIResult<()> {
                 let index_dir = self.index_dir(i);
@@ -179,7 +203,7 @@ impl KmerIndex {
                 }
                 let n_layers = self.n_layers(i)?;
                 for l in 0..n_layers {
-                    let layer_dir = self.layer_dir(i, l);
+                    let layer_dir = self.layer_dir(i, l)?;
                     let presence_dir = layer_dir.join("presence");
                     let counts_dir = layer_dir.join("counts");
                     if presence_dir.exists() {
@@ -207,7 +231,8 @@ impl KmerIndex {
     ///
     /// Old indexes were built before this file was required.  The number of
     /// kmers is recovered from `unitigs.bin`, which is always present.
-    pub fn upgrade_layer_meta(&self) -> OKIResult<()> {
+    /// TODO: Should not exist anymore
+    pub(crate) fn upgrade_layer_meta(&self) -> OKIResult<()> {
         use obicompactvec::LayerMeta;
         use obiskio::UnitigFileReader;
 
@@ -229,7 +254,15 @@ impl KmerIndex {
                     }
                 };
                 for l in 0..n_layers {
-                    let layer_dir = self.layer_dir(i, l);
+                    let layer_dir = match self.layer_dir(i, l) {
+                        Ok(d) => d,
+                        Err(e) => {
+                            return Some(OKIError::Io(std::io::Error::new(
+                                std::io::ErrorKind::Other,
+                                e.to_string(),
+                            )));
+                        }
+                    };
                     let meta_path = layer_dir.join(LayerMeta::FILENAME);
                     if meta_path.exists() {
                         continue;
diff --git a/src/obikindex/src/index/meta.rs b/src/obikindex/src/index/meta.rs
index 7e272e20..f4a2a4d3 100644
--- a/src/obikindex/src/index/meta.rs
+++ b/src/obikindex/src/index/meta.rs
@@ -21,8 +21,44 @@ pub struct GenomeInfo {
 }
 
 impl GenomeInfo {
+    /// Panics if `label` fails [`validate_label`](Self::validate_label) —
+    /// an invalid genome label is a programmer error to surface loudly,
+    /// not a recoverable condition for this constructor to paper over.
+    /// Callers that need a graceful error (e.g. a CLI parsing user input)
+    /// should call [`validate_label`](Self::validate_label) themselves
+    /// first and report it their own way.
     pub fn new(label: impl Into<String>) -> Self {
-        Self { label: label.into(), meta: HashMap::new() }
+        let label = label.into();
+        Self::validate_label(&label).expect("invalid genome label");
+        Self {
+            label,
+            meta: HashMap::new(),
+        }
+    }
+
+    /// Validate a user-supplied genome label.
+    ///
+    /// Forbidden: `/` (filesystem separator), `=` (--new-label parser separator),
+    /// `\0` (null byte), `\n`, `\r`, `\t` (break CSV output).
+    /// Empty labels are also rejected.
+    pub fn validate_label(label: &str) -> Result<(), String> {
+        if label.is_empty() {
+            return Err("genome label must not be empty".into());
+        }
+        const FORBIDDEN: &[char] = &['/', '=', '\0', '\n', '\r', '\t'];
+        if let Some(c) = label.chars().find(|c| FORBIDDEN.contains(c)) {
+            let display = match c {
+                '\0' => "\\0 (null)".to_string(),
+                '\n' => "\\n (newline)".to_string(),
+                '\r' => "\\r (carriage return)".to_string(),
+                '\t' => "\\t (tab)".to_string(),
+                c => format!("'{c}'"),
+            };
+            return Err(format!(
+                "genome label contains forbidden character {display}"
+            ));
+        }
+        Ok(())
     }
 }
 
@@ -47,7 +83,7 @@ pub struct IndexConfig {
 /// On-disk shape of `index.meta` — the whole file, read/written atomically
 /// as one JSON blob by every `IndexMeta` operation below.
 #[derive(Debug, Clone, Serialize, Deserialize)]
-struct OnDiskMeta {
+struct IndexMetadata {
     version: u32,
     config: IndexConfig,
     #[serde(default)]
@@ -81,21 +117,29 @@ impl IndexMeta {
     /// Create a brand-new `index.meta` for `index`, from fixed config and
     /// an initial genome list (often empty — genomes are usually added
     /// later via [`push_genome`](Self::push_genome)).
-    pub fn create(index: &KmerIndex, config: IndexConfig, genomes: Vec<GenomeInfo>) -> io::Result<Self> {
-        Self::create_at(index.root_path(), config, genomes)
+    pub fn create(
+        index: &KmerIndex,
+        config: IndexConfig,
+        genomes: Vec<GenomeInfo>,
+    ) -> io::Result<Self> {
+        Self::create_at(index.dir(), config, genomes)
     }
 
     /// Same as [`create`](Self::create), taking a raw path — for
     /// `KmerIndex::create` itself, which doesn't have a complete
     /// `KmerIndex` yet to hand in (it's what's being built).
-    pub(crate) fn create_at(root_path: &Path, config: IndexConfig, genomes: Vec<GenomeInfo>) -> io::Result<Self> {
+    pub(crate) fn create_at(
+        root_path: &Path,
+        config: IndexConfig,
+        genomes: Vec<GenomeInfo>,
+    ) -> io::Result<Self> {
         let meta = Self {
             root_path: root_path.to_owned(),
             version: META_VERSION,
             config,
             lock: RwLock::new(()),
         };
-        meta.write_full(&OnDiskMeta {
+        meta.write_full(&IndexMetadata {
             version: meta.version,
             config: meta.config.clone(),
             genomes,
@@ -106,7 +150,7 @@ impl IndexMeta {
 
     /// Reopen the `index.meta` already on disk for `index`.
     pub fn open(index: &KmerIndex) -> io::Result<Self> {
-        Self::open_at(index.root_path())
+        Self::open_at(index.dir())
     }
 
     pub(crate) fn open_at(root_path: &Path) -> io::Result<Self> {
@@ -119,7 +163,12 @@ impl IndexMeta {
         })
     }
 
-    pub fn exists(root: &Path) -> bool {
+    /// Returns whether the metadata file exists in `root`.
+    ///
+    /// This is a static method (associated function), not an instance method.
+    /// It only checks for the presence of the `index.meta` file inside the
+    /// given directory; it does not validate its contents.
+    pub(crate) fn exists(root: &Path) -> bool {
         root.join(META_FILENAME).exists()
     }
 
@@ -177,10 +226,20 @@ impl IndexMeta {
     /// (e.g. `self.meta = Arc::new(IndexMeta::open(self)?)`) — this method
     /// only updates the file, it has no way to reach back into whatever
     /// `KmerIndex` holds it.
-    pub fn rewrite_config(&self, config: IndexConfig, genomes: Vec<GenomeInfo>) -> io::Result<()> {
+    /// TODO: This methode is strange
+    pub(crate) fn rewrite_config(
+        &self,
+        config: IndexConfig,
+        genomes: Vec<GenomeInfo>,
+    ) -> io::Result<()> {
         let _guard = self.lock.write().unwrap();
         let state = Self::read_full(&self.root_path)?.state;
-        self.write_full(&OnDiskMeta { version: self.version, config, genomes, state })
+        self.write_full(&IndexMetadata {
+            version: self.version,
+            config,
+            genomes,
+            state,
+        })
     }
 
     pub fn set_state(&self, state: IndexState) -> io::Result<()> {
@@ -221,12 +280,12 @@ impl IndexMeta {
 
     // ── private ──────────────────────────────────────────────────────────────
 
-    fn read_full(root: &Path) -> io::Result<OnDiskMeta> {
+    fn read_full(root: &Path) -> io::Result<IndexMetadata> {
         let file = fs::File::open(root.join(META_FILENAME))?;
         serde_json::from_reader(file).map_err(io::Error::other)
     }
 
-    fn write_full(&self, on_disk: &OnDiskMeta) -> io::Result<()> {
+    fn write_full(&self, on_disk: &IndexMetadata) -> io::Result<()> {
         let file = fs::File::create(self.root_path.join(META_FILENAME))?;
         serde_json::to_writer_pretty(file, on_disk).map_err(io::Error::other)
     }
@@ -248,26 +307,3 @@ fn label_from_path(path: &Path) -> String {
         s
     }
 }
-
-/// Validate a user-supplied genome label.
-///
-/// Forbidden: `/` (filesystem separator), `=` (--new-label parser separator),
-/// `\0` (null byte), `\n`, `\r`, `\t` (break CSV output).
-/// Empty labels are also rejected.
-pub fn validate_label(label: &str) -> Result<(), String> {
-    if label.is_empty() {
-        return Err("genome label must not be empty".into());
-    }
-    const FORBIDDEN: &[char] = &['/', '=', '\0', '\n', '\r', '\t'];
-    if let Some(c) = label.chars().find(|c| FORBIDDEN.contains(c)) {
-        let display = match c {
-            '\0' => "\\0 (null)".to_string(),
-            '\n' => "\\n (newline)".to_string(),
-            '\r' => "\\r (carriage return)".to_string(),
-            '\t' => "\\t (tab)".to_string(),
-            c    => format!("'{c}'"),
-        };
-        return Err(format!("genome label contains forbidden character {display}"));
-    }
-    Ok(())
-}
diff --git a/src/obikindex/src/index/mod.rs b/src/obikindex/src/index/mod.rs
index 975676f0..8a60f488 100644
--- a/src/obikindex/src/index/mod.rs
+++ b/src/obikindex/src/index/mod.rs
@@ -1,39 +1,13 @@
 pub mod error;
 pub mod meta;
-pub mod predicate;
 pub mod state;
 mod builder;
 mod common;
-mod distance;
-mod dump;
-mod dump_layer;
-pub mod filter;
-mod graph_pipeline;
 mod kmer_index;
-mod matrix_store;
-mod merge;
-mod merge_layer;
-mod numa;
-mod query_layer;
-mod rebuild;
-mod rebuild_layer;
-mod reindex;
-mod select;
-mod select_layer;
-mod stats;
 
 pub use error::{OKIError, OKIResult};
 pub use builder::IndexBuilder;
-pub use common::olm_to_sk;
-pub use graph_pipeline::{materialize_layer, write_graph_as_unitigs};
-pub use distance::{DistanceMetric, DistanceOutput};
-pub use filter::{GroupQuorumFilter, KmerFilter, passes_all};
+pub use common::ColBuilder;
 pub use kmer_index::KmerIndex;
-pub use merge_layer::MergeMode;
-pub use meta::{validate_label, GenomeInfo, IndexConfig, IndexMeta, META_FILENAME};
-pub use predicate::{GroupFilterParams, MetaPred};
-pub use query_layer::{KmerDesc, QueryHit, QueryStats};
-pub use select_layer::{AggOp, OutputCol};
+pub use meta::{GenomeInfo, IndexConfig, IndexMeta, META_FILENAME};
 pub use state::IndexState;
-pub use stats::IndexBitsPerKmer;
-pub use numa::PartitionRunner;
diff --git a/src/obikindex/src/index/progress.rs b/src/obikindex/src/index/progress.rs
deleted file mode 100644
index 1c3f948c..00000000
--- a/src/obikindex/src/index/progress.rs
+++ /dev/null
@@ -1,21 +0,0 @@
-use std::sync::OnceLock;
-
-use indicatif::MultiProgress;
-
-static MULTI: OnceLock<MultiProgress> = OnceLock::new();
-
-/// Initialise the shared progress display.  Call once from the binary before
-/// any index operation.  Subsequent calls are silently ignored.
-pub fn init(multi: MultiProgress) {
-    let _ = MULTI.set(multi);
-}
-
-/// Return the shared `MultiProgress`, creating a plain default one if the
-/// binary never called [`init`].
-pub fn get() -> &'static MultiProgress {
-    MULTI.get_or_init(MultiProgress::new)
-}
-
-pub(crate) fn multi() -> &'static MultiProgress {
-    get()
-}
diff --git a/src/obikindex/src/layer/content_layer.rs b/src/obikindex/src/layer/content_layer.rs
index 0222368d..c318ab0a 100644
--- a/src/obikindex/src/layer/content_layer.rs
+++ b/src/obikindex/src/layer/content_layer.rs
@@ -24,77 +24,103 @@
 
 use std::path::{Path, PathBuf};
 
+use crate::layer::utils::LAYERNAME_SUFFIX;
+
 use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix};
 use obikseq::CanonicalKmer;
 
-use crate::layer::error::OLMResult;
-use crate::layer::meta::IndexMode;
+use crate::index::error::OKIResult;
 use crate::layer::mphf_layer::{
     EVIDENCE_FILE, EvidenceKind, FINGERPRINT_FILE, MPHF_FILE, UNITIGS_FILE,
 };
 use crate::layer::typed_layer::{COUNTS_DIR, LayerContent, PRESENCE_DIR, TypedLayer};
+use crate::partition::KmerPartition;
+
+/// Superkmer-file extension used by every pre-layer-construction artifact
+/// below (`raw`/`dereplicated`) — an implementation detail of the SK file
+/// format, never meant to leak as a literal string past this module.
+const SK_EXT: &str = "skmer.zst";
 
 /// One layer, at any point in its life — see the module docs. Only
 /// [`Empty`](Layer::Empty) and the two ready-to-read states
 /// (`Count`/`Presence`) exist so far; states in between (unitigs written,
 /// MPHF built, evidence built, matrix not yet built) are not represented
 /// yet.
+///
+/// Every variant carries `dir` + `id` — a layer's identity never depends on
+/// its state. `id` is its own layer number within the partition (not yet
+/// consumed by anything — `KmerPartition` doesn't hand it in yet, that
+/// lands with `KmerLayer::at`). `dir`/`id` live here, on `KmerLayer` itself,
+/// not inside `TypedLayer`/`MphfLayer`: those only ever need `dir`
+/// transiently, while opening — `KmerLayer` is what has to answer `.dir()`
+/// after the fact, so it's the one that keeps it.
 pub enum KmerLayer {
     /// Just a directory — nothing built yet. Every method below other than
     /// the path accessors panics on this variant: calling them means the
     /// caller assumed a layer was ready when it wasn't, an implementation
     /// error to surface loudly, not paper over with a default value.
-    Empty {
+    Empty { dir: PathBuf, id: usize },
+    Count {
         dir: PathBuf,
+        id: usize,
+        layer: TypedLayer<PersistentCompactIntMatrix>,
+    },
+    Presence {
+        dir: PathBuf,
+        id: usize,
+        layer: TypedLayer<PersistentBitMatrix>,
     },
-    Count(TypedLayer<PersistentCompactIntMatrix>),
-    Presence(TypedLayer<PersistentBitMatrix>),
 }
 
 impl KmerLayer {
+    pub fn new(partition: &KmerPartition, i: usize) -> Self {
+        let dir = partition.dir().join(format!("{LAYERNAME_SUFFIX}{i}"));
+        KmerLayer::Empty { dir, id: i }
+    }
     /// An empty shell at `dir` — creates the directory (if it doesn't
     /// already exist) but nothing inside it. The starting state for
     /// building a new layer.
-    pub fn create(dir: &Path) -> std::io::Result<Self> {
-        std::fs::create_dir_all(dir)?;
-        Ok(KmerLayer::Empty {
-            dir: dir.to_owned(),
-        })
+    pub fn create(self) -> std::io::Result<Self> {
+        std::fs::create_dir_all(self.dir())?;
+        Ok(self)
     }
 
-    /// Open one layer, auto-detecting count vs. presence from what is
-    /// actually on disk (`counts/` present and wanted, else presence) — the
-    /// single source of truth every caller that opens a layer's own matrix
-    /// should share, so two callers can never disagree about which format a
-    /// given layer is. Dense vs. sparse presence storage is
-    /// `PersistentBitMatrix::open`'s own concern (it's a 4-way
-    /// `Columnar`/`Packed`/`Sparse`/`Implicit` enum internally), not decided
-    /// here.
-    pub fn open(layer_dir: &Path, mode: &IndexMode, with_counts: bool) -> OLMResult<Self> {
-        if with_counts && layer_dir.join(COUNTS_DIR).exists() {
-            return TypedLayer::<PersistentCompactIntMatrix>::open(layer_dir, mode)
-                .map(KmerLayer::Count);
+    /// Open this layer, auto-detecting count vs. presence from what is
+    /// actually on disk (`counts/` present, else presence) — the single
+    /// source of truth every caller that opens a layer's own matrix should
+    /// share, so two callers can never disagree about which format a given
+    /// layer is. Dense vs. sparse presence storage is `PersistentBitMatrix::
+    /// open`'s own concern (a 4-way `Columnar`/`Packed`/`Sparse`/`Implicit`
+    /// enum internally), not decided here. A no-op (returns `self`
+    /// unchanged) if already `Count`/`Presence`.
+    pub fn open(self) -> OKIResult<Self> {
+        let (dir, id) = match self {
+            KmerLayer::Empty { dir, id } => (dir, id),
+            already_open => return Ok(already_open),
+        };
+        if dir.join(COUNTS_DIR).exists() {
+            let layer = TypedLayer::<PersistentCompactIntMatrix>::open(&dir)?;
+            Ok(KmerLayer::Count { dir, id, layer })
+        } else {
+            let layer = TypedLayer::<PersistentBitMatrix>::open(&dir)?;
+            Ok(KmerLayer::Presence { dir, id, layer })
         }
-        TypedLayer::<PersistentBitMatrix>::open(layer_dir, mode).map(KmerLayer::Presence)
     }
 
-    // ── Paths — only meaningful before anything is built ───────────────
+    // ── Paths — valid in every state, `dir` never varies with content ────
     //
-    // Once a layer is `Count`/`Presence`, its caller already knows the
-    // directory (it had to pass it to `open`) — these exist for builder
-    // code holding an `Empty` layer, so the `mphf.bin`/`unitigs.bin`/…
-    // naming stays defined once, here, rather than re-declared as
-    // string literals at every call site that writes into a layer
-    // directory (the same duplication `layer_dir`/`index_dir` fixed one
-    // level up — see `DevDocMD/implementation/partition_layer_cache.md`).
+    // The `mphf.bin`/`unitigs.bin`/… naming stays defined once, here,
+    // rather than re-declared as string literals at every call site that
+    // writes into a layer directory (the same duplication `layer_dir`/
+    // `index_dir` fixed one level up — see `DevDocMD/implementation/
+    // partition_layer_cache.md`).
 
-    /// This layer's own directory. Panics on `Count`/`Presence` — by that
-    /// point the caller already has the directory it opened with; asking
-    /// again here would mean it lost track of its own state.
+    /// This layer's own directory.
     pub fn dir(&self) -> &Path {
         match self {
-            KmerLayer::Empty { dir } => dir,
-            _ => panic!("Layer::dir() only available on Empty — caller already has this path"),
+            KmerLayer::Empty { dir, .. } => dir,
+            KmerLayer::Count { dir, .. } => dir,
+            KmerLayer::Presence { dir, .. } => dir,
         }
     }
 
@@ -117,36 +143,57 @@ impl KmerLayer {
         self.dir().join(PRESENCE_DIR)
     }
 
+    /// Path of this layer's raw, not-yet-dereplicated superkmer file —
+    /// written by whichever algorithm routes superkmers into this layer
+    /// (today: `obikindexer::algorithms::partitionner::PartitionRouter`),
+    /// read by whichever algorithm dereplicates it (today:
+    /// `obikindexer::algorithms::dereplicator::Dereplicator`). Naming this
+    /// once here, rather than in either algorithm submodule, is what lets
+    /// the two agree on the filename without depending on each other
+    /// directly — see `DevDocMD/implementation/partition_layer_cache.md`.
+    pub fn raw_superkmers_path(&self) -> PathBuf {
+        self.dir().join(format!("raw.{SK_EXT}"))
+    }
+
+    /// Path of this layer's dereplicated superkmer file — written by
+    /// `obikindexer::algorithms::dereplicator::Dereplicator`, read by
+    /// whichever algorithm counts kmer abundances from it (today:
+    /// `obikindexer::algorithms::partitionner::PartitionRouter::count_kmer`)
+    /// and, later, by `obikindex::build_index_layer` to build the real layer.
+    pub fn dereplicated_superkmers_path(&self) -> PathBuf {
+        self.dir().join(format!("dereplicated.{SK_EXT}"))
+    }
+
     // ── Ready-only surface ───────────────────────────────────────────────
 
     pub fn content(&self) -> LayerContent {
         match self {
-            KmerLayer::Count(_) => LayerContent::Count,
-            KmerLayer::Presence(_) => LayerContent::Presence,
+            KmerLayer::Count { .. } => LayerContent::Count,
+            KmerLayer::Presence { .. } => LayerContent::Presence,
             KmerLayer::Empty { .. } => panic!("Layer::content() called on an Empty layer"),
         }
     }
 
     pub fn evidence_kind(&self) -> EvidenceKind {
         match self {
-            KmerLayer::Count(l) => l.evidence_kind(),
-            KmerLayer::Presence(l) => l.evidence_kind(),
+            KmerLayer::Count { layer, .. } => layer.evidence_kind(),
+            KmerLayer::Presence { layer, .. } => layer.evidence_kind(),
             KmerLayer::Empty { .. } => panic!("Layer::evidence_kind() called on an Empty layer"),
         }
     }
 
     pub fn n(&self) -> usize {
         match self {
-            KmerLayer::Count(l) => l.n(),
-            KmerLayer::Presence(l) => l.n(),
+            KmerLayer::Count { layer, .. } => layer.n(),
+            KmerLayer::Presence { layer, .. } => layer.n(),
             KmerLayer::Empty { .. } => panic!("Layer::n() called on an Empty layer"),
         }
     }
 
     pub fn find_slot(&self, kmer: CanonicalKmer) -> Option<usize> {
         match self {
-            KmerLayer::Count(l) => l.find_slot(kmer),
-            KmerLayer::Presence(l) => l.find_slot(kmer),
+            KmerLayer::Count { layer, .. } => layer.find_slot(kmer),
+            KmerLayer::Presence { layer, .. } => layer.find_slot(kmer),
             KmerLayer::Empty { .. } => panic!("Layer::find_slot() called on an Empty layer"),
         }
     }
@@ -156,16 +203,16 @@ impl KmerLayer {
     /// the evidence check `find_slot`/`find` would perform is redundant.
     pub fn hash_batch(&self, kmers: &[CanonicalKmer]) -> Vec<usize> {
         match self {
-            KmerLayer::Count(l) => l.hash_batch(kmers),
-            KmerLayer::Presence(l) => l.hash_batch(kmers),
+            KmerLayer::Count { layer, .. } => layer.hash_batch(kmers),
+            KmerLayer::Presence { layer, .. } => layer.hash_batch(kmers),
             KmerLayer::Empty { .. } => panic!("Layer::hash_batch() called on an Empty layer"),
         }
     }
 
     pub fn n_cols(&self) -> usize {
         match self {
-            KmerLayer::Count(l) => l.n_cols(),
-            KmerLayer::Presence(l) => l.n_cols(),
+            KmerLayer::Count { layer, .. } => layer.n_cols(),
+            KmerLayer::Presence { layer, .. } => layer.n_cols(),
             KmerLayer::Empty { .. } => panic!("Layer::n_cols() called on an Empty layer"),
         }
     }
@@ -178,10 +225,10 @@ impl KmerLayer {
     /// presence (`!= 0`) in place.
     pub fn fill_sub_matrix_carries(&self, slots: &[usize], out: &mut [Vec<bool>]) {
         match self {
-            KmerLayer::Presence(l) => l.fill_sub_matrix(slots, out),
-            KmerLayer::Count(l) => {
+            KmerLayer::Presence { layer, .. } => layer.fill_sub_matrix(slots, out),
+            KmerLayer::Count { layer, .. } => {
                 let mut counts: Vec<Vec<u32>> = out.iter().map(|_| Vec::new()).collect();
-                l.fill_sub_matrix(slots, &mut counts);
+                layer.fill_sub_matrix(slots, &mut counts);
                 for (o, c) in out.iter_mut().zip(counts.iter()) {
                     o.clear();
                     o.extend(c.iter().map(|&v| v != 0));
@@ -196,8 +243,8 @@ impl KmerLayer {
     /// Raw MPHF lookup: kmer → slot, no membership check.
     pub fn hash(&self, kmer: CanonicalKmer) -> usize {
         match self {
-            KmerLayer::Count(l) => l.hash(kmer),
-            KmerLayer::Presence(l) => l.hash(kmer),
+            KmerLayer::Count { layer, .. } => layer.hash(kmer),
+            KmerLayer::Presence { layer, .. } => layer.hash(kmer),
             KmerLayer::Empty { .. } => panic!("Layer::hash() called on an Empty layer"),
         }
     }
@@ -205,8 +252,8 @@ impl KmerLayer {
     /// Iterate over all canonical kmers in the layer, in deterministic order.
     pub fn iter_kmers(&self) -> crate::layer::mphf_layer::KmerIter {
         match self {
-            KmerLayer::Count(l) => l.iter_kmers(),
-            KmerLayer::Presence(l) => l.iter_kmers(),
+            KmerLayer::Count { layer, .. } => layer.iter_kmers(),
+            KmerLayer::Presence { layer, .. } => layer.iter_kmers(),
             KmerLayer::Empty { .. } => panic!("Layer::iter_kmers() called on an Empty layer"),
         }
     }
@@ -215,8 +262,8 @@ impl KmerLayer {
     /// sequence index in `unitigs.bin`.
     pub fn enumerate_kmers(&self) -> std::iter::Enumerate<crate::layer::mphf_layer::KmerIter> {
         match self {
-            KmerLayer::Count(l) => l.enumerate_kmers(),
-            KmerLayer::Presence(l) => l.enumerate_kmers(),
+            KmerLayer::Count { layer, .. } => layer.enumerate_kmers(),
+            KmerLayer::Presence { layer, .. } => layer.enumerate_kmers(),
             KmerLayer::Empty { .. } => panic!("Layer::enumerate_kmers() called on an Empty layer"),
         }
     }
@@ -224,8 +271,8 @@ impl KmerLayer {
     /// Iterate over the layer's canonical kmers in batches of `n`.
     pub fn iter_kmers_batch(&self, n: usize) -> crate::layer::mphf_layer::KmerBatchIter {
         match self {
-            KmerLayer::Count(l) => l.iter_kmers_batch(n),
-            KmerLayer::Presence(l) => l.iter_kmers_batch(n),
+            KmerLayer::Count { layer, .. } => layer.iter_kmers_batch(n),
+            KmerLayer::Presence { layer, .. } => layer.iter_kmers_batch(n),
             KmerLayer::Empty { .. } => panic!("Layer::iter_kmers_batch() called on an Empty layer"),
         }
     }
@@ -237,8 +284,8 @@ impl KmerLayer {
         n: usize,
     ) -> Box<dyn Iterator<Item = (usize, Vec<CanonicalKmer>)> + Send + 'static> {
         match self {
-            KmerLayer::Count(l) => Box::new(l.enumerate_kmers_batch(n)),
-            KmerLayer::Presence(l) => Box::new(l.enumerate_kmers_batch(n)),
+            KmerLayer::Count { layer, .. } => Box::new(layer.enumerate_kmers_batch(n)),
+            KmerLayer::Presence { layer, .. } => Box::new(layer.enumerate_kmers_batch(n)),
             KmerLayer::Empty { .. } => {
                 panic!("Layer::enumerate_kmers_batch() called on an Empty layer")
             }
diff --git a/src/obikindex/src/layer/error.rs b/src/obikindex/src/layer/error.rs
deleted file mode 100644
index c6b4e7f6..00000000
--- a/src/obikindex/src/layer/error.rs
+++ /dev/null
@@ -1,50 +0,0 @@
-use std::fmt;
-use std::io;
-
-#[derive(Debug)]
-pub enum OLMError {
-    Io(io::Error),
-    Json(serde_json::Error),
-    Mphf(String),
-    InvalidLayer(String),
-}
-
-pub type OLMResult<T> = Result<T, OLMError>;
-
-impl fmt::Display for OLMError {
-    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
-        match self {
-            OLMError::Io(e)            => write!(f, "I/O error: {e}"),
-            OLMError::Json(e)          => write!(f, "JSON error: {e}"),
-            OLMError::Mphf(s)          => write!(f, "MPHF error: {s}"),
-            OLMError::InvalidLayer(s)  => write!(f, "invalid layer: {s}"),
-        }
-    }
-}
-
-impl std::error::Error for OLMError {
-    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
-        match self {
-            OLMError::Io(e)   => Some(e),
-            OLMError::Json(e) => Some(e),
-            _                 => None,
-        }
-    }
-}
-
-impl From<io::Error> for OLMError {
-    fn from(e: io::Error) -> Self { OLMError::Io(e) }
-}
-
-impl From<serde_json::Error> for OLMError {
-    fn from(e: serde_json::Error) -> Self { OLMError::Json(e) }
-}
-
-impl From<obiskio::SKError> for OLMError {
-    fn from(e: obiskio::SKError) -> Self {
-        match e {
-            obiskio::SKError::Io(io_err) => OLMError::Io(io_err),
-            other => OLMError::InvalidLayer(other.to_string()),
-        }
-    }
-}
diff --git a/src/obikindex/src/layer/evidence.rs b/src/obikindex/src/layer/evidence.rs
index 48736e42..15cd126c 100644
--- a/src/obikindex/src/layer/evidence.rs
+++ b/src/obikindex/src/layer/evidence.rs
@@ -6,14 +6,14 @@ use std::path::Path;
 
 use memmap2::Mmap;
 
-use crate::layer::error::{OLMError, OLMResult};
+use crate::index::error::{OKIError, OKIResult};
 
 pub struct Evidence {
     mmap: Mmap,
 }
 
 impl Evidence {
-    pub fn open(path: &Path) -> OLMResult<Self> {
+    pub fn open(path: &Path) -> OKIResult<Self> {
         let f = File::open(path)?;
         let mmap = unsafe { Mmap::map(&f)? };
         Ok(Self { mmap })
@@ -26,16 +26,16 @@ impl Evidence {
         (raw >> 7, (raw & 0x7F) as u8)
     }
 
+    #[inline]
+    pub fn encode(chunk_id: u32, rank: u8) -> u32 {
+        (chunk_id << 7) | (rank as u32 & 0x7F)
+    }
+
     pub fn len(&self) -> usize {
         self.mmap.len() / 4
     }
 }
 
-#[inline]
-pub fn encode(chunk_id: u32, rank: u8) -> u32 {
-    (chunk_id << 7) | (rank as u32 & 0x7F)
-}
-
 pub struct EvidenceWriter {
     buf: Vec<u32>,
 }
@@ -47,13 +47,13 @@ impl EvidenceWriter {
 
     #[inline]
     pub fn set(&mut self, slot: usize, chunk_id: u32, rank: u8) {
-        self.buf[slot] = encode(chunk_id, rank);
+        self.buf[slot] = Evidence::encode(chunk_id, rank);
     }
 
-    pub fn write(self, path: &Path) -> OLMResult<()> {
+    pub fn write(self, path: &Path) -> OKIResult<()> {
         let mut f = BufWriter::new(File::create(path)?);
         for v in self.buf {
-            f.write_all(&v.to_le_bytes()).map_err(OLMError::Io)?;
+            f.write_all(&v.to_le_bytes()).map_err(OKIError::Io)?;
         }
         Ok(())
     }
diff --git a/src/obikindex/src/layer/fingerprint.rs b/src/obikindex/src/layer/fingerprint.rs
index 2297fc15..c5c199d8 100644
--- a/src/obikindex/src/layer/fingerprint.rs
+++ b/src/obikindex/src/layer/fingerprint.rs
@@ -14,7 +14,7 @@ use std::path::Path;
 use bitvec::prelude::*;
 use memmap2::Mmap;
 
-use crate::layer::error::{OLMError, OLMResult};
+use crate::index::error::{OKIError, OKIResult};
 
 const MAGIC: &[u8; 4] = b"FPVF";
 const HEADER: usize = 16;
@@ -30,15 +30,15 @@ pub struct FingerprintVec {
 }
 
 impl FingerprintVec {
-    pub fn open(path: &Path) -> OLMResult<Self> {
+    pub fn open(path: &Path) -> OKIResult<Self> {
         let f = File::open(path)?;
         let mmap = unsafe { Mmap::map(&f)? };
         if mmap.len() < HEADER || &mmap[..4] != MAGIC {
-            return Err(OLMError::InvalidLayer("bad fingerprint magic".into()));
+            return Err(OKIError::InvalidData { context: "layer", detail: "bad fingerprint magic".into() });
         }
         let b = mmap[4];
         if b == 0 || b > 64 {
-            return Err(OLMError::InvalidLayer("invalid fingerprint width".into()));
+            return Err(OKIError::InvalidData { context: "layer", detail: "invalid fingerprint width".into() });
         }
         let n = u64::from_le_bytes(mmap[8..16].try_into().unwrap()) as usize;
         let mask: u64 = if b == 64 { u64::MAX } else { (1u64 << b) - 1 };
@@ -89,12 +89,12 @@ impl FingerprintVecWriter {
         bits[lo .. lo + self.b as usize].store_le(fingerprint);
     }
 
-    pub fn write(self, path: &Path) -> OLMResult<()> {
-        let mut f = BufWriter::new(File::create(path).map_err(OLMError::Io)?);
-        f.write_all(MAGIC).map_err(OLMError::Io)?;
-        f.write_all(&[self.b, 0, 0, 0]).map_err(OLMError::Io)?;
-        f.write_all(&(self.n as u64).to_le_bytes()).map_err(OLMError::Io)?;
-        f.write_all(&self.buf).map_err(OLMError::Io)?;
+    pub fn write(self, path: &Path) -> OKIResult<()> {
+        let mut f = BufWriter::new(File::create(path).map_err(OKIError::Io)?);
+        f.write_all(MAGIC).map_err(OKIError::Io)?;
+        f.write_all(&[self.b, 0, 0, 0]).map_err(OKIError::Io)?;
+        f.write_all(&(self.n as u64).to_le_bytes()).map_err(OKIError::Io)?;
+        f.write_all(&self.buf).map_err(OKIError::Io)?;
         Ok(())
     }
 }
diff --git a/src/obikindex/src/layer/map.rs b/src/obikindex/src/layer/map.rs
deleted file mode 100644
index 5e77c1cc..00000000
--- a/src/obikindex/src/layer/map.rs
+++ /dev/null
@@ -1,128 +0,0 @@
-use std::collections::HashMap;
-use std::fs;
-use std::path::{Path, PathBuf};
-
-use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix, StorageKind};
-use obikseq::CanonicalKmer;
-use obiskio::{UnitigFileWriter, DEFAULT_BLOCK_BITS};
-
-use crate::layer::error::{OLMError, OLMResult};
-use crate::layer::typed_layer::{layer_dir, Hit, TypedLayer, LayerContent, LayerData};
-use crate::layer::meta::{IndexMode, PartitionMeta};
-use crate::layer::mphf_layer::EvidenceKind;
-
-/// Layered kmer index for a single partition.
-///
-/// Each layer covers a disjoint kmer set. Queries probe layers in order;
-/// the first match wins. Adding a dataset appends a new layer without
-/// rebuilding existing ones.
-pub struct LayeredMap<D: LayerData = ()> {
-    root:   PathBuf,
-    meta:   PartitionMeta,
-    layers: Vec<TypedLayer<D>>,
-}
-
-// ── Common methods ────────────────────────────────────────────────────────────
-
-impl<D: LayerData> LayeredMap<D> {
-    /// Open an existing layered index at `root`.
-    /// The mode is read once from `PartitionMeta` and applied to all layers.
-    pub fn open(root: &Path) -> OLMResult<Self> {
-        let meta = PartitionMeta::load(root)?;
-        let layers = (0..meta.n_layers)
-            .map(|i| TypedLayer::<D>::open(&layer_dir(root, i), &meta.mode))
-            .collect::<OLMResult<Vec<_>>>()?;
-        Ok(Self { root: root.to_owned(), meta, layers })
-    }
-
-    /// Create a new, empty layered index at `root` with the given mode.
-    pub fn create(root: &Path, mode: IndexMode) -> OLMResult<Self> {
-        fs::create_dir_all(root)?;
-        let meta = PartitionMeta::new(mode);
-        meta.save(root)?;
-        Ok(Self { root: root.to_owned(), meta, layers: Vec::new() })
-    }
-
-    pub fn n_layers(&self) -> usize { self.layers.len() }
-    pub fn layer(&self, i: usize) -> &TypedLayer<D> { &self.layers[i] }
-    pub fn mode(&self) -> &IndexMode { &self.meta.mode }
-
-    /// TypedLayer `i`'s content (`Count`/`Presence`) — a lightweight disk probe,
-    /// no MPHF/matrix opened. For picking a `D` before committing to
-    /// [`TypedLayer::open`], or for reporting/diagnostics over an already-open
-    /// `LayeredMap` without re-opening every layer under a different `D`.
-    pub fn detect_layer_content(&self, i: usize) -> LayerContent {
-        LayerContent::detect(&layer_dir(&self.root, i))
-    }
-
-    /// TypedLayer `i`'s storage format — a lightweight disk probe, no
-    /// MPHF/matrix opened. See [`detect_layer_content`](Self::detect_layer_content).
-    pub fn detect_layer_storage(&self, i: usize) -> OLMResult<StorageKind> {
-        let dir = layer_dir(&self.root, i);
-        match LayerContent::detect(&dir) {
-            LayerContent::Count    => PersistentCompactIntMatrix::detect_storage(&dir).map_err(OLMError::Io),
-            LayerContent::Presence => PersistentBitMatrix::detect_storage(&dir).map_err(OLMError::Io),
-        }
-    }
-
-    /// TypedLayer `i`'s evidence mode — a lightweight disk probe, no
-    /// MPHF/matrix opened. See [`detect_layer_content`](Self::detect_layer_content).
-    pub fn detect_layer_evidence(&self, i: usize) -> OLMResult<EvidenceKind> {
-        EvidenceKind::detect(&layer_dir(&self.root, i))
-    }
-
-    /// Query `kmer` across all layers. Returns `(layer_index, Hit)` on match.
-    pub fn query(&self, kmer: CanonicalKmer) -> Option<(usize, Hit<D::Item>)> {
-        self.layers
-            .iter()
-            .enumerate()
-            .find_map(|(i, layer)| layer.query(kmer).map(|hit| (i, hit)))
-    }
-
-    pub fn next_layer_writer(&self) -> OLMResult<UnitigFileWriter> {
-        let dir = layer_dir(&self.root, self.layers.len());
-        TypedLayer::<D>::unitig_writer(&dir)
-    }
-
-    fn append_layer(&mut self) -> OLMResult<()> {
-        let i = self.layers.len();
-        let dir = layer_dir(&self.root, i);
-        self.layers.push(TypedLayer::<D>::open(&dir, &self.meta.mode)?);
-        self.meta.n_layers = self.layers.len();
-        self.meta.save(&self.root)?;
-        Ok(())
-    }
-}
-
-// ── Mode 1 — set membership ───────────────────────────────────────────────────
-
-impl LayeredMap<()> {
-    pub fn push_layer(&mut self) -> OLMResult<usize> {
-        let i = self.layers.len();
-        let dir = layer_dir(&self.root, i);
-        TypedLayer::<()>::build(&dir, DEFAULT_BLOCK_BITS, &self.meta.mode)?;
-        self.append_layer()?;
-        Ok(i)
-    }
-}
-
-// ── Mode 2 — count matrix ─────────────────────────────────────────────────────
-
-impl LayeredMap<PersistentCompactIntMatrix> {
-    pub fn push_layer(&mut self, count_of: impl Fn(CanonicalKmer) -> u32) -> OLMResult<usize> {
-        let i = self.layers.len();
-        let dir = layer_dir(&self.root, i);
-        TypedLayer::<PersistentCompactIntMatrix>::build(&dir, DEFAULT_BLOCK_BITS, &self.meta.mode, count_of)?;
-        self.append_layer()?;
-        Ok(i)
-    }
-
-    pub fn push_layer_from_map(&mut self, counts: &HashMap<CanonicalKmer, u32>) -> OLMResult<usize> {
-        self.push_layer(|kmer| counts.get(&kmer).copied().unwrap_or(0))
-    }
-}
-
-#[cfg(test)]
-#[path = "tests/map.rs"]
-mod tests;
-
diff --git a/src/obikindex/src/layer/meta.rs b/src/obikindex/src/layer/meta.rs
index 8c4de76b..935ae45f 100644
--- a/src/obikindex/src/layer/meta.rs
+++ b/src/obikindex/src/layer/meta.rs
@@ -1,18 +1,20 @@
-use std::fs::File;
 use std::path::Path;
 
 use serde::{Deserialize, Serialize};
 
-use crate::layer::error::OLMResult;
-
-const META_FILE: &str = "meta.json";
+use crate::index::error::OKIResult;
+use crate::layer::fingerprint::FingerprintVec;
+use crate::layer::mphf_layer::{EvidenceKind, FINGERPRINT_FILE};
+use crate::layer::utils::layer_dir;
 
 // ── IndexMode ─────────────────────────────────────────────────────────────────
 
 /// Evidence mode for an entire partitioned index — homogeneous across all layers.
 ///
-/// Determined once at build time; stored in `PartitionMeta` (`meta.json`).
-/// All layers within an index share the same mode.
+/// Determined once at build time. All layers within an index share the same
+/// mode, so it is never persisted separately — [`IndexMode::detect`] reads it
+/// straight back from whichever evidence file(s) layer 0 actually has on
+/// disk, the same probe [`EvidenceKind::detect`] uses.
 #[derive(Debug, Clone, Serialize, Deserialize)]
 #[serde(tag = "type", rename_all = "snake_case")]
 pub enum IndexMode {
@@ -20,7 +22,9 @@ pub enum IndexMode {
     Exact,
     /// Approximate evidence: `fingerprint.bin` only.
     /// `b` — fingerprint bits per slot; false-positive rate ≈ 1/2^b per query.
-    /// `z` — Findere consecutive-kmer parameter (build-time only; not used at query time).
+    /// `z` — Findere consecutive-kmer parameter (build-time only; not used at
+    /// query time, not stored on disk, always `0` once recovered via
+    /// [`detect`](Self::detect)).
     Approx { b: u8, z: u8 },
     /// Hybrid: both `fingerprint.bin` and `evidence.bin` + `unitigs.bin.idx`.
     /// `find()` uses the fingerprint (O(1), approx); `find_strict()` uses exact evidence.
@@ -31,33 +35,24 @@ impl Default for IndexMode {
     fn default() -> Self { Self::Exact }
 }
 
-// ── PartitionMeta ─────────────────────────────────────────────────────────────
-
-/// Index-level metadata stored in `meta.json` at the root of a partition index.
-#[derive(Debug, Clone, Serialize, Deserialize)]
-pub struct PartitionMeta {
-    pub n_layers: usize,
-    #[serde(default)]
-    pub mode: IndexMode,
-}
-
-impl PartitionMeta {
-    pub fn new(mode: IndexMode) -> Self {
-        Self { n_layers: 0, mode }
-    }
-
-    pub fn load(dir: &Path) -> OLMResult<Self> {
-        let f = File::open(dir.join(META_FILE))?;
-        Ok(serde_json::from_reader(f)?)
-    }
-
-    pub fn save(&self, dir: &Path) -> OLMResult<()> {
-        let f = File::create(dir.join(META_FILE))?;
-        serde_json::to_writer_pretty(f, self)?;
-        Ok(())
+impl IndexMode {
+    /// Detect the evidence mode actually on disk, from partition index
+    /// directory `root`'s layer 0 — the single source of truth every layer
+    /// in the same index shares. `z` cannot be recovered (not stored in
+    /// `fingerprint.bin`, and irrelevant past build time), so it comes back
+    /// as `0`.
+    pub fn detect(root: &Path) -> OKIResult<Self> {
+        let layer0 = layer_dir(root, 0);
+        Ok(match EvidenceKind::detect(&layer0)? {
+            EvidenceKind::Exact => IndexMode::Exact,
+            EvidenceKind::Approx => IndexMode::Approx {
+                b: FingerprintVec::open(&layer0.join(FINGERPRINT_FILE))?.b(),
+                z: 0,
+            },
+            EvidenceKind::Hybrid => IndexMode::Hybrid {
+                b: FingerprintVec::open(&layer0.join(FINGERPRINT_FILE))?.b(),
+                z: 0,
+            },
+        })
     }
 }
-
-impl Default for PartitionMeta {
-    fn default() -> Self { Self::new(IndexMode::Exact) }
-}
diff --git a/src/obikindex/src/layer/mod.rs b/src/obikindex/src/layer/mod.rs
index a3021aee..fb0b1334 100644
--- a/src/obikindex/src/layer/mod.rs
+++ b/src/obikindex/src/layer/mod.rs
@@ -1,20 +1,13 @@
 pub mod content_layer;
-pub mod error;
 pub mod evidence;
 pub mod fingerprint;
-pub mod layered_store;
-pub mod map;
 pub mod meta;
 pub(crate) mod mphf_layer;
 pub mod typed_layer;
+pub(crate) mod utils;
 
+pub use crate::index::error::{OKIError, OKIResult};
 pub use content_layer::KmerLayer;
-pub use error::{OLMError, OLMResult};
-pub use layered_store::LayeredStore;
-pub use map::LayeredMap;
-pub use meta::{IndexMode, PartitionMeta};
+pub use meta::IndexMode;
 pub use mphf_layer::{EvidenceKind, KmerBatchIter, KmerIter, MphfLayer, MphfOnly};
-pub use typed_layer::{
-    HasLayerContent, HasStorageKind, Hit, LayerContent, LayerData, TypedLayer,
-    dereplicated_superkmers_path, layer_dir, open_data, raw_superkmers_path,
-};
+pub use typed_layer::{HasLayerContent, HasStorageKind, Hit, LayerContent, LayerData, TypedLayer};
diff --git a/src/obikindex/src/layer/mphf_layer.rs b/src/obikindex/src/layer/mphf_layer.rs
index a75e9340..ce16c2b7 100644
--- a/src/obikindex/src/layer/mphf_layer.rs
+++ b/src/obikindex/src/layer/mphf_layer.rs
@@ -10,7 +10,7 @@ use obikseq::CanonicalKmer;
 use obiskio::{CanonicalKmerIter, UnitigFileReader, UnitigFileWriter, build_unitig_idx};
 use ptr_hash::{PtrHash, PtrHashParams, bucket_fn::CubicEps, hash::Xx64};
 
-use crate::layer::error::{OLMError, OLMResult};
+use crate::index::error::{OKIError, OKIResult};
 use crate::layer::evidence::{Evidence, EvidenceWriter};
 use crate::layer::fingerprint::{FingerprintVec, FingerprintVecWriter};
 use crate::layer::meta::IndexMode;
@@ -76,7 +76,7 @@ impl LayerEvidence {
     /// Identify which evidence files exist at `layer_dir`, without opening
     /// any of them — a lightweight disk probe (same signal `EvidenceKind::
     /// detect` uses), not a load.
-    fn at(layer_dir: &Path) -> OLMResult<Self> {
+    fn at(layer_dir: &Path) -> OKIResult<Self> {
         Ok(match EvidenceKind::detect(layer_dir)? {
             EvidenceKind::Exact => LayerEvidence::Exact {
                 evidence_path: layer_dir.join(EVIDENCE_FILE),
@@ -94,7 +94,7 @@ impl LayerEvidence {
     }
 
     /// mmap the evidence file(s) this identity points to.
-    fn open(self) -> OLMResult<OpenLayerEvidence> {
+    fn open(self) -> OKIResult<OpenLayerEvidence> {
         Ok(match self {
             LayerEvidence::Exact { evidence_path, unitig_path } => OpenLayerEvidence::Exact {
                 evidence: Evidence::open(&evidence_path)?,
@@ -127,17 +127,17 @@ impl EvidenceKind {
     /// Lightweight disk probe, no MPHF/evidence opened: presence of
     /// `evidence.bin`/`fingerprint.bin` alone determines the mode — same
     /// signal `MphfLayer::open` uses, just without opening either file.
-    pub fn detect(layer_dir: &Path) -> OLMResult<EvidenceKind> {
+    pub fn detect(layer_dir: &Path) -> OKIResult<EvidenceKind> {
         let has_evidence = layer_dir.join(EVIDENCE_FILE).exists();
         let has_fingerprint = layer_dir.join(FINGERPRINT_FILE).exists();
         match (has_evidence, has_fingerprint) {
             (true, false) => Ok(EvidenceKind::Exact),
             (false, true) => Ok(EvidenceKind::Approx),
             (true, true) => Ok(EvidenceKind::Hybrid),
-            (false, false) => Err(OLMError::InvalidLayer(format!(
+            (false, false) => Err(OKIError::InvalidData { context: "layer", detail: format!(
                 "no evidence.bin or fingerprint.bin in {}",
                 layer_dir.display()
-            ))),
+            ) }),
         }
     }
 }
@@ -151,7 +151,6 @@ impl EvidenceKind {
 /// - [`find_strict`](Self::find_strict) — always exact; O(1) on Exact/Hybrid layers,
 ///   O(n) sequential scan on Approx layers.
 pub struct MphfLayer {
-    layer_dir: PathBuf,
     /// Mode-independent copy — used by `iter_kmers` (all modes) and
     /// `find_strict`'s `Approx` fallback. `Exact`/`Hybrid` additionally
     /// carry their own inside `OpenLayerEvidence` (see its doc comment for
@@ -165,18 +164,15 @@ pub struct MphfLayer {
 }
 
 impl MphfLayer {
-    /// This layer's own directory.
-    pub(crate) fn dir(&self) -> &Path {
-        &self.layer_dir
-    }
-
     /// Open a layer — evidence mode is auto-detected from which files exist
     /// on disk (see [`LayerEvidence::at`]), never passed in: the caller
     /// can't tell this type anything about its own mode that isn't already
-    /// derivable from `layer_dir` itself.
-    pub fn open(dir: &Path) -> OLMResult<Self> {
+    /// derivable from `dir` itself. `dir` is only used transiently here —
+    /// not stored: `KmerLayer`, one level up, is what owns a layer's
+    /// directory as a persistent field, not this type.
+    pub fn open(dir: &Path) -> OKIResult<Self> {
         let mphf: MemCase<MphfEps> = Mphf::mmap(&dir.join(MPHF_FILE), Flags::empty())
-            .map_err(|e| OLMError::InvalidLayer(e.to_string()))?;
+            .map_err(|e| OKIError::InvalidData { context: "layer", detail: e.to_string() })?;
         let unitigs = Arc::new(UnitigFileReader::open(&dir.join(UNITIGS_FILE))?);
         let ev = LayerEvidence::at(dir)?.open()?;
         let n = match &ev {
@@ -184,7 +180,7 @@ impl MphfLayer {
             OpenLayerEvidence::Hybrid { evidence, .. } => evidence.len(),
             OpenLayerEvidence::Approx { fingerprint } => fingerprint.n(),
         };
-        Ok(Self { layer_dir: dir.to_owned(), unitigs, mphf, ev, n })
+        Ok(Self { unitigs, mphf, ev, n })
     }
 
     // ── Query API ─────────────────────────────────────────────────────────────
@@ -405,9 +401,9 @@ impl Iterator for KmerBatchIter {
 pub struct MphfOnly(MemCase<MphfEps>);
 
 impl MphfOnly {
-    pub fn open(dir: &Path) -> OLMResult<Self> {
+    pub fn open(dir: &Path) -> OKIResult<Self> {
         let mphf: MemCase<MphfEps> = Mphf::mmap(&dir.join(MPHF_FILE), Flags::empty())
-            .map_err(|e| OLMError::InvalidLayer(e.to_string()))?;
+            .map_err(|e| OKIError::InvalidData { context: "layer", detail: e.to_string() })?;
         Ok(Self(mphf))
     }
 
@@ -421,13 +417,13 @@ impl MphfOnly {
 impl MphfLayer {
     // ── Build helpers ─────────────────────────────────────────────────────────
 
-    pub fn unitig_writer(dir: &Path) -> OLMResult<UnitigFileWriter> {
+    pub fn unitig_writer(dir: &Path) -> OKIResult<UnitigFileWriter> {
         fs::create_dir_all(dir)?;
         Ok(UnitigFileWriter::create(&dir.join(UNITIGS_FILE))?)
     }
 
     /// Build `evidence.bin` + `unitigs.bin.idx` from `unitigs.bin` + `mphf.bin`.
-    pub fn build_exact_evidence(dir: &Path, block_bits: u8) -> OLMResult<usize> {
+    pub fn build_exact_evidence(dir: &Path, block_bits: u8) -> OKIResult<usize> {
         let unitig_path = dir.join(UNITIGS_FILE);
         let unitigs = UnitigFileReader::open_sequential(&unitig_path)?;
         let n = unitigs.n_kmers();
@@ -439,7 +435,7 @@ impl MphfLayer {
         }
 
         let mphf: Mphf = Mphf::load_full(&dir.join(MPHF_FILE))
-            .map_err(|e| OLMError::InvalidLayer(e.to_string()))?;
+            .map_err(|e| OKIError::InvalidData { context: "layer", detail: e.to_string() })?;
 
         let mut ev = EvidenceWriter::new(n);
         let mut seen = vec![0u8; (n + 7) / 8];
@@ -447,12 +443,12 @@ impl MphfLayer {
         for (kmer, chunk_id, rank) in unitigs.iter_indexed_canonical_kmers() {
             let slot = mphf.index(&kmer.raw());
             if slot >= n {
-                return Err(OLMError::Mphf("slot out of bounds".into()));
+                return Err(OKIError::InvalidData { context: "mphf", detail: "slot out of bounds".into() });
             }
             let byte = slot / 8;
             let bit = 1u8 << (slot % 8);
             if seen[byte] & bit != 0 {
-                return Err(OLMError::Mphf("duplicate slot".into()));
+                return Err(OKIError::InvalidData { context: "mphf", detail: "duplicate slot".into() });
             }
             seen[byte] |= bit;
             ev.set(slot, chunk_id as u32, rank as u8);
@@ -464,14 +460,14 @@ impl MphfLayer {
     }
 
     /// Build `fingerprint.bin` from `unitigs.bin` + `mphf.bin`.
-    pub fn build_approx_evidence(dir: &Path, b: u8, z: u8) -> OLMResult<usize> {
+    pub fn build_approx_evidence(dir: &Path, b: u8, z: u8) -> OKIResult<usize> {
         if b == 0 || b > 64 {
-            return Err(OLMError::InvalidLayer(
+            return Err(OKIError::InvalidData { context: "layer", detail: 
                 "fingerprint width must be 1..=64".into(),
-            ));
+             });
         }
         if z == 0 {
-            return Err(OLMError::InvalidLayer("z must be ≥ 1".into()));
+            return Err(OKIError::InvalidData { context: "layer", detail: "z must be ≥ 1".into() });
         }
         let unitig_path = dir.join(UNITIGS_FILE);
         let unitigs = UnitigFileReader::open_sequential(&unitig_path)?;
@@ -483,14 +479,14 @@ impl MphfLayer {
         }
 
         let mphf: Mphf = Mphf::load_full(&dir.join(MPHF_FILE))
-            .map_err(|e| OLMError::InvalidLayer(e.to_string()))?;
+            .map_err(|e| OKIError::InvalidData { context: "layer", detail: e.to_string() })?;
 
         let mut fw = FingerprintVecWriter::new(n, b);
 
         for (kmer, _, _) in unitigs.iter_indexed_canonical_kmers() {
             let slot = mphf.index(&kmer.raw());
             if slot >= n {
-                return Err(OLMError::Mphf("slot out of bounds".into()));
+                return Err(OKIError::InvalidData { context: "mphf", detail: "slot out of bounds".into() });
             }
             fw.set(slot, kmer.seq_hash());
         }
@@ -508,24 +504,24 @@ impl MphfLayer {
         dir: &Path,
         block_bits: u8,
         mode: &IndexMode,
-        fill_slot: &mut impl FnMut(usize, CanonicalKmer) -> OLMResult<()>,
-    ) -> OLMResult<usize> {
+        fill_slot: &mut impl FnMut(usize, CanonicalKmer) -> OKIResult<()>,
+    ) -> OKIResult<usize> {
         use rayon::prelude::*;
 
         let unitig_path = dir.join(UNITIGS_FILE);
         let n = UnitigFileReader::open_sequential(&unitig_path)?.n_kmers();
 
         let sk_to_olm = |e: obiskio::SKError| match e {
-            obiskio::SKError::Io(io) => OLMError::Io(io),
-            e => OLMError::InvalidLayer(e.to_string()),
+            obiskio::SKError::Io(io) => OKIError::Io(io),
+            e => OKIError::InvalidData { context: "layer", detail: e.to_string() },
         };
 
         // ── Empty layer ───────────────────────────────────────────────────────
         if n == 0 {
             let mphf: Mphf = Mphf::try_new(&[] as &[u64], PtrHashParams::<CubicEps>::default())
-                .ok_or_else(|| OLMError::Mphf("construction failed".into()))?;
+                .ok_or_else(|| OKIError::InvalidData { context: "mphf", detail: "construction failed".into() })?;
             mphf.store(&dir.join(MPHF_FILE))
-                .map_err(|e| OLMError::InvalidLayer(e.to_string()))?;
+                .map_err(|e| OKIError::InvalidData { context: "layer", detail: e.to_string() })?;
             match mode {
                 IndexMode::Exact | IndexMode::Hybrid { .. } => {
                     fs::File::create(dir.join(EVIDENCE_FILE))?;
@@ -549,7 +545,7 @@ impl MphfLayer {
             PtrHashParams::<CubicEps>::default(),
         );
         mphf.store(&dir.join(MPHF_FILE))
-            .map_err(|e| OLMError::InvalidLayer(e.to_string()))?;
+            .map_err(|e| OKIError::InvalidData { context: "layer", detail: e.to_string() })?;
 
         // ── Pass 2: fill evidence files + callback ────────────────────────────
         let unitigs2 = UnitigFileReader::open_sequential(&unitig_path)?;
@@ -561,12 +557,12 @@ impl MphfLayer {
                 for (kmer, chunk_id, rank) in unitigs2.iter_indexed_canonical_kmers() {
                     let slot = mphf.index(&kmer.raw());
                     if slot >= n {
-                        return Err(OLMError::Mphf("slot out of bounds".into()));
+                        return Err(OKIError::InvalidData { context: "mphf", detail: "slot out of bounds".into() });
                     }
                     let byte = slot / 8;
                     let bit = 1u8 << (slot % 8);
                     if seen[byte] & bit != 0 {
-                        return Err(OLMError::Mphf("duplicate slot".into()));
+                        return Err(OKIError::InvalidData { context: "mphf", detail: "duplicate slot".into() });
                     }
                     seen[byte] |= bit;
                     ev.set(slot, chunk_id as u32, rank as u8);
@@ -581,12 +577,12 @@ impl MphfLayer {
                 for (kmer, _, _) in unitigs2.iter_indexed_canonical_kmers() {
                     let slot = mphf.index(&kmer.raw());
                     if slot >= n {
-                        return Err(OLMError::Mphf("slot out of bounds".into()));
+                        return Err(OKIError::InvalidData { context: "mphf", detail: "slot out of bounds".into() });
                     }
                     let byte = slot / 8;
                     let bit = 1u8 << (slot % 8);
                     if seen[byte] & bit != 0 {
-                        return Err(OLMError::Mphf("duplicate slot".into()));
+                        return Err(OKIError::InvalidData { context: "mphf", detail: "duplicate slot".into() });
                     }
                     seen[byte] |= bit;
                     fw.set(slot, kmer.seq_hash());
@@ -601,12 +597,12 @@ impl MphfLayer {
                 for (kmer, chunk_id, rank) in unitigs2.iter_indexed_canonical_kmers() {
                     let slot = mphf.index(&kmer.raw());
                     if slot >= n {
-                        return Err(OLMError::Mphf("slot out of bounds".into()));
+                        return Err(OKIError::InvalidData { context: "mphf", detail: "slot out of bounds".into() });
                     }
                     let byte = slot / 8;
                     let bit = 1u8 << (slot % 8);
                     if seen[byte] & bit != 0 {
-                        return Err(OLMError::Mphf("duplicate slot".into()));
+                        return Err(OKIError::InvalidData { context: "mphf", detail: "duplicate slot".into() });
                     }
                     seen[byte] |= bit;
                     ev.set(slot, chunk_id as u32, rank as u8);
diff --git a/src/obikindex/src/layer/tests/map.rs b/src/obikindex/src/layer/tests/map.rs
deleted file mode 100644
index 38bc6edd..00000000
--- a/src/obikindex/src/layer/tests/map.rs
+++ /dev/null
@@ -1,219 +0,0 @@
-use super::*;
-use obicompactvec::{pack_bit_matrix, pack_sparse_bit_matrix, PersistentBitMatrix, PersistentCompactIntMatrix, StorageKind};
-use obikseq::{set_k, Sequence as _, Unitig};
-use obiskio::DEFAULT_BLOCK_BITS;
-use crate::layer::typed_layer::LayerContent;
-use crate::layer::meta::IndexMode;
-use crate::layer::mphf_layer::EvidenceKind;
-use tempfile::tempdir;
-
-fn push_unitigs_and_layer(
-    map: &mut LayeredMap<PersistentCompactIntMatrix>,
-    seqs: &[&[u8]],
-    count: u32,
-) {
-    let mut w = map.next_layer_writer().unwrap();
-    for s in seqs {
-        w.write(&Unitig::from_ascii(s)).unwrap();
-    }
-    w.close().unwrap();
-    map.push_layer(|_| count).unwrap();
-}
-
-fn canonical(ascii: &[u8]) -> CanonicalKmer {
-    obikseq::Kmer::from_ascii(ascii).unwrap().canonical()
-}
-
-#[test]
-fn create_empty_map() {
-    set_k(4);
-    let dir = tempdir().unwrap();
-    let map = LayeredMap::<()>::create(dir.path(), IndexMode::Exact).unwrap();
-    assert_eq!(map.n_layers(), 0);
-}
-
-#[test]
-fn open_reloads_layer_count() {
-    set_k(4);
-    let dir = tempdir().unwrap();
-    {
-        let mut map = LayeredMap::<PersistentCompactIntMatrix>::create(dir.path(), IndexMode::Exact).unwrap();
-        push_unitigs_and_layer(&mut map, &[b"AAAACGT"], 1);
-    }
-    let map = LayeredMap::<PersistentCompactIntMatrix>::open(dir.path()).unwrap();
-    assert_eq!(map.n_layers(), 1);
-}
-
-#[test]
-fn query_finds_kmer_in_layer_zero() {
-    set_k(4);
-    let dir = tempdir().unwrap();
-    let mut map = LayeredMap::<PersistentCompactIntMatrix>::create(dir.path(), IndexMode::Exact).unwrap();
-    push_unitigs_and_layer(&mut map, &[b"AAAACGT"], 3);
-    let kmer = canonical(b"AAAC");
-    let (layer_idx, hit) = map.query(kmer).expect("kmer must be found");
-    assert_eq!(layer_idx, 0);
-    assert_eq!(hit.data[0], 3);
-}
-
-#[test]
-fn query_finds_kmer_in_correct_layer() {
-    set_k(4);
-    let dir = tempdir().unwrap();
-    let mut map = LayeredMap::<PersistentCompactIntMatrix>::create(dir.path(), IndexMode::Exact).unwrap();
-    push_unitigs_and_layer(&mut map, &[b"AAAACGT"], 1);
-    push_unitigs_and_layer(&mut map, &[b"GGGACGT"], 2);
-    assert_eq!(map.n_layers(), 2);
-
-    let (li, hit) = map.query(canonical(b"AAAA")).expect("AAAA must be found");
-    assert_eq!(li, 0);
-    assert_eq!(hit.data[0], 1);
-
-    let (li, hit) = map.query(canonical(b"GGGA")).expect("GGGA must be found");
-    assert_eq!(li, 1);
-    assert_eq!(hit.data[0], 2);
-}
-
-#[test]
-fn query_absent_returns_none() {
-    set_k(4);
-    let dir = tempdir().unwrap();
-    let mut map = LayeredMap::<PersistentCompactIntMatrix>::create(dir.path(), IndexMode::Exact).unwrap();
-    push_unitigs_and_layer(&mut map, &[b"AAAACGT"], 1);
-    let absent = canonical(b"CCCC");
-    assert!(map.query(absent).is_none());
-}
-
-#[test]
-fn push_layer_from_map_convenience() {
-    set_k(4);
-    let dir = tempdir().unwrap();
-    let mut map = LayeredMap::<PersistentCompactIntMatrix>::create(dir.path(), IndexMode::Exact).unwrap();
-    let mut w = map.next_layer_writer().unwrap();
-    w.write(&Unitig::from_ascii(b"AAAACGT")).unwrap();
-    w.close().unwrap();
-    let counts: HashMap<CanonicalKmer, u32> = vec![
-        (canonical(b"AAAA"), 10u32),
-    ].into_iter().collect();
-    map.push_layer_from_map(&counts).unwrap();
-    let (_, hit) = map.query(canonical(b"AAAA")).unwrap();
-    assert_eq!(hit.data[0], 10);
-}
-
-// ── detect_layer_content / detect_layer_storage / detect_layer_evidence ────
-//
-// All built directly on disk (bypassing `LayeredMap<D>::push_layer`, which
-// only exists for modes 1 and 2 — mode 3/presence has no `push_layer`),
-// then reopened as `LayeredMap<()>` — `()`'s `LayerData::open` never reads
-// the matrix, so it's the right handle for exercising the disk-probing
-// `detect_layer_*` methods regardless of what content/storage the layer
-// underneath actually is.
-
-fn write_unitigs_at(dir: &Path, seqs: &[&[u8]]) {
-    fs::create_dir_all(dir).unwrap();
-    let mut w = UnitigFileWriter::create(&dir.join(crate::layer::typed_layer::UNITIGS_FILE)).unwrap();
-    for s in seqs {
-        w.write(&Unitig::from_ascii(s)).unwrap();
-    }
-    w.close().unwrap();
-}
-
-/// Build a one-layer partition root with a presence layer at `layer_0`,
-/// save `meta.json`, and reopen it as `LayeredMap<()>`.
-fn presence_partition(seqs: &[&[u8]], n_genomes: usize) -> (tempfile::TempDir, LayeredMap<()>) {
-    let dir = tempdir().unwrap();
-    let mode = IndexMode::Exact;
-    let layer0 = layer_dir(dir.path(), 0);
-    write_unitigs_at(&layer0, seqs);
-    TypedLayer::<PersistentBitMatrix>::build_presence(&layer0, DEFAULT_BLOCK_BITS, &mode, n_genomes, |kmer, g| {
-        (kmer.raw().wrapping_add(g as u64)) % 2 == 0
-    }).unwrap();
-    PartitionMeta { n_layers: 1, mode }.save(dir.path()).unwrap();
-    let map = LayeredMap::<()>::open(dir.path()).unwrap();
-    (dir, map)
-}
-
-#[test]
-fn detect_layer_content_count() {
-    set_k(4);
-    let dir = tempdir().unwrap();
-    let mut map = LayeredMap::<PersistentCompactIntMatrix>::create(dir.path(), IndexMode::Exact).unwrap();
-    push_unitigs_and_layer(&mut map, &[b"AAAACGT"], 3);
-    assert_eq!(map.detect_layer_content(0), LayerContent::Count);
-}
-
-#[test]
-fn detect_layer_content_presence() {
-    set_k(4);
-    let (_dir, map) = presence_partition(&[b"AAATCTA", b"CTTCGCC", b"TGATACG"], 3);
-    assert_eq!(map.detect_layer_content(0), LayerContent::Presence);
-}
-
-#[test]
-fn detect_layer_storage_columnar_then_packed_then_sparse() {
-    set_k(4);
-    let (dir, map) = presence_partition(&[b"AAATCTA", b"CTTCGCC", b"TGATACG"], 3);
-    let presence_dir = layer_dir(dir.path(), 0).join("presence");
-
-    assert_eq!(map.detect_layer_storage(0).unwrap(), StorageKind::Columnar);
-
-    pack_bit_matrix(&presence_dir).unwrap();
-    assert_eq!(map.detect_layer_storage(0).unwrap(), StorageKind::Packed);
-
-    pack_sparse_bit_matrix(&presence_dir).unwrap();
-    assert_eq!(map.detect_layer_storage(0).unwrap(), StorageKind::Sparse);
-}
-
-#[test]
-fn detect_layer_storage_implicit() {
-    set_k(4);
-    let dir = tempdir().unwrap();
-    let mode = IndexMode::Exact;
-    let layer0 = layer_dir(dir.path(), 0);
-    write_unitigs_at(&layer0, &[b"AAAACGT"]);
-    // Mode-1 build: MPHF + evidence + `layer_meta.json`, no matrix at all —
-    // must read back as `Presence`/`Implicit`, never a third "empty" state.
-    TypedLayer::<()>::build(&layer0, DEFAULT_BLOCK_BITS, &mode).unwrap();
-    PartitionMeta { n_layers: 1, mode }.save(dir.path()).unwrap();
-    let map = LayeredMap::<()>::open(dir.path()).unwrap();
-
-    assert_eq!(map.detect_layer_content(0), LayerContent::Presence);
-    assert_eq!(map.detect_layer_storage(0).unwrap(), StorageKind::Implicit);
-}
-
-#[test]
-fn detect_layer_storage_count_columnar_then_packed() {
-    set_k(4);
-    let dir = tempdir().unwrap();
-    let mut map = LayeredMap::<PersistentCompactIntMatrix>::create(dir.path(), IndexMode::Exact).unwrap();
-    push_unitigs_and_layer(&mut map, &[b"AAAACGT"], 3);
-    drop(map);
-
-    let map = LayeredMap::<()>::open(dir.path()).unwrap();
-    assert_eq!(map.detect_layer_storage(0).unwrap(), StorageKind::Columnar);
-
-    obicompactvec::pack_compact_int_matrix(&layer_dir(dir.path(), 0).join("counts")).unwrap();
-    assert_eq!(map.detect_layer_storage(0).unwrap(), StorageKind::Packed);
-}
-
-#[test]
-fn detect_layer_evidence_exact_vs_approx() {
-    set_k(4);
-
-    let dir = tempdir().unwrap();
-    let layer0 = layer_dir(dir.path(), 0);
-    write_unitigs_at(&layer0, &[b"AAAACGT"]);
-    TypedLayer::<()>::build(&layer0, DEFAULT_BLOCK_BITS, &IndexMode::Exact).unwrap();
-    PartitionMeta { n_layers: 1, mode: IndexMode::Exact }.save(dir.path()).unwrap();
-    let map = LayeredMap::<()>::open(dir.path()).unwrap();
-    assert_eq!(map.detect_layer_evidence(0).unwrap(), EvidenceKind::Exact);
-
-    let approx = IndexMode::Approx { b: 8, z: 1 };
-    let dir2 = tempdir().unwrap();
-    let layer0b = layer_dir(dir2.path(), 0);
-    write_unitigs_at(&layer0b, &[b"AAAACGT"]);
-    TypedLayer::<()>::build(&layer0b, DEFAULT_BLOCK_BITS, &approx).unwrap();
-    PartitionMeta { n_layers: 1, mode: approx }.save(dir2.path()).unwrap();
-    let map2 = LayeredMap::<()>::open(dir2.path()).unwrap();
-    assert_eq!(map2.detect_layer_evidence(0).unwrap(), EvidenceKind::Approx);
-}
diff --git a/src/obikindex/src/layer/typed_layer.rs b/src/obikindex/src/layer/typed_layer.rs
index 11809725..ec1d27d0 100644
--- a/src/obikindex/src/layer/typed_layer.rs
+++ b/src/obikindex/src/layer/typed_layer.rs
@@ -1,19 +1,18 @@
-use std::collections::HashMap;
-use std::fs;
-use std::path::{Path, PathBuf};
-
+use crate::layer::utils::layer_dir;
 use obicompactvec::{
     BinaryMatrix, PersistentBitMatrix, PersistentBitMatrixBuilder, PersistentCompactIntMatrix,
     PersistentCompactIntMatrixBuilder, PersistentSparseBitMatrix,
 };
 use obikseq::CanonicalKmer;
 use obiskio::{UnitigFileReader, UnitigFileWriter};
+use std::collections::HashMap;
+use std::fs;
+use std::path::Path;
 
-use crate::layer::error::{OLMError, OLMResult};
+use crate::index::error::{OKIError, OKIResult};
 use crate::layer::meta::IndexMode;
 use crate::layer::mphf_layer::MphfLayer;
 pub(crate) use crate::layer::mphf_layer::UNITIGS_FILE;
-
 pub(crate) const COUNTS_DIR: &str = "counts";
 pub(crate) const PRESENCE_DIR: &str = "presence";
 
@@ -21,48 +20,10 @@ pub(crate) const PRESENCE_DIR: &str = "presence";
 
 pub trait LayerData: Sized {
     type Item;
-    fn open(layer_dir: &Path) -> OLMResult<Self>;
+    fn open(layer_dir: &Path) -> OKIResult<Self>;
     fn read(&self, slot: usize) -> Self::Item;
 }
 
-/// TypedLayer directory path for layer `i` within a partition's index root — the
-/// single source of truth for the on-disk `layer_N` naming convention
-/// (mirrors what `LayeredMap::open`/`push_layer` already use internally).
-///
-/// `obikindex::layer` operates within a single partition's index root; it has
-/// no notion of "partition" at all. Turning a partition number into that
-/// root is `obikindex::KmerIndex::index_dir`'s job, one layer up — callers
-/// here only ever name a layer *number*, never build the path themselves.
-pub fn layer_dir(root: &Path, i: usize) -> PathBuf {
-    root.join(format!("layer_{i}"))
-}
-
-/// Superkmer-file extension used by every pre-layer-construction artifact
-/// below (`raw`/`dereplicated`) — an implementation detail of the SK file
-/// format, never meant to leak as a literal string past this module.
-const SK_EXT: &str = "skmer.zst";
-
-/// Path of a layer's raw, not-yet-dereplicated superkmer file — written by
-/// whichever algorithm routes superkmers into this layer (today:
-/// `obikindexer::algorithms::partitionner::PartitionRouter`), read by whichever
-/// algorithm dereplicates it (today:
-/// `obikindexer::algorithms::dereplicator::Dereplicator`). Naming this once
-/// here, rather than in either algorithm submodule, is what lets the two
-/// agree on the filename without depending on each other directly — see
-/// `DevDocMD/implementation/partition_layer_cache.md`.
-pub fn raw_superkmers_path(layer_dir: &Path) -> PathBuf {
-    layer_dir.join(format!("raw.{SK_EXT}"))
-}
-
-/// Path of a layer's dereplicated superkmer file — written by
-/// `obikindexer::algorithms::dereplicator::Dereplicator`, read by whichever
-/// algorithm counts kmer abundances from it (today:
-/// `obikindexer::algorithms::partitionner::PartitionRouter::count_kmer`) and,
-/// later, by `obikindex::build_index_layer` to build the real layer.
-pub fn dereplicated_superkmers_path(layer_dir: &Path) -> PathBuf {
-    layer_dir.join(format!("dereplicated.{SK_EXT}"))
-}
-
 /// Opens layer `i`'s data only, skipping the MPHF — for callers that only
 /// need matrix-level operations (distance traits, column weights, group
 /// filters, sub-matrix extraction) and never look up a kmer for this layer.
@@ -74,13 +35,13 @@ pub fn dereplicated_superkmers_path(layer_dir: &Path) -> PathBuf {
 /// *number* within a partition it already knows the root of, the same
 /// vocabulary as [`layer_dir`] and `LayeredMap` — never the `layer_N`
 /// naming convention itself, which stays private to this crate.
-pub fn open_data<D: LayerData>(root: &Path, i: usize) -> OLMResult<D> {
+pub(crate) fn open_data<D: LayerData>(root: &Path, i: usize) -> OKIResult<D> {
     D::open(&layer_dir(root, i))
 }
 
 impl LayerData for () {
     type Item = ();
-    fn open(_layer_dir: &Path) -> OLMResult<Self> {
+    fn open(_layer_dir: &Path) -> OKIResult<Self> {
         Ok(())
     }
     fn read(&self, _slot: usize) {}
@@ -88,8 +49,8 @@ impl LayerData for () {
 
 impl LayerData for PersistentCompactIntMatrix {
     type Item = Box<[u32]>;
-    fn open(layer_dir: &Path) -> OLMResult<Self> {
-        PersistentCompactIntMatrix::open(layer_dir).map_err(OLMError::Io)
+    fn open(layer_dir: &Path) -> OKIResult<Self> {
+        PersistentCompactIntMatrix::open(layer_dir).map_err(OKIError::Io)
     }
     fn read(&self, slot: usize) -> Box<[u32]> {
         self.row(slot)
@@ -98,8 +59,8 @@ impl LayerData for PersistentCompactIntMatrix {
 
 impl LayerData for PersistentBitMatrix {
     type Item = Box<[bool]>;
-    fn open(layer_dir: &Path) -> OLMResult<Self> {
-        PersistentBitMatrix::open(layer_dir).map_err(OLMError::Io)
+    fn open(layer_dir: &Path) -> OKIResult<Self> {
+        PersistentBitMatrix::open(layer_dir).map_err(OKIError::Io)
     }
     fn read(&self, slot: usize) -> Box<[bool]> {
         self.row(slot)
@@ -108,8 +69,8 @@ impl LayerData for PersistentBitMatrix {
 
 impl LayerData for PersistentSparseBitMatrix {
     type Item = Box<[bool]>;
-    fn open(layer_dir: &Path) -> OLMResult<Self> {
-        PersistentSparseBitMatrix::open(&layer_dir.join(PRESENCE_DIR)).map_err(OLMError::Io)
+    fn open(layer_dir: &Path) -> OKIResult<Self> {
+        PersistentSparseBitMatrix::open(&layer_dir.join(PRESENCE_DIR)).map_err(OKIError::Io)
     }
     fn read(&self, slot: usize) -> Box<[bool]> {
         self.row(slot)
@@ -201,11 +162,7 @@ pub struct Hit<T = ()> {
 // ── Common read path ──────────────────────────────────────────────────────────
 
 impl<D: LayerData> TypedLayer<D> {
-    // `mode` no longer forwarded to `MphfLayer::open` (auto-detected from
-    // disk now, see mphf_layer.rs) — kept as a parameter for the moment so
-    // this signature (and `KmerLayer::open`'s call to it) doesn't have to
-    // change in the same step; dropped when content_layer.rs is done.
-    pub fn open(path: &Path, _mode: &IndexMode) -> OLMResult<Self> {
+    pub fn open(path: &Path) -> OKIResult<Self> {
         let mphf = MphfLayer::open(path)?;
         let data = D::open(path)?;
         Ok(Self { mphf, data })
@@ -266,21 +223,21 @@ impl<D: LayerData> TypedLayer<D> {
         self.mphf.enumerate_kmers_batch(n)
     }
 
-    pub fn unitig_writer(out_dir: &Path) -> OLMResult<UnitigFileWriter> {
+    pub fn unitig_writer(out_dir: &Path) -> OKIResult<UnitigFileWriter> {
         MphfLayer::unitig_writer(out_dir)
     }
 
     /// Build `unitigs.bin.idx` and `evidence.bin` from `unitigs.bin` and
     /// `mphf.bin` already present in `layer_dir`.
     /// `block_bits` controls the `.idx` block size (2^block_bits chunks/block).
-    pub fn build_exact_evidence(layer_dir: &Path, block_bits: u8) -> OLMResult<usize> {
+    pub fn build_exact_evidence(layer_dir: &Path, block_bits: u8) -> OKIResult<usize> {
         MphfLayer::build_exact_evidence(layer_dir, block_bits)
     }
 
     /// Build `fingerprint.bin` from `unitigs.bin` and `mphf.bin` already
     /// present in `layer_dir`.  `b` — fingerprint bits (1..=64); `z` — Findere
     /// consecutive k-mer parameter (≥1).
-    pub fn build_approx_evidence(layer_dir: &Path, b: u8, z: u8) -> OLMResult<usize> {
+    pub fn build_approx_evidence(layer_dir: &Path, b: u8, z: u8) -> OKIResult<usize> {
         MphfLayer::build_approx_evidence(layer_dir, b, z)
     }
 
@@ -311,21 +268,21 @@ impl<D: LayerData + HasStorageKind> TypedLayer<D> {
 // ── Mode 1 — set membership ───────────────────────────────────────────────────
 
 impl TypedLayer<()> {
-    pub fn build(out_dir: &Path, block_bits: u8, mode: &IndexMode) -> OLMResult<usize> {
+    pub fn build(out_dir: &Path, block_bits: u8, mode: &IndexMode) -> OKIResult<usize> {
         MphfLayer::build(out_dir, block_bits, mode, &mut |_, _| Ok(()))
     }
 
     /// Create a presence matrix for a set-membership layer (first merge).
-    pub fn init_presence_matrix(layer_dir: &Path, n_kmers: usize) -> OLMResult<()> {
+    pub fn init_presence_matrix(layer_dir: &Path, n_kmers: usize) -> OKIResult<()> {
         let presence_dir = layer_dir.join(PRESENCE_DIR);
-        fs::create_dir_all(&presence_dir).map_err(OLMError::Io)?;
+        fs::create_dir_all(&presence_dir).map_err(OKIError::Io)?;
         let mut mb =
-            PersistentBitMatrixBuilder::new(n_kmers, &presence_dir).map_err(OLMError::Io)?;
+            PersistentBitMatrixBuilder::new(n_kmers, &presence_dir).map_err(OKIError::Io)?;
         mb.add_col_ones()
-            .map_err(OLMError::Io)?
+            .map_err(OKIError::Io)?
             .close()
-            .map_err(OLMError::Io)?;
-        mb.close().map_err(OLMError::Io)
+            .map_err(OKIError::Io)?;
+        mb.close().map_err(OKIError::Io)
     }
 }
 
@@ -337,18 +294,18 @@ impl TypedLayer<PersistentCompactIntMatrix> {
         block_bits: u8,
         mode: &IndexMode,
         count_of: impl Fn(CanonicalKmer) -> u32,
-    ) -> OLMResult<usize> {
+    ) -> OKIResult<usize> {
         let n = UnitigFileReader::open_sequential(&out_dir.join(UNITIGS_FILE))?.n_kmers();
         let counts_dir = out_dir.join(COUNTS_DIR);
         let mut mb =
-            PersistentCompactIntMatrixBuilder::new(n, &counts_dir).map_err(OLMError::Io)?;
-        let mut col = mb.add_col().map_err(OLMError::Io)?;
+            PersistentCompactIntMatrixBuilder::new(n, &counts_dir).map_err(OKIError::Io)?;
+        let mut col = mb.add_col().map_err(OKIError::Io)?;
         let n_built = MphfLayer::build(out_dir, block_bits, mode, &mut |slot, kmer| {
             col.set(slot, count_of(kmer));
             Ok(())
         })?;
-        col.close().map_err(OLMError::Io)?;
-        mb.close().map_err(OLMError::Io)?;
+        col.close().map_err(OKIError::Io)?;
+        mb.close().map_err(OKIError::Io)?;
         Ok(n_built)
     }
 
@@ -357,7 +314,7 @@ impl TypedLayer<PersistentCompactIntMatrix> {
         block_bits: u8,
         mode: &IndexMode,
         counts: &HashMap<CanonicalKmer, u32>,
-    ) -> OLMResult<usize> {
+    ) -> OKIResult<usize> {
         Self::build(out_dir, block_bits, mode, |kmer| {
             counts.get(&kmer).copied().unwrap_or(0)
         })
@@ -370,9 +327,9 @@ impl TypedLayer<PersistentCompactIntMatrix> {
     pub fn append_genome_column(
         layer_dir: &Path,
         value_of: impl Fn(usize) -> u32,
-    ) -> OLMResult<()> {
+    ) -> OKIResult<()> {
         PersistentCompactIntMatrix::append_column(&layer_dir.join(COUNTS_DIR), value_of)
-            .map_err(OLMError::Io)
+            .map_err(OKIError::Io)
     }
 
     /// Number of genome columns in this layer's count matrix.
@@ -448,9 +405,9 @@ impl TypedLayer<PersistentBitMatrix> {
     pub fn append_genome_column(
         layer_dir: &Path,
         value_of: impl Fn(usize) -> bool,
-    ) -> OLMResult<()> {
+    ) -> OKIResult<()> {
         PersistentBitMatrix::append_column(&layer_dir.join(PRESENCE_DIR), value_of)
-            .map_err(OLMError::Io)
+            .map_err(OKIError::Io)
     }
 
     pub fn build_presence(
@@ -459,13 +416,13 @@ impl TypedLayer<PersistentBitMatrix> {
         mode: &IndexMode,
         n_genomes: usize,
         present_in: impl Fn(CanonicalKmer, usize) -> bool,
-    ) -> OLMResult<usize> {
+    ) -> OKIResult<usize> {
         let n = UnitigFileReader::open_sequential(&out_dir.join(UNITIGS_FILE))?.n_kmers();
         let presence_dir = out_dir.join(PRESENCE_DIR);
-        let mut mb = PersistentBitMatrixBuilder::new(n, &presence_dir).map_err(OLMError::Io)?;
+        let mut mb = PersistentBitMatrixBuilder::new(n, &presence_dir).map_err(OKIError::Io)?;
         let mut cols: Vec<_> = (0..n_genomes)
-            .map(|_| mb.add_col().map_err(OLMError::Io))
-            .collect::<OLMResult<_>>()?;
+            .map(|_| mb.add_col().map_err(OKIError::Io))
+            .collect::<OKIResult<_>>()?;
         let n_built = MphfLayer::build(out_dir, block_bits, mode, &mut |slot, kmer| {
             for (g, col) in cols.iter_mut().enumerate() {
                 col.set(slot, present_in(kmer, g));
@@ -473,9 +430,9 @@ impl TypedLayer<PersistentBitMatrix> {
             Ok(())
         })?;
         for col in cols {
-            col.close().map_err(OLMError::Io)?;
+            col.close().map_err(OKIError::Io)?;
         }
-        mb.close().map_err(OLMError::Io)?;
+        mb.close().map_err(OKIError::Io)?;
         Ok(n_built)
     }
 }
diff --git a/src/obikindex/src/layer/utils.rs b/src/obikindex/src/layer/utils.rs
new file mode 100644
index 00000000..2a17313b
--- /dev/null
+++ b/src/obikindex/src/layer/utils.rs
@@ -0,0 +1,14 @@
+pub(crate) const LAYERNAME_SUFFIX: &str = "layer_";
+use std::path::{Path, PathBuf};
+
+/// TypedLayer directory path for layer `i` within a partition's index root — the
+/// single source of truth for the on-disk `layer_N` naming convention
+/// (mirrors what `LayeredMap::open`/`push_layer` already use internally).
+///
+/// `obikindex::layer` operates within a single partition's index root; it has
+/// no notion of "partition" at all. Turning a partition number into that
+/// root is `obikindex::KmerIndex::index_dir`'s job, one layer up — callers
+/// here only ever name a layer *number*, never build the path themselves.
+pub(crate) fn layer_dir(root: &Path, i: usize) -> PathBuf {
+    root.join(format!("{LAYERNAME_SUFFIX}{i}"))
+}
diff --git a/src/obikindex/src/lib.rs b/src/obikindex/src/lib.rs
index 093ed46b..2ab9c8fe 100644
--- a/src/obikindex/src/lib.rs
+++ b/src/obikindex/src/lib.rs
@@ -13,10 +13,10 @@ pub mod layer;
 pub mod partition;
 
 pub use index::{
-    validate_label, AggOp, DistanceMetric, DistanceOutput, GenomeInfo, GroupFilterParams,
-    GroupQuorumFilter, IndexBitsPerKmer, IndexBuilder, IndexConfig, IndexMeta, IndexState,
-    KmerDesc, KmerFilter, KmerIndex, MergeMode, MetaPred, OKIError, OKIResult, OutputCol,
-    PartitionRunner, QueryHit, QueryStats, META_FILENAME,
-    materialize_layer, olm_to_sk, write_graph_as_unitigs, passes_all,
+    GenomeInfo,
+    IndexBuilder, IndexConfig, IndexMeta, IndexState,
+    KmerIndex, OKIError, OKIResult,
+    META_FILENAME,
+    ColBuilder,
 };
-pub use index::{filter, meta};
+pub use index::meta;
diff --git a/src/obikindex/src/partition/mod.rs b/src/obikindex/src/partition/mod.rs
index 34da67f0..87b31152 100644
--- a/src/obikindex/src/partition/mod.rs
+++ b/src/obikindex/src/partition/mod.rs
@@ -19,10 +19,13 @@
 //! entirely) both still reinvent a fragment of this. Migrating them is a
 //! separate, deferred step.
 
+use crate::OKIError;
 use std::path::{Path, PathBuf};
 
-use crate::layer::{IndexMode, KmerLayer, OLMResult, layer_dir};
-use obikseq::CanonicalKmer;
+use crate::{
+    KmerIndex, OKIResult,
+    layer::{KmerLayer, utils::layer_dir},
+};
 
 /// Partition subdirectory name, under an index's root — the single source
 /// of truth for the on-disk `partitions/part_NNNNN` naming convention.
@@ -34,59 +37,77 @@ pub const PARTITIONS_SUBDIR: &str = "partitions";
 
 /// Path of partition `i`'s directory under `root` — `<root>/partitions/part_NNNNN`,
 /// zero-padded to 5 digits.
-pub fn partition_dir(root: &Path, i: usize) -> PathBuf {
+pub(crate) fn partition_dir(root: &Path, i: usize) -> PathBuf {
     root.join(PARTITIONS_SUBDIR).join(format!("part_{i:05}"))
 }
 
 /// Path of partition `i`'s layered-index directory — `<partition_dir>/index`,
 /// the root every [`crate::layer::layer_dir`] call for this partition is
 /// relative to.
-pub fn index_dir(root: &Path, i: usize) -> PathBuf {
+pub(crate) fn index_dir(root: &Path, i: usize) -> PathBuf {
     partition_dir(root, i).join("index")
 }
 
-/// One partition's open layers, in layer order (layer 0 first).
+/// One partition — identified by its own `index_dir`, nothing eagerly
+/// opened. `index_dir` is the minimal field `KmerLayer::at` needs to
+/// compute a layer's path; added here only because `layer()` below can't
+/// be written without it now that there's no `layers: Vec<KmerLayer>` to
+/// index into.
 pub struct KmerPartition {
-    layers: Vec<KmerLayer>,
+    partition_dir: PathBuf,
+    id: usize,
 }
 
 impl KmerPartition {
-    /// Open every layer under `index_dir` (`index_dir/layer_0`,
-    /// `index_dir/layer_1`, ... up to `n_layers`), eagerly — not lazily on
-    /// first access, so the caller pays the mmap cost once, up front,
-    /// rather than at an unpredictable point during later lookups.
-    pub fn open(
-        index_dir: &Path,
-        mode: &IndexMode,
-        n_layers: usize,
-        with_counts: bool,
-    ) -> OLMResult<Self> {
-        let layers = (0..n_layers)
-            .map(|l| KmerLayer::open(&layer_dir(index_dir, l), mode, with_counts))
-            .collect::<OLMResult<Vec<_>>>()?;
-        Ok(Self { layers })
+    pub fn new(index: &KmerIndex, i: usize) -> Self {
+        KmerPartition {
+            partition_dir: index.dir().join(format!("part_{i:05}")),
+            id: i,
+        }
     }
 
+    pub fn id(&self) -> usize {
+        self.id
+    }
+
+    /// Number of layers under `index_dir`, found by probing `layer_0`,
+    /// `layer_1`, ... sequentially until one is missing — a disk scan, not
+    /// a metadata read (`meta.json`'s own `n_layers` is redundant with this
+    /// and no longer consulted here). Intrinsically slow: one `exists()`
+    /// syscall per layer, called fresh every time — not for a hot path.
     pub fn n_layers(&self) -> usize {
-        self.layers.len()
+        let mut n = 0;
+        while layer_dir(&self.partition_dir, n).exists() {
+            n += 1;
+        }
+        n
     }
 
-    pub fn layer(&self, i: usize) -> &KmerLayer {
-        &self.layers[i]
+    /// Evidence mode for this partition, detected from layer 0's evidence
+    /// files on disk (see [`crate::layer::IndexMode::detect`]) — never
+    /// persisted as metadata, since every layer within a partition shares
+    /// the same mode by construction.
+    pub fn mode(&self) -> OKIResult<crate::layer::IndexMode> {
+        crate::layer::IndexMode::detect(&self.partition_dir)
     }
 
-    pub fn layers(&self) -> &[KmerLayer] {
-        &self.layers
+    pub fn layer(&self, i: usize) -> OKIResult<KmerLayer> {
+        let n = self.n_layers();
+        if i >= n {
+            return Err(OKIError::Layer(obiskio::SKError::InvalidData {
+                context: "New KmerLayer",
+                detail: format!("Layer {i} requested, Partition has only {n} layer"),
+            }));
+        }
+        Ok(KmerLayer::new(self, i))
     }
 
-    /// Existence lookup of `kmer` across this partition's layers: tries
-    /// each in turn, stopping at the first hit and reporting which layer it
-    /// was — `find_slot`, not a data read, for a plain existence check.
-    pub fn find(&self, kmer: CanonicalKmer) -> Option<usize> {
-        self.layers
-            .iter()
-            .enumerate()
-            .find_map(|(li, layer)| layer.find_slot(kmer).map(|_| li))
+    pub fn layer_dir(&self, i: usize) -> OKIResult<PathBuf> {
+        Ok(self.layer(i)?.dir().to_path_buf())
+    }
+
+    pub fn dir(&self) -> &Path {
+        &self.partition_dir
     }
 }
 
diff --git a/src/obikindexer/src/algorithms/layer_builder/mod.rs b/src/obikindexer/src/algorithms/layer_builder/mod.rs
index eb4225ef..1522e975 100644
--- a/src/obikindexer/src/algorithms/layer_builder/mod.rs
+++ b/src/obikindexer/src/algorithms/layer_builder/mod.rs
@@ -18,7 +18,7 @@ use crate::extensions::PrivateBuilder;
 /// The actual per-partition construction (De Bruijn graph, unitigs, MPHF,
 /// matrix) stays a `KmerIndex` method (`build_index_layer`) — unlike
 /// `Dereplicator`/`Counter`, it depends on several `obikindex`-internal
-/// helpers (`graph_pipeline`, `common::olm_to_sk`) already shared with
+/// helpers (`graph_pipeline`) already shared with
 /// `merge`/`select`/`rebuild`'s own layer-construction paths, so it
 /// belongs there, not duplicated or newly exported just for this
 /// algorithm. `LayerBuilder`'s own job is the orchestration around it:
diff --git a/src/obikindexer/src/extensions/mod.rs b/src/obikindexer/src/extensions/mod.rs
index 3ac6b003..4fce656e 100644
--- a/src/obikindexer/src/extensions/mod.rs
+++ b/src/obikindexer/src/extensions/mod.rs
@@ -36,7 +36,7 @@ use obidebruinj::GraphDeBruijn;
 use obikindex::layer::IndexMode;
 use obikindex::layer::{TypedLayer, meta::PartitionMeta};
 use obikindex::{KmerIndex, OKIError, OKIResult};
-use obikindex::{materialize_layer, olm_to_sk, write_graph_as_unitigs};
+use crate::graph_pipeline::{materialize_layer, write_graph_as_unitigs};
 use obiskio::{SKError, SKFileMeta, SKFileReader};
 use ptr_hash::{PtrHash, bucket_fn::CubicEps, hash::Xx64};
 
@@ -109,7 +109,7 @@ impl PrivateBuilder for KmerIndex {
             .map(|g| g.label.as_str())
             .unwrap_or("unknown")
             .to_owned();
-        let spectrums_dir = self.root_path().join("spectrums");
+        let spectrums_dir = self.dir().join("spectrums");
         fs::create_dir_all(&spectrums_dir)?;
         let path = spectrums_dir.join(format!("{label}.json"));
         let spectrum_map: BTreeMap<String, u64> = counts
@@ -194,8 +194,7 @@ impl PrivateBuilder for KmerIndex {
                     (Some(mphf), Some(counts)) => counts.get(mphf.index(&kmer.raw())),
                     _ => 1,
                 },
-            )
-            .map_err(|e| olm_to_sk(e, "layer build"))?;
+            )?;
             n
         } else {
             materialize_layer(g, &layer0_dir, block_bits, mode)?
@@ -206,8 +205,7 @@ impl PrivateBuilder for KmerIndex {
             n_layers: 1,
             mode: mode.clone(),
         }
-        .save(index_dir)
-        .map_err(|e| olm_to_sk(e, "layer build"))?;
+        .save(index_dir)?;
 
         Ok(n_kmers)
     }
diff --git a/src/obikindex/src/index/graph_pipeline.rs b/src/obikindexer/src/graph_pipeline.rs
similarity index 89%
rename from src/obikindex/src/index/graph_pipeline.rs
rename to src/obikindexer/src/graph_pipeline.rs
index 307436d8..5fd9ae9b 100644
--- a/src/obikindex/src/index/graph_pipeline.rs
+++ b/src/obikindexer/src/graph_pipeline.rs
@@ -10,10 +10,9 @@ use obipipeline::{
 
 use obidebruinj::GraphDeBruijn;
 use obikseq::CanonicalKmer;
-use crate::layer::{IndexMode, TypedLayer};
-use obiskio::{SKError, SKResult};
+use obikindex::layer::{IndexMode, TypedLayer};
+use obikindex::{OKIError, OKIResult};
 
-use crate::index::common::olm_to_sk;
 
 // ── KmerGraphData ─────────────────────────────────────────────────────────────
 
@@ -29,16 +28,16 @@ enum KmerGraphData {
 ///
 /// `flat_fn(path, emit)`: opens path, iterates kmers, calls `emit(batch)` for each batch.
 /// `filter(kmer) -> bool`: secondary filter applied in the Transform stage.
-pub(crate) fn build_graph<I, F, G>(
+pub fn build_graph<I, F, G>(
     file_source: I,
     flat_fn: F,
     filter: G,
     n_workers: usize,
     max_open: usize,
-) -> SKResult<GraphDeBruijn>
+) -> OKIResult<GraphDeBruijn>
 where
     I: Iterator<Item = PathBuf> + Send + 'static,
-    F: Fn(PathBuf, &mut dyn FnMut(Vec<CanonicalKmer>)) -> SKResult<()> + Send + Sync + 'static,
+    F: Fn(PathBuf, &mut dyn FnMut(Vec<CanonicalKmer>)) -> OKIResult<()> + Send + Sync + 'static,
     G: Fn(CanonicalKmer) -> bool + Send + Sync + 'static,
 {
     let capacity = 2;
@@ -47,7 +46,7 @@ where
     let filter = Arc::new(filter);
     let g_shared = Arc::new(Mutex::new(GraphDeBruijn::new()));
     let g_sink = Arc::clone(&g_shared);
-    let err_cap: Arc<Mutex<Option<SKError>>> = Arc::new(Mutex::new(None));
+    let err_cap: Arc<Mutex<Option<OKIError>>> = Arc::new(Mutex::new(None));
     let err_flat = Arc::clone(&err_cap);
 
     let throttled = throttle(file_source, max_open);
@@ -121,11 +120,11 @@ where
 /// Phase 2 (write unitigs only): compute degrees, write unitigs to `layer_dir`, drop graph.
 ///
 /// Returns n_kmers. Does NOT build the MPHF — caller does it.
-pub fn write_graph_as_unitigs(g: GraphDeBruijn, layer_dir: &Path) -> SKResult<usize> {
+pub fn write_graph_as_unitigs(g: GraphDeBruijn, layer_dir: &Path) -> OKIResult<usize> {
     let n_kmers = g.len();
     g.compute_degrees_and_mark_starts();
     std::fs::create_dir_all(layer_dir)?;
-    let mut uw = TypedLayer::<()>::unitig_writer(layer_dir).map_err(|e| olm_to_sk(e, "graph pipeline"))?;
+    let mut uw = TypedLayer::<()>::unitig_writer(layer_dir)?;
     g.try_for_each_unitig(|unitig| uw.write(unitig))?;
     uw.close()?;
     drop(g);
@@ -142,11 +141,10 @@ pub fn materialize_layer(
     layer_dir: &Path,
     block_bits: u8,
     evidence: &IndexMode,
-) -> SKResult<usize> {
+) -> OKIResult<usize> {
     let n = write_graph_as_unitigs(g, layer_dir)?;
     debug!("materialize_layer: unitigs written ({n} kmers), building MPHF");
-    TypedLayer::<()>::build(layer_dir, block_bits, evidence)
-        .map_err(|e| olm_to_sk(e, "graph pipeline"))?;
+    TypedLayer::<()>::build(layer_dir, block_bits, evidence)?;
     debug!("materialize_layer: MPHF build done");
     Ok(n)
 }
diff --git a/src/obikindexer/src/lib.rs b/src/obikindexer/src/lib.rs
index 0bda5d6d..2b6bc442 100644
--- a/src/obikindexer/src/lib.rs
+++ b/src/obikindexer/src/lib.rs
@@ -14,3 +14,6 @@
 
 pub mod algorithms;
 pub(crate) mod extensions;
+pub mod graph_pipeline;
+
+pub use graph_pipeline::{build_graph, materialize_layer, write_graph_as_unitigs};
diff --git a/src/obikmer/Cargo.toml b/src/obikmer/Cargo.toml
index 3aaedb34..0ff9dcb1 100644
--- a/src/obikmer/Cargo.toml
+++ b/src/obikmer/Cargo.toml
@@ -19,6 +19,8 @@ obisys        = { path = "../obisys" }
 obiskio       = { path = "../obiskio" }
 obikindex     = { path = "../obikindex", default-features = false }
 obikindexer   = { path = "../obikindexer" }
+obikfilter    = { path = "../obikfilter" }
+obikstats     = { path = "../obikstats" }
 obikalgorithm = { path = "../obikalgorithm" }
 obikphylo     = { path = "../obikphylo" }
 obitaxonomy   = { path = "../obitaxonomy" }
@@ -37,5 +39,5 @@ pprof = { version = "0.15", features = ["prost-codec"], optional = true }
 
 [features]
 default   = ["numa"]
-numa      = ["obikindex/numa"]
+numa      = ["obisys/numa"]
 profiling = ["dep:pprof"]
diff --git a/src/obikmer/src/cmd/index/mod.rs b/src/obikmer/src/cmd/index/mod.rs
index 3159b250..642dc0b8 100644
--- a/src/obikmer/src/cmd/index/mod.rs
+++ b/src/obikmer/src/cmd/index/mod.rs
@@ -2,13 +2,13 @@ use std::path::PathBuf;
 use std::time::Instant;
 
 use clap::Args;
+use obikalgorithm::Algorithm;
+use obikindex::layer::IndexMode;
+use obikindex::{GenomeInfo, IndexBuilder, IndexConfig, IndexState, KmerIndex};
 use obikindexer::algorithms::counter::Counter;
 use obikindexer::algorithms::dereplicator::Dereplicator;
 use obikindexer::algorithms::layer_builder::LayerBuilder;
 use obikindexer::algorithms::partitionner::PartitionRouter;
-use obikindex::{validate_label, GenomeInfo, IndexBuilder, IndexConfig, IndexState, KmerIndex};
-use obikindex::layer::IndexMode;
-use obikalgorithm::Algorithm;
 
 fn current_state(idx: &KmerIndex) -> IndexState {
     idx.state().unwrap_or_else(|e| {
@@ -18,10 +18,12 @@ fn current_state(idx: &KmerIndex) -> IndexState {
 }
 
 fn parse_key_value(s: &str) -> Result<(String, String), String> {
-    let pos = s.find('=').ok_or_else(|| format!("invalid key=value: no '=' in '{s}'"))?;
+    let pos = s
+        .find('=')
+        .ok_or_else(|| format!("invalid key=value: no '=' in '{s}'"))?;
     Ok((s[..pos].to_string(), s[pos + 1..].to_string()))
 }
-use obisys::{progress_bar, spinner, Progress, Reporter, Stage};
+use obisys::{Progress, Reporter, Stage, progress_bar, spinner};
 use tracing::info;
 
 use crate::cli::{CommonArgs, block_size_to_bits, partitions_to_bits};
@@ -108,9 +110,7 @@ pub(crate) fn resolve_approx_params(
     const DEFAULT_B: u8 = 8;
     const DEFAULT_Z: u8 = 1;
 
-    let bits_needed = |fp: f64| -> u8 {
-        (-fp.log2()).ceil() as u8
-    };
+    let bits_needed = |fp: f64| -> u8 { (-fp.log2()).ceil() as u8 };
 
     match (z_opt, b_opt, fp_opt) {
         // All three given: use b and z, recompute fp conservatively.
@@ -198,7 +198,7 @@ pub fn run(args: IndexArgs) {
     };
 
     // ── Open or create the index ─────────────────────────────────────────────
-    if KmerIndex::exists(&output) {
+    if KmerIndex::is_an_index(&output) {
         if !args.force {
             eprintln!(
                 "error: an index already exists at {} (use --force to overwrite it)",
@@ -211,23 +211,32 @@ pub fn run(args: IndexArgs) {
             eprintln!("error removing existing index: {e}");
             std::process::exit(1);
         });
+    } else if output.exists() {
+        eprintln!(
+            "error: {} exists but is not an obikmer index, it cannot be deleted",
+            output.display()
+        );
+        std::process::exit(1);
     }
     let n_bits = partitions_to_bits(args.common.partitions);
     let effective = 1usize << n_bits;
     if effective != args.common.partitions {
-        info!("partitions: {} → {} (next power of 2)", args.common.partitions, effective);
+        info!(
+            "partitions: {} → {} (next power of 2)",
+            args.common.partitions, effective
+        );
     }
     let block_bits = block_size_to_bits(args.block_size);
     let config = IndexConfig {
-        kmer_size:      effective_kmer_size,
+        kmer_size: effective_kmer_size,
         minimizer_size: args.common.minimizer_size,
         n_bits,
-        with_counts:    args.with_counts,
-        evidence:       evidence.clone(),
+        with_counts: args.with_counts,
+        evidence: evidence.clone(),
         block_bits,
     };
     let genome_info = args.label.as_ref().map(|label| {
-        validate_label(label).unwrap_or_else(|e| {
+        GenomeInfo::validate_label(label).unwrap_or_else(|e| {
             eprintln!("error: --label: {e}");
             std::process::exit(1);
         });
@@ -242,7 +251,6 @@ pub fn run(args: IndexArgs) {
         std::process::exit(1);
     });
 
-
     // ── Stage 1: scatter ─────────────────────────────────────────────────────
     if current_state(&idx) < IndexState::Scattered {
         let n_workers = args.common.threads.max(1);
@@ -272,19 +280,23 @@ pub fn run(args: IndexArgs) {
                 last_bases = p.position;
                 let bp = p.position as f64;
                 let (count_str, rate_str) = if bp >= 1e9 {
-                    (format!("{:.2} Gbp", bp / 1e9), format!("{:.0} Mbp/s", ema_rate / 1e6))
+                    (
+                        format!("{:.2} Gbp", bp / 1e9),
+                        format!("{:.0} Mbp/s", ema_rate / 1e6),
+                    )
                 } else {
-                    (format!("{:.0} Mbp", bp / 1e6), format!("{:.0} Mbp/s", ema_rate / 1e6))
+                    (
+                        format!("{:.0} Mbp", bp / 1e6),
+                        format!("{:.0} Mbp/s", ema_rate / 1e6),
+                    )
                 };
                 pb.set_message(format!("{count_str}  {rate_str}"));
             });
 
-        router
-            .run()
-            .unwrap_or_else(|e| {
-                eprintln!("error: {e}");
-                std::process::exit(1);
-            });
+        router.run().unwrap_or_else(|e| {
+            eprintln!("error: {e}");
+            std::process::exit(1);
+        });
         pb.finish_and_clear();
         rep.push(t.stop());
         drop(router); // ends the borrow of `idx` early — `PartitionRouter`'s `Drop` impl would otherwise extend it to the end of scope (`run()` already called `close()`, which marks scatter done, internally)
diff --git a/src/obikmer/src/cmd/predicate.rs b/src/obikmer/src/cmd/predicate.rs
index 87e0e653..0633cacc 100644
--- a/src/obikmer/src/cmd/predicate.rs
+++ b/src/obikmer/src/cmd/predicate.rs
@@ -1,6 +1,6 @@
 use clap::Args;
-use obikindex::{GroupFilterParams, IndexMeta, MetaPred};
-use obikindex::KmerFilter;
+use obikindex::IndexMeta;
+use obikfilter::{GroupFilterParams, KmerFilter, MetaPred};
 
 /// CLI args for ingroup/outgroup filtering — embeddable in any command via `#[command(flatten)]`.
 #[derive(Args)]
diff --git a/src/obikmer/src/cmd/utils/maintenance.rs b/src/obikmer/src/cmd/utils/maintenance.rs
index fd805759..fa464b2b 100644
--- a/src/obikmer/src/cmd/utils/maintenance.rs
+++ b/src/obikmer/src/cmd/utils/maintenance.rs
@@ -1,6 +1,7 @@
 use std::path::PathBuf;
 
-use obikindex::{validate_label, IndexBitsPerKmer, KmerIndex};
+use obikindex::{GenomeInfo, KmerIndex};
+use obikstats::IndexBitsPerKmer;
 use tracing::info;
 
 pub(super) fn run_stats(index_path: &PathBuf) {
@@ -76,7 +77,7 @@ pub(super) fn run_rename(index_path: &PathBuf, spec: &str) {
             std::process::exit(1);
         });
 
-    validate_label(&new_label).unwrap_or_else(|e| {
+    GenomeInfo::validate_label(&new_label).unwrap_or_else(|e| {
         eprintln!("error: --new-label: {e}");
         std::process::exit(1);
     });
diff --git a/src/obikmer2/Cargo.toml b/src/obikmer2/Cargo.toml
new file mode 100644
index 00000000..f332c200
--- /dev/null
+++ b/src/obikmer2/Cargo.toml
@@ -0,0 +1,24 @@
+[package]
+name = "obikmer2"
+version = "1.2.2"
+edition = "2024"
+
+[[bin]]
+name = "obikmer2"
+path = "src/main.rs"
+
+[dependencies]
+obikseq       = { path = "../obikseq" }
+obiread       = { path = "../obiread" }
+obipipeline   = { path = "../obipipeline" }
+obisys        = { path = "../obisys" }
+obikindex     = { path = "../obikindex", default-features = false }
+obikindexer   = { path = "../obikindexer" }
+obikalgorithm = { path = "../obikalgorithm" }
+clap          = { version = "4", features = ["derive"] }
+tracing       = "0.1.44"
+tracing-subscriber = { version = "0.3", features = ["fmt", "env-filter"] }
+
+[features]
+default   = ["numa"]
+numa      = ["obisys/numa"]
diff --git a/src/obikmer2/src/cli.rs b/src/obikmer2/src/cli.rs
new file mode 100644
index 00000000..6018ad30
--- /dev/null
+++ b/src/obikmer2/src/cli.rs
@@ -0,0 +1,114 @@
+use std::path::PathBuf;
+
+use clap::Args;
+use obiread::NucPage;
+use obikseq::RoutableSuperKmer;
+use obipipeline::Throttled;
+
+// ── Shared arguments ──────────────────────────────────────────────────────────
+
+#[derive(Args)]
+pub struct CommonArgs {
+    /// Input files or directories (FASTA/FASTQ, optionally gzip-compressed).
+    /// If omitted, reads from stdin.
+    #[arg(num_args = 0..)]
+    pub inputs: Vec<String>,
+
+    /// k-mer size
+    #[arg(short, long, default_value_t = 31)]
+    pub kmer_size: usize,
+
+    /// Minimizer size
+    #[arg(short, long, default_value_t = 11)]
+    pub minimizer_size: usize,
+
+    /// Entropy threshold (k-mers with score ≤ theta are rejected)
+    #[arg(long, default_value_t = 0.7)]
+    pub theta: f64,
+
+    /// Maximum sub-word size for entropy computation
+    #[arg(long, default_value_t = 6)]
+    pub level_max: usize,
+
+    /// Number of partitions (rounded up to the next power of 2)
+    #[arg(short, long, default_value_t = 256)]
+    pub partitions: usize,
+
+    /// Number of worker threads
+    #[arg(
+        short = 'T',
+        long,
+        default_value_t = obisys::effective_parallelism()
+    )]
+    pub threads: usize,
+
+    /// Maximum number of input files open simultaneously.
+    /// Defaults to threads/4 (minimum 1). Keep below the number of workers
+    /// to ensure CPU workers are always available for the transform stage.
+    #[arg(long)]
+    pub max_open_files: Option<usize>,
+}
+
+/// Smallest `b` such that `2^b >= n` (i.e. `n.next_power_of_two().ilog2()`).
+/// Minimum 1 (degenerate n=0 or n=1 → 1 partition).
+pub fn partitions_to_bits(n: usize) -> usize {
+    n.max(1).next_power_of_two().trailing_zeros() as usize
+}
+
+/// Convert a block size (number of unitigs per block) to its `block_bits` exponent.
+/// `block_size=1` → `block_bits=0` (one entry per unitig, O(1) random access).
+pub fn block_size_to_bits(n: usize) -> u8 {
+    n.max(1).next_power_of_two().trailing_zeros() as u8
+}
+
+impl CommonArgs {
+    /// Validate k and m constraints. Exits on error.
+    pub fn validate(&self) {
+        let k = self.kmer_size;
+        let m = self.minimizer_size;
+
+        if k < 11 || k > 31 {
+            eprintln!("error: --kmer-size must be in [11, 31] (got {k})");
+            std::process::exit(1);
+        }
+        if k % 2 == 0 {
+            eprintln!("error: --kmer-size must be odd (got {k}); even k allows palindromic k-mers");
+            std::process::exit(1);
+        }
+        if m < 3 || m >= k {
+            eprintln!("error: --minimizer-size must be in [3, k−1] = [3, {}] (got {m})", k - 1);
+            std::process::exit(1);
+        }
+        if m % 2 == 0 {
+            eprintln!("error: --minimizer-size must be odd (got {m})");
+            std::process::exit(1);
+        }
+    }
+
+    pub fn effective_max_open(&self) -> usize {
+        self.max_open_files
+            .unwrap_or_else(|| (self.threads / 4).max(1))
+            .max(1)
+    }
+
+    pub fn seqfile_paths(&self) -> obiread::PathIter {
+        let paths: Vec<PathBuf> = if self.inputs.is_empty() {
+            vec![PathBuf::from("-")]
+        } else {
+            self.inputs.iter().map(PathBuf::from).collect()
+        };
+        obiread::PathIter::new(paths)
+    }
+}
+
+// ── Pipeline data carrier ─────────────────────────────────────────────────────
+
+pub enum PipelineData {
+    Path(Throttled<PathBuf>),
+    NucPage(NucPage),
+    Batch(Vec<RoutableSuperKmer>),
+}
+
+unsafe impl Send for PipelineData {}
+unsafe impl Sync for PipelineData {}
+
diff --git a/src/obikmer2/src/cmd/index/mod.rs b/src/obikmer2/src/cmd/index/mod.rs
new file mode 100644
index 00000000..642dc0b8
--- /dev/null
+++ b/src/obikmer2/src/cmd/index/mod.rs
@@ -0,0 +1,363 @@
+use std::path::PathBuf;
+use std::time::Instant;
+
+use clap::Args;
+use obikalgorithm::Algorithm;
+use obikindex::layer::IndexMode;
+use obikindex::{GenomeInfo, IndexBuilder, IndexConfig, IndexState, KmerIndex};
+use obikindexer::algorithms::counter::Counter;
+use obikindexer::algorithms::dereplicator::Dereplicator;
+use obikindexer::algorithms::layer_builder::LayerBuilder;
+use obikindexer::algorithms::partitionner::PartitionRouter;
+
+fn current_state(idx: &KmerIndex) -> IndexState {
+    idx.state().unwrap_or_else(|e| {
+        eprintln!("error reading index metadata: {e}");
+        std::process::exit(1);
+    })
+}
+
+fn parse_key_value(s: &str) -> Result<(String, String), String> {
+    let pos = s
+        .find('=')
+        .ok_or_else(|| format!("invalid key=value: no '=' in '{s}'"))?;
+    Ok((s[..pos].to_string(), s[pos + 1..].to_string()))
+}
+use obisys::{Progress, Reporter, Stage, progress_bar, spinner};
+use tracing::info;
+
+use crate::cli::{CommonArgs, block_size_to_bits, partitions_to_bits};
+
+#[derive(Args)]
+pub struct IndexArgs {
+    /// Output index directory
+    #[arg(short, long)]
+    pub output: PathBuf,
+
+    /// Overwrite output directory if it already exists
+    #[arg(long, default_value_t = false)]
+    pub force: bool,
+
+    /// Genome label (default: input filename without path/extension)
+    #[arg(long)]
+    pub label: Option<String>,
+
+    /// Genome categorical metadata as key=value pairs (repeatable)
+    #[arg(long = "meta", value_parser = parse_key_value)]
+    pub meta: Vec<(String, String)>,
+
+    /// Minimum kmer abundance (inclusive)
+    #[arg(long, default_value_t = 1)]
+    pub min_abundance: u32,
+
+    /// Maximum kmer abundance (inclusive)
+    #[arg(long)]
+    pub max_abundance: Option<u32>,
+
+    /// Store kmer counts in the index (default: set membership only)
+    #[arg(long, default_value_t = false)]
+    pub with_counts: bool,
+
+    /// Keep intermediate build files (dereplicated superkmers, mphf1, counts1)
+    #[arg(long, default_value_t = false)]
+    pub keep_intermediate: bool,
+
+    /// Use approximate (fingerprint-based) evidence instead of exact evidence.
+    /// False-positive rate per z-window: 1/2^(b·z).
+    #[arg(long, default_value_t = false)]
+    pub approx: bool,
+
+    /// Findere z parameter: number of consecutive k-mers that must all match.
+    /// Effective indexed k-mer size is kmer_size - z + 1.
+    #[arg(short = 'z', long, default_value = None)]
+    pub findere_z: Option<u8>,
+
+    /// Fingerprint bits per slot (b).  FP per z-window = 1/2^(b·z).
+    #[arg(long, default_value = None)]
+    pub evidence_bits: Option<u8>,
+
+    /// Target false-positive rate per z-window (e.g. 0.01).
+    /// Used to derive missing b or z.
+    #[arg(long, default_value = None)]
+    pub fp: Option<f64>,
+
+    /// Block size for exact evidence `.idx` (number of unitigs per block).
+    /// Must be a power of two; rounded up if not. Default 1 = O(1) random access.
+    #[arg(long, default_value_t = 1)]
+    pub block_size: usize,
+
+    #[command(flatten)]
+    pub common: CommonArgs,
+}
+
+/// Resolve the (z, b, fp) triplet from the user-supplied subset.
+///
+/// Model: FP = 1/2^(b·z)  ⟹  b·z = ⌈-log₂(fp)⌉
+///
+/// Rules when one value is missing (conservative = ceiling):
+///   given z, b       → fp = 1/2^(b·z)
+///   given z, fp      → b  = ⌈-log₂(fp) / z⌉
+///   given b, fp      → z  = ⌈-log₂(fp) / b⌉
+///   given z only     → b  = 8  (default),  fp derived
+///   given b only     → z  = 1  (default),  fp derived
+///   given fp only    → b  = 8  (default),  z  derived
+///   none given       → z  = 1, b = 8, fp = 1/256
+pub(crate) fn resolve_approx_params(
+    z_opt: Option<u8>,
+    b_opt: Option<u8>,
+    fp_opt: Option<f64>,
+) -> (u8, u8, f64) {
+    const DEFAULT_B: u8 = 8;
+    const DEFAULT_Z: u8 = 1;
+
+    let bits_needed = |fp: f64| -> u8 { (-fp.log2()).ceil() as u8 };
+
+    match (z_opt, b_opt, fp_opt) {
+        // All three given: use b and z, recompute fp conservatively.
+        (Some(z), Some(b), Some(_fp)) => {
+            let fp = 1.0_f64 / (1u64 << (b as u32 * z as u32)) as f64;
+            (z, b, fp)
+        }
+        // Two given, derive third.
+        (Some(z), Some(b), None) => {
+            let fp = 1.0_f64 / (1u64 << (b as u32 * z as u32)) as f64;
+            (z, b, fp)
+        }
+        (Some(z), None, Some(fp)) => {
+            let bz = (-fp.log2()).ceil() as u32;
+            let b = ((bz + z as u32 - 1) / z as u32).max(1) as u8;
+            let actual_fp = 1.0_f64 / (1u64 << (b as u32 * z as u32)) as f64;
+            (z, b, actual_fp)
+        }
+        (None, Some(b), Some(fp)) => {
+            let bz = (-fp.log2()).ceil() as u32;
+            let z = ((bz + b as u32 - 1) / b as u32).max(1) as u8;
+            let actual_fp = 1.0_f64 / (1u64 << (b as u32 * z as u32)) as f64;
+            (z, b, actual_fp)
+        }
+        // One given, apply defaults for the other.
+        (Some(z), None, None) => {
+            let b = DEFAULT_B;
+            let fp = 1.0_f64 / (1u64 << (b as u32 * z as u32)) as f64;
+            (z, b, fp)
+        }
+        (None, Some(b), None) => {
+            let z = DEFAULT_Z;
+            let fp = 1.0_f64 / (1u64 << (b as u32 * z as u32)) as f64;
+            (z, b, fp)
+        }
+        (None, None, Some(fp)) => {
+            let b = DEFAULT_B;
+            let z = ((bits_needed(fp) as u32 + b as u32 - 1) / b as u32).max(1) as u8;
+            let actual_fp = 1.0_f64 / (1u64 << (b as u32 * z as u32)) as f64;
+            (z, b, actual_fp)
+        }
+        // None given: defaults.
+        (None, None, None) => {
+            let b = DEFAULT_B;
+            let z = DEFAULT_Z;
+            let fp = 1.0_f64 / (1u64 << (b as u32 * z as u32)) as f64;
+            (z, b, fp)
+        }
+    }
+}
+
+pub fn run(args: IndexArgs) {
+    args.common.validate();
+
+    let output = args.output.clone();
+    let mut rep = Reporter::new();
+
+    // Locked for the whole build (including a possible --force removal +
+    // recreation below): a second `index` run resuming/overwriting the same
+    // output directory concurrently would otherwise corrupt it. Unlinking
+    // the lock file via --force's remove_dir_all is safe — the held file
+    // descriptor keeps the lock regardless of the directory entry.
+    let _lock = obisys::DirLock::acquire(&output).unwrap_or_else(|e| {
+        eprintln!("error locking output directory {}: {e}", output.display());
+        std::process::exit(1);
+    });
+
+    // ── Resolve evidence kind ────────────────────────────────────────────────
+    let (evidence, effective_kmer_size) = if args.approx {
+        let (z, b, fp) = resolve_approx_params(args.findere_z, args.evidence_bits, args.fp);
+        let k = args.common.kmer_size;
+        if z as usize >= k {
+            eprintln!(
+                "error: Findere z={z} must be < kmer-size={k} \
+                 (effective kmer size k−z+1 = {} ≤ 0)",
+                k as isize - z as isize + 1
+            );
+            std::process::exit(1);
+        }
+        let s = k - z as usize + 1;
+        info!("approximate evidence: b={b}, z={z}, fp={fp:.2e}, indexed kmer size={s}");
+        (IndexMode::Approx { b, z }, s)
+    } else {
+        (IndexMode::Exact, args.common.kmer_size)
+    };
+
+    // ── Open or create the index ─────────────────────────────────────────────
+    if KmerIndex::is_an_index(&output) {
+        if !args.force {
+            eprintln!(
+                "error: an index already exists at {} (use --force to overwrite it)",
+                output.display()
+            );
+            std::process::exit(1);
+        }
+        info!("--force: removing existing index at {}", output.display());
+        std::fs::remove_dir_all(&output).unwrap_or_else(|e| {
+            eprintln!("error removing existing index: {e}");
+            std::process::exit(1);
+        });
+    } else if output.exists() {
+        eprintln!(
+            "error: {} exists but is not an obikmer index, it cannot be deleted",
+            output.display()
+        );
+        std::process::exit(1);
+    }
+    let n_bits = partitions_to_bits(args.common.partitions);
+    let effective = 1usize << n_bits;
+    if effective != args.common.partitions {
+        info!(
+            "partitions: {} → {} (next power of 2)",
+            args.common.partitions, effective
+        );
+    }
+    let block_bits = block_size_to_bits(args.block_size);
+    let config = IndexConfig {
+        kmer_size: effective_kmer_size,
+        minimizer_size: args.common.minimizer_size,
+        n_bits,
+        with_counts: args.with_counts,
+        evidence: evidence.clone(),
+        block_bits,
+    };
+    let genome_info = args.label.as_ref().map(|label| {
+        GenomeInfo::validate_label(label).unwrap_or_else(|e| {
+            eprintln!("error: --label: {e}");
+            std::process::exit(1);
+        });
+        let mut info = GenomeInfo::new(label.clone());
+        for (k, v) in &args.meta {
+            info.meta.insert(k.clone(), v.clone());
+        }
+        info
+    });
+    let idx = KmerIndex::create(&output, config, genome_info).unwrap_or_else(|e| {
+        eprintln!("error creating index: {e}");
+        std::process::exit(1);
+    });
+
+    // ── Stage 1: scatter ─────────────────────────────────────────────────────
+    if current_state(&idx) < IndexState::Scattered {
+        let n_workers = args.common.threads.max(1);
+        let max_open = args.common.effective_max_open();
+
+        let t = Stage::start("scatter");
+        let pb = spinner("scatter");
+        let mut ema_rate: f64 = 0.0;
+        let mut last_t = Instant::now();
+        let mut last_bases: u64 = 0;
+        const ALPHA: f64 = 0.15;
+
+        let mut router = PartitionRouter::new(&idx)
+            .level_max(args.common.level_max)
+            .theta(args.common.theta)
+            .workers(n_workers)
+            .max_open(max_open)
+            .files(args.common.seqfile_paths())
+            .on_progress(|p: Progress| {
+                let now = Instant::now();
+                let dt = now.duration_since(last_t).as_secs_f64();
+                if dt > 0.0 {
+                    let instant = (p.position - last_bases) as f64 / dt;
+                    ema_rate = ALPHA * instant + (1.0 - ALPHA) * ema_rate;
+                }
+                last_t = now;
+                last_bases = p.position;
+                let bp = p.position as f64;
+                let (count_str, rate_str) = if bp >= 1e9 {
+                    (
+                        format!("{:.2} Gbp", bp / 1e9),
+                        format!("{:.0} Mbp/s", ema_rate / 1e6),
+                    )
+                } else {
+                    (
+                        format!("{:.0} Mbp", bp / 1e6),
+                        format!("{:.0} Mbp/s", ema_rate / 1e6),
+                    )
+                };
+                pb.set_message(format!("{count_str}  {rate_str}"));
+            });
+
+        router.run().unwrap_or_else(|e| {
+            eprintln!("error: {e}");
+            std::process::exit(1);
+        });
+        pb.finish_and_clear();
+        rep.push(t.stop());
+        drop(router); // ends the borrow of `idx` early — `PartitionRouter`'s `Drop` impl would otherwise extend it to the end of scope (`run()` already called `close()`, which marks scatter done, internally)
+    } else {
+        info!("scatter already done, skipping");
+    }
+
+    // ── Stage 2: dereplicate + count ─────────────────────────────────────────
+    if current_state(&idx) < IndexState::Counted {
+        let t = Stage::start("dereplicate");
+        let pb = progress_bar("dereplication", idx.n_partitions() as u64, "partitions");
+        Dereplicator::new(&idx)
+            .on_progress(|_: Progress| pb.inc(1))
+            .run()
+            .unwrap_or_else(|e| {
+                eprintln!("error: {e}");
+                std::process::exit(1);
+            });
+        pb.finish_and_clear();
+        rep.push(t.stop());
+
+        let t = Stage::start("count_kmer");
+        let pb = progress_bar("counting", idx.n_partitions() as u64, "partitions");
+        // `Counter::run` writes `spectrums/{label}.json` and marks count
+        // done (`count.done`) internally once every partition succeeds.
+        Counter::new(&idx)
+            .keep_partial(args.keep_intermediate)
+            .on_progress(|_: Progress| pb.inc(1))
+            .run()
+            .unwrap_or_else(|e| {
+                eprintln!("error: {e}");
+                std::process::exit(1);
+            });
+        pb.finish_and_clear();
+        rep.push(t.stop());
+    } else {
+        info!("dereplicate+count already done, skipping");
+    }
+
+    // ── Stage 3: build layered index ─────────────────────────────────────────
+    if current_state(&idx) < IndexState::Indexed {
+        let t = Stage::start("index");
+        let pb = progress_bar("index", idx.n_partitions() as u64, "partitions");
+        let total_kmers = LayerBuilder::new(&idx)
+            .min_abundance(args.min_abundance)
+            .max_abundance(args.max_abundance)
+            .keep_intermediate(args.keep_intermediate)
+            .on_progress(|_: Progress| pb.inc(1))
+            .run()
+            .unwrap_or_else(|e| {
+                eprintln!("error: {e}");
+                std::process::exit(1);
+            });
+        pb.finish_and_clear();
+        info!("done — {total_kmers} total kmers indexed");
+        rep.push(t.stop());
+        // `LayerBuilder::run` marks the index done (`index.done`) internally
+        // once every partition succeeds.
+    } else {
+        info!("index already built, skipping");
+    }
+
+    rep.print();
+}
diff --git a/src/obikmer2/src/cmd/mod.rs b/src/obikmer2/src/cmd/mod.rs
new file mode 100644
index 00000000..33edc959
--- /dev/null
+++ b/src/obikmer2/src/cmd/mod.rs
@@ -0,0 +1 @@
+pub mod index;
diff --git a/src/obikmer2/src/main.rs b/src/obikmer2/src/main.rs
new file mode 100644
index 00000000..a490edd1
--- /dev/null
+++ b/src/obikmer2/src/main.rs
@@ -0,0 +1,32 @@
+mod cli;
+mod cmd;
+
+use clap::{Parser, Subcommand};
+use tracing_subscriber::{EnvFilter, fmt};
+
+#[derive(Parser)]
+#[command(name = "obikmer2", about = "DNA k-mer tools", version)]
+struct Cli {
+    #[command(subcommand)]
+    command: Commands,
+}
+
+#[derive(Subcommand)]
+enum Commands {
+    /// Build the complete genome index (scatter → dereplicate → count → layered MPHF)
+    Index(cmd::index::IndexArgs),
+}
+
+fn main() {
+    fmt()
+        .with_env_filter(
+            EnvFilter::try_from_default_env().unwrap_or_else(|_| EnvFilter::new("info")),
+        )
+        .with_writer(std::io::stderr)
+        .init();
+
+    let cli = Cli::parse();
+    match cli.command {
+        Commands::Index(args) => cmd::index::run(args),
+    }
+}
diff --git a/src/obikmerge/Cargo.toml b/src/obikmerge/Cargo.toml
new file mode 100644
index 00000000..1620c3c9
--- /dev/null
+++ b/src/obikmerge/Cargo.toml
@@ -0,0 +1,15 @@
+[package]
+name = "obikmerge"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+obikindex     = { path = "../obikindex" }
+obikindexer   = { path = "../obikindexer" }
+obikfilter    = { path = "../obikfilter" }
+obicompactvec = { path = "../obicompactvec" }
+obiskio       = { path = "../obiskio" }
+obikseq       = { path = "../obikseq" }
+obipipeline   = { path = "../obipipeline" }
+obisys        = { path = "../obisys" }
+tracing       = "0.1.44"
diff --git a/src/obikmerge/src/lib.rs b/src/obikmerge/src/lib.rs
new file mode 100644
index 00000000..89d51a79
--- /dev/null
+++ b/src/obikmerge/src/lib.rs
@@ -0,0 +1,17 @@
+//! Merging multiple `obikindex::KmerIndex` sources into one: bootstrapping
+//! from the first source, then merging each remaining source partition by
+//! partition (de Bruijn graph union + column fill). A maintenance/
+//! transformation operation on already-built indexes, not part of the
+//! `Index { Partition { Layer } }` data model itself — kept out of
+//! `obikindex` so that crate doesn't grow this algorithm's own
+//! dependencies (`obipipeline`, `obidebruinj` via `obikindexer`).
+//!
+//! [`merge_layer`] holds the per-partition merge primitive
+//! (`merge_partition`, `SrcLayerData`) — the latter is also reused by
+//! `obikrebuild::rebuild_layer`, hence its `pub` visibility here.
+
+mod merge;
+mod merge_layer;
+
+pub use merge::*;
+pub use merge_layer::{MergeMode, SrcLayerData};
diff --git a/src/obikindex/src/index/merge.rs b/src/obikmerge/src/merge.rs
similarity index 97%
rename from src/obikindex/src/index/merge.rs
rename to src/obikmerge/src/merge.rs
index 4ddad6eb..c428a94b 100644
--- a/src/obikindex/src/index/merge.rs
+++ b/src/obikmerge/src/merge.rs
@@ -3,19 +3,20 @@ use std::fs;
 use std::io;
 use std::path::Path;
 
-use crate::index::builder::IndexBuilder;
+use obikindex::IndexBuilder;
 
 use obisys::{Reporter, Stage, progress_bar, spinner};
 use tracing::{debug, info};
 
-use crate::layer::IndexMode;
+use obikindex::layer::IndexMode;
 
-use crate::index::error::{OKIError, OKIResult};
-use crate::index::kmer_index::KmerIndex;
-use crate::index::meta::{GenomeInfo, IndexMeta};
-use crate::index::state::IndexState;
+use obikindex::{OKIError, OKIResult};
+use obikindex::KmerIndex;
+use obikindex::{GenomeInfo, IndexMeta};
+use obikindex::IndexState;
+use obisys::PartitionRunner;
 
-pub use crate::index::merge_layer::MergeMode;
+pub use crate::merge_layer::MergeMode;
 
 // ── per-partition diagnostic record ──────────────────────────────────────────
 
@@ -229,7 +230,7 @@ impl KmerIndex {
             let srcs = &srcs;
             let evidence = &evidence;
 
-            let runner = crate::index::numa::PartitionRunner::new();
+            let runner = PartitionRunner::new();
             let mut part_stats: Vec<PartStat> = Vec::with_capacity(n_partitions);
 
             runner
diff --git a/src/obikindex/src/index/merge_layer/mod.rs b/src/obikmerge/src/merge_layer/mod.rs
similarity index 94%
rename from src/obikindex/src/index/merge_layer/mod.rs
rename to src/obikmerge/src/merge_layer/mod.rs
index f8a35849..f659327c 100644
--- a/src/obikindex/src/index/merge_layer/mod.rs
+++ b/src/obikmerge/src/merge_layer/mod.rs
@@ -20,16 +20,18 @@ use tracing::debug;
 
 use obicompactvec::{PersistentBitMatrixBuilder, PersistentCompactIntMatrixBuilder};
 use obikseq::CanonicalKmer;
-use crate::layer::{IndexMode, TypedLayer, LayeredMap, MphfOnly, layer_dir};
-use obiskio::{SKError, SKResult, UnitigFileReader};
+use obikindex::layer::{IndexMode, TypedLayer, LayeredMap, MphfOnly};
+use obikindex::layer::utils::layer_dir;
+use obiskio::UnitigFileReader;
+use obikindex::{OKIError, OKIResult};
 
-use crate::index::common::{ColBuilder, load_meta, olm_to_sk};
-use crate::index::graph_pipeline::{build_graph, materialize_layer};
-use crate::index::kmer_index::KmerIndex;
+use obikindex::{ColBuilder, load_meta};
+use obikindexer::{build_graph, materialize_layer};
+use obikindex::KmerIndex;
 
 mod src_layer;
 
-pub(crate) use src_layer::SrcLayerData;
+pub use src_layer::SrcLayerData;
 
 // ── MergeMode ─────────────────────────────────────────────────────────────────
 
@@ -216,15 +218,14 @@ impl KmerIndex {
         n_dst_genomes: usize,
         block_bits: u8,
         evidence: &IndexMode,
-    ) -> SKResult<usize> {
+    ) -> OKIResult<usize> {
         let dst_index_dir = self.index_dir(i);
         if !dst_index_dir.exists() {
             return Ok(0);
         }
 
-        load_meta(&dst_index_dir, "merge")?; // ensure meta.json exists before LayeredMap::open
-        let dst_map =
-            Arc::new(LayeredMap::<()>::open(&dst_index_dir).map_err(|e| olm_to_sk(e, "merge"))?);
+        load_meta(&dst_index_dir)?; // ensure meta.json exists before LayeredMap::open
+        let dst_map = Arc::new(LayeredMap::<()>::open(&dst_index_dir)?);
         let n_dst_layers = dst_map.n_layers();
         let n_src_total: usize = sources.iter().map(|(_, n)| *n).sum();
 
@@ -235,8 +236,7 @@ impl KmerIndex {
                 TypedLayer::<()>::init_presence_matrix(
                     &layer_dir(&dst_index_dir, l),
                     dst_map.layer(l).n(),
-                )
-                .map_err(|e| olm_to_sk(e, "merge"))?;
+                )?;
             }
         }
 
@@ -254,7 +254,7 @@ impl KmerIndex {
             if !src_index_dir.exists() {
                 continue;
             }
-            let src_meta = load_meta(&src_index_dir, "merge")?;
+            let src_meta = load_meta(&src_index_dir)?;
             for l in 0..src_meta.n_layers {
                 let p = layer_dir(&src_index_dir, l).join("unitigs.bin");
                 if p.exists() {
@@ -277,7 +277,7 @@ impl KmerIndex {
 
         let g = build_graph(
             unitig_paths.into_iter(),
-            move |path: PathBuf, emit: &mut dyn FnMut(Vec<CanonicalKmer>)| -> SKResult<()> {
+            move |path: PathBuf, emit: &mut dyn FnMut(Vec<CanonicalKmer>)| -> OKIResult<()> {
                 let reader = UnitigFileReader::open_sequential(&path)?;
                 let mut batch: Vec<CanonicalKmer> = Vec::with_capacity(BATCH);
                 for (kmer, _, _) in reader.iter_indexed_canonical_kmers() {
@@ -318,9 +318,7 @@ impl KmerIndex {
 
         let t_open = std::time::Instant::now();
         let new_mphf: Option<Arc<MphfOnly>> = if any_new {
-            Some(Arc::new(
-                MphfOnly::open(&new_layer_dir).map_err(|e| olm_to_sk(e, "merge"))?,
-            ))
+            Some(Arc::new(MphfOnly::open(&new_layer_dir)?))
         } else {
             None
         };
@@ -340,13 +338,13 @@ impl KmerIndex {
                 MergeMode::Count => new_layer_dir.join("counts"),
             };
             fs::create_dir_all(&data_dir)?;
-            let mut mb = MatrixBuilder::new(mode, n_new, &data_dir).map_err(SKError::Io)?;
+            let mut mb = MatrixBuilder::new(mode, n_new, &data_dir).map_err(OKIError::Io)?;
             for _ in 0..n_dst_genomes {
-                mb.add_absent_col().map_err(SKError::Io)?;
+                mb.add_absent_col().map_err(OKIError::Io)?;
             }
             let cols = (0..n_src_total)
-                .map(|_| mb.add_col().map_err(SKError::Io))
-                .collect::<SKResult<Vec<_>>>()?;
+                .map(|_| mb.add_col().map_err(OKIError::Io))
+                .collect::<OKIResult<Vec<_>>>()?;
             (cols, Some(mb))
         } else {
             (vec![], None)
@@ -365,10 +363,10 @@ impl KmerIndex {
                 MergeMode::Presence => layer_dir.join("presence"),
                 MergeMode::Count => layer_dir.join("counts"),
             };
-            let mut mb = MatrixBuilder::resume(mode, &data_dir).map_err(SKError::Io)?;
+            let mut mb = MatrixBuilder::resume(mode, &data_dir).map_err(OKIError::Io)?;
             let cols = (0..n_src_total)
-                .map(|_| mb.add_col().map_err(SKError::Io))
-                .collect::<SKResult<Vec<_>>>()?;
+                .map(|_| mb.add_col().map_err(OKIError::Io))
+                .collect::<OKIResult<Vec<_>>>()?;
             exist_mbs.push(mb);
             exist_builders.push(cols);
         }
@@ -391,7 +389,7 @@ impl KmerIndex {
                     col_offset += src_n;
                     continue;
                 }
-                let src_meta = load_meta(&src_index_dir, "merge")?;
+                let src_meta = load_meta(&src_index_dir)?;
                 for l in 0..src_meta.n_layers {
                     let src_layer_dir = layer_dir(&src_index_dir, l);
                     if src_layer_dir.join("unitigs.bin").exists() {
@@ -552,7 +550,7 @@ impl KmerIndex {
             .into_inner()
             .unwrap_or_else(|e| e.into_inner())
         {
-            return Err(SKError::InvalidData {
+            return Err(OKIError::InvalidData {
                 context: "merge pass2",
                 detail: msg,
             });
@@ -568,7 +566,7 @@ impl KmerIndex {
                     .unwrap_or_else(|e| e.into_inner())
                     .close()?;
             }
-            mb.close().map_err(SKError::Io)?;
+            mb.close().map_err(OKIError::Io)?;
         }
 
         for b in new_locked {
@@ -579,13 +577,12 @@ impl KmerIndex {
                 .close()?;
         }
         if let Some(mb) = new_mb {
-            mb.close().map_err(SKError::Io)?;
+            mb.close().map_err(OKIError::Io)?;
 
             let mut part_meta = self.partition_meta(i)?;
             part_meta.n_layers = new_layer_idx + 1;
             part_meta
-                .save(&dst_index_dir)
-                .map_err(|e| olm_to_sk(e, "merge"))?;
+                .save(&dst_index_dir)?;
         }
 
         debug!(
diff --git a/src/obikindex/src/index/merge_layer/src_layer.rs b/src/obikmerge/src/merge_layer/src_layer.rs
similarity index 76%
rename from src/obikindex/src/index/merge_layer/src_layer.rs
rename to src/obikmerge/src/merge_layer/src_layer.rs
index 4f104b6f..5078f735 100644
--- a/src/obikindex/src/index/merge_layer/src_layer.rs
+++ b/src/obikmerge/src/merge_layer/src_layer.rs
@@ -2,44 +2,43 @@ use std::path::Path;
 
 use obicompactvec::{MatrixGroupOps, PersistentBitMatrix, PersistentCompactIntMatrix};
 use obikseq::CanonicalKmer;
-use crate::layer::MphfOnly;
-use obiskio::{SKError, SKResult};
+use obikindex::layer::MphfOnly;
+use obikindex::{OKIError, OKIResult};
 
-use crate::index::common::olm_to_sk;
 
 use super::MergeMode;
 
 // ── SrcLayerData — opened source matrix for pass-2 lookup ─────────────────────
 
-pub(crate) enum SrcLayerData {
+pub enum SrcLayerData {
     Presence(MphfOnly, PersistentBitMatrix),
     Count(MphfOnly, PersistentCompactIntMatrix),
 }
 
 impl SrcLayerData {
-    pub(crate) fn open(layer_dir: &Path, merge_mode: MergeMode) -> SKResult<Self> {
+    pub fn open(layer_dir: &Path, merge_mode: MergeMode) -> OKIResult<Self> {
         let counts_dir = layer_dir.join("counts");
         match merge_mode {
             MergeMode::Presence => {
                 if counts_dir.exists() && !layer_dir.join("presence").exists() {
-                    let mphf = MphfOnly::open(layer_dir).map_err(|e| olm_to_sk(e, "merge"))?;
-                    let mat = PersistentCompactIntMatrix::open(layer_dir).map_err(SKError::Io)?;
+                    let mphf = MphfOnly::open(layer_dir)?;
+                    let mat = PersistentCompactIntMatrix::open(layer_dir).map_err(OKIError::Io)?;
                     Ok(SrcLayerData::Count(mphf, mat))
                 } else {
                     // presence dir exists, or neither exists → Implicit handled by open()
-                    let mphf = MphfOnly::open(layer_dir).map_err(|e| olm_to_sk(e, "merge"))?;
-                    let mat = PersistentBitMatrix::open(layer_dir).map_err(SKError::Io)?;
+                    let mphf = MphfOnly::open(layer_dir)?;
+                    let mat = PersistentBitMatrix::open(layer_dir).map_err(OKIError::Io)?;
                     Ok(SrcLayerData::Presence(mphf, mat))
                 }
             }
             MergeMode::Count => {
-                let mphf = MphfOnly::open(layer_dir).map_err(|e| olm_to_sk(e, "merge"))?;
+                let mphf = MphfOnly::open(layer_dir)?;
                 if counts_dir.exists() {
-                    let mat = PersistentCompactIntMatrix::open(layer_dir).map_err(SKError::Io)?;
+                    let mat = PersistentCompactIntMatrix::open(layer_dir).map_err(OKIError::Io)?;
                     Ok(SrcLayerData::Count(mphf, mat))
                 } else {
                     // No counts → treat as implicit presence (all 1s)
-                    let mat = PersistentBitMatrix::open(layer_dir).map_err(SKError::Io)?;
+                    let mat = PersistentBitMatrix::open(layer_dir).map_err(OKIError::Io)?;
                     Ok(SrcLayerData::Presence(mphf, mat))
                 }
             }
@@ -58,7 +57,7 @@ impl SrcLayerData {
         buf
     }
 
-    pub(crate) fn n_slots(&self) -> usize {
+    pub fn n_slots(&self) -> usize {
         match self {
             SrcLayerData::Presence(_, mat) => mat.n(),
             SrcLayerData::Count(_, mat) => mat.n(),
@@ -67,7 +66,7 @@ impl SrcLayerData {
 
     /// MPHF lookup: returns the slot index for `kmer` (kmer must be in the domain).
     #[inline]
-    pub(crate) fn slot(&self, kmer: CanonicalKmer) -> usize {
+    pub fn slot(&self, kmer: CanonicalKmer) -> usize {
         match self {
             SrcLayerData::Presence(mphf, _) => mphf.index(kmer),
             SrcLayerData::Count(mphf, _) => mphf.index(kmer),
@@ -76,7 +75,7 @@ impl SrcLayerData {
 
     /// Row lookup by slot index, bypassing the MPHF.
     #[inline]
-    pub(crate) fn fill_row_by_slot(&self, slot: usize, n_genomes: usize) -> Vec<u32> {
+    pub fn fill_row_by_slot(&self, slot: usize, n_genomes: usize) -> Vec<u32> {
         let mut buf = vec![0u32; n_genomes];
         match self {
             SrcLayerData::Presence(_, mat) => mat.fill_row(slot, &mut buf),
@@ -86,7 +85,7 @@ impl SrcLayerData {
     }
 
     /// Call `f` with a reference to the underlying matrix as `&dyn MatrixGroupOps`.
-    pub(crate) fn with_matrix<R>(&self, f: impl FnOnce(&dyn MatrixGroupOps) -> R) -> R {
+    pub fn with_matrix<R>(&self, f: impl FnOnce(&dyn MatrixGroupOps) -> R) -> R {
         match self {
             SrcLayerData::Presence(_, mat) => f(mat),
             SrcLayerData::Count(_, mat) => f(mat),
diff --git a/src/obikphylo/Cargo.toml b/src/obikphylo/Cargo.toml
index df689c2c..ee7a5593 100644
--- a/src/obikphylo/Cargo.toml
+++ b/src/obikphylo/Cargo.toml
@@ -9,6 +9,7 @@ obikseq          = { path = "../obikseq" }
 obiskio          = { path = "../obiskio" }
 obisys           = { path = "../obisys" }
 obicompactvec    = { path = "../obicompactvec" }
+obikidxcache     = { path = "../obikidxcache" }
 obiskbuilder     = { path = "../obiskbuilder" }
 obipipeline      = { path = "../obipipeline" }
 memmap2          = "0.9"
diff --git a/src/obikindex/src/index/distance.rs b/src/obikphylo/src/distance.rs
similarity index 97%
rename from src/obikindex/src/index/distance.rs
rename to src/obikphylo/src/distance.rs
index 957ed055..00e0f3a2 100644
--- a/src/obikindex/src/index/distance.rs
+++ b/src/obikphylo/src/distance.rs
@@ -1,10 +1,10 @@
 use ndarray::Array2;
 use obicompactvec::traits::{BitPartials, CountPartials};
-use crate::layer::LayeredStore;
+use obikidxcache::LayeredStore;
 use rayon::prelude::*;
 
-use crate::index::error::{OKIError, OKIResult};
-use crate::index::kmer_index::KmerIndex;
+use obikindex::{OKIError, OKIResult};
+use obikindex::KmerIndex;
 
 // ── Public API ────────────────────────────────────────────────────────────────
 
diff --git a/src/obikphylo/src/lib.rs b/src/obikphylo/src/lib.rs
index 53a34bab..653c2cb3 100644
--- a/src/obikphylo/src/lib.rs
+++ b/src/obikphylo/src/lib.rs
@@ -9,6 +9,9 @@
 //! incrementally.
 
 mod cardcomp;
+mod distance;
+mod matrix_store;
 pub mod siblings;
 
 pub use cardcomp::{cardinality_transition_probs, composition_transition_probs, pairwise_cost_matrix};
+pub use distance::{DistanceMetric, DistanceOutput};
diff --git a/src/obikindex/src/index/matrix_store.rs b/src/obikphylo/src/matrix_store.rs
similarity index 58%
rename from src/obikindex/src/index/matrix_store.rs
rename to src/obikphylo/src/matrix_store.rs
index 0d92a5c1..74ddf02f 100644
--- a/src/obikindex/src/index/matrix_store.rs
+++ b/src/obikphylo/src/matrix_store.rs
@@ -1,46 +1,47 @@
+use obikindex::OKIResult;
+use obikindex::layer::open_data;
+use obikidxcache::LayeredStore;
 use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix};
-use crate::layer::{LayeredStore, open_data};
-use obiskio::SKResult;
 
-use crate::index::common::{load_meta, olm_to_sk};
-use crate::index::kmer_index::KmerIndex;
+use obikindex::load_meta;
+use obikindex::KmerIndex;
 
 impl KmerIndex {
     /// Open all count matrices for partition `part`, one per layer.
     /// Layers without a `counts/` directory are skipped.
-    pub fn count_store(&self, part: usize) -> SKResult<LayeredStore<PersistentCompactIntMatrix>> {
+    pub fn count_store(&self, part: usize) -> OKIResult<LayeredStore<PersistentCompactIntMatrix>> {
         let index_dir = self.index_dir(part);
         if !index_dir.exists() {
             return Ok(LayeredStore::new(vec![]));
         }
-        let n_layers = load_meta(&index_dir, "distance")?.n_layers;
+        let n_layers = self.n_layers();
         let matrices = (0..n_layers)
             .filter_map(|l| {
                 self.layer_dir(part, l)
                     .join("counts")
                     .exists()
-                    .then(|| open_data(&index_dir, l).map_err(|e| olm_to_sk(e, "distance")))
+                    .then(|| open_data(&index_dir, l))
             })
-            .collect::<SKResult<Vec<_>>>()?;
+            .collect::<OKIResult<Vec<_>>>()?;
         Ok(LayeredStore::new(matrices))
     }
 
     /// Open all presence matrices for partition `part`, one per layer.
     /// Layers without a `presence/` directory are skipped.
-    pub fn presence_store(&self, part: usize) -> SKResult<LayeredStore<PersistentBitMatrix>> {
+    pub fn presence_store(&self, part: usize) -> OKIResult<LayeredStore<PersistentBitMatrix>> {
         let index_dir = self.index_dir(part);
         if !index_dir.exists() {
             return Ok(LayeredStore::new(vec![]));
         }
-        let n_layers = load_meta(&index_dir, "distance")?.n_layers;
+        let n_layers = load_meta(&index_dir)?.n_layers;
         let matrices = (0..n_layers)
             .filter_map(|l| {
                 self.layer_dir(part, l)
                     .join("presence")
                     .exists()
-                    .then(|| open_data(&index_dir, l).map_err(|e| olm_to_sk(e, "distance")))
+                    .then(|| open_data(&index_dir, l))
             })
-            .collect::<SKResult<Vec<_>>>()?;
+            .collect::<OKIResult<Vec<_>>>()?;
         Ok(LayeredStore::new(matrices))
     }
 }
diff --git a/src/obikphylo/src/siblings/build.rs b/src/obikphylo/src/siblings/build.rs
index 090ea03b..4ed4a703 100644
--- a/src/obikphylo/src/siblings/build.rs
+++ b/src/obikphylo/src/siblings/build.rs
@@ -14,7 +14,7 @@ use obikindex::OKIResult;
 
 use super::cache::PartitionCache;
 use super::helpers::{central_base, is_minorant};
-use super::{ANNEX_FILE_NAME, FamilyMask, SiblingAnnexBuilder, olm_to_ok};
+use super::{ANNEX_FILE_NAME, FamilyMask, SiblingAnnexBuilder};
 
 // ── obipipeline data types ─────────────────────────────────────────────────
 
@@ -125,7 +125,7 @@ fn build_layer_sibling_annex(
     l: usize,
     cache: &Arc<PartitionCache>,
 ) -> OKIResult<u64> {
-    let mphf = MphfLayer::open(layer_dir).map_err(olm_to_ok)?;
+    let mphf = MphfLayer::open(layer_dir)?;
     let k = index.kmer_size();
     let n = mphf.n();
 
diff --git a/src/obikphylo/src/siblings/family_scan.rs b/src/obikphylo/src/siblings/family_scan.rs
index 572ade99..59d4d368 100644
--- a/src/obikphylo/src/siblings/family_scan.rs
+++ b/src/obikphylo/src/siblings/family_scan.rs
@@ -65,7 +65,7 @@ use obikindex::layer::KmerLayer;
 use super::cache::PartitionCache;
 use super::helpers::central_base;
 use super::iter::{SiblingEntry, SiblingLayerExt};
-use super::{ANNEX_FILE_NAME, FamilyMask, SiblingAnnex, olm_to_ok};
+use super::{ANNEX_FILE_NAME, FamilyMask, SiblingAnnex};
 
 /// Families per batch — see the module docs for the memory-vs-per-partition-
 /// density trade-off this picks a point on. At ~90 genomes and a few
@@ -194,10 +194,10 @@ pub(super) fn scan_layer_families(
     let index_dir = layer_dir
         .parent()
         .expect("layer_dir has a parent index dir");
-    let meta = PartitionMeta::load(index_dir).map_err(olm_to_ok)?;
+    let meta = PartitionMeta::load(index_dir)?;
     let annex = Arc::new(SiblingAnnex::open(&layer_dir.join(ANNEX_FILE_NAME))?);
 
-    let mat = KmerLayer::open(layer_dir, &meta.mode, with_counts).map_err(olm_to_ok)?;
+    let mat = KmerLayer::open(layer_dir, &meta.mode, with_counts)?;
     let n_cols = mat.n_cols().min(n_genomes);
 
     let ctx = Arc::new(LayerCtx {
diff --git a/src/obikphylo/src/siblings/mod.rs b/src/obikphylo/src/siblings/mod.rs
index 6556bedd..bbd03b59 100644
--- a/src/obikphylo/src/siblings/mod.rs
+++ b/src/obikphylo/src/siblings/mod.rs
@@ -71,15 +71,4 @@ pub(crate) use siblingannex::{FamilyMask, SiblingAnnex, SiblingAnnexBuilder};
 pub use stats::{SiblingAnnexStats, SiblingStatsExt};
 pub use subsample::EntropyBias;
 
-use obikindex::layer::OLMError;
-
-use obikindex::OKIError;
-
 pub(super) const ANNEX_FILE_NAME: &str = "siblings.psib";
-
-pub(super) fn olm_to_ok(e: OLMError) -> OKIError {
-    match e {
-        OLMError::Io(e) => OKIError::Io(e),
-        other => OKIError::InvalidInput(format!("layered-map error: {other}")),
-    }
-}
diff --git a/src/obikquery/Cargo.toml b/src/obikquery/Cargo.toml
new file mode 100644
index 00000000..df532dc6
--- /dev/null
+++ b/src/obikquery/Cargo.toml
@@ -0,0 +1,7 @@
+[package]
+name = "obikquery"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+obikindex = { path = "../obikindex" }
diff --git a/src/obikquery/src/lib.rs b/src/obikquery/src/lib.rs
new file mode 100644
index 00000000..4201bd0a
--- /dev/null
+++ b/src/obikquery/src/lib.rs
@@ -0,0 +1,10 @@
+//! Query-side operations on an `obikindex::KmerIndex`: staging ground for
+//! code currently living in `obikindex::index`'s query path, to be migrated
+//! here to lighten that crate — see `DevDocMD/` for the rationale. Kept as
+//! a separate crate — not a module of `obikindex` — so the dependency runs
+//! one way only (query code depends on the data model, never the reverse),
+//! same pattern as `obikindexer` for the build side.
+
+mod query_layer;
+
+pub use query_layer::{KmerDesc, QueryHit, QueryStats};
diff --git a/src/obikindex/src/index/query_layer.rs b/src/obikquery/src/query_layer.rs
similarity index 93%
rename from src/obikindex/src/index/query_layer.rs
rename to src/obikquery/src/query_layer.rs
index 7cfd3084..2684408e 100644
--- a/src/obikindex/src/index/query_layer.rs
+++ b/src/obikquery/src/query_layer.rs
@@ -3,20 +3,10 @@ use std::path::Path;
 
 use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix};
 use obikseq::CanonicalKmer;
-use crate::layer::{MphfLayer, OLMError};
-use obiskio::{SKError, SKResult};
+use obikindex::layer::MphfLayer;
+use obikindex::{OKIError, OKIResult};
 
-use crate::index::kmer_index::KmerIndex;
-
-fn olm_to_sk(e: OLMError) -> SKError {
-    match e {
-        OLMError::Io(io_err) => SKError::Io(io_err),
-        other => SKError::InvalidData {
-            context: "query",
-            detail: other.to_string(),
-        },
-    }
-}
+use obikindex::KmerIndex;
 
 // ── per-layer query handle ────────────────────────────────────────────────────
 
@@ -26,21 +16,21 @@ enum QueryLayer {
 }
 
 impl QueryLayer {
-    fn open(layer_dir: &Path, with_counts: bool) -> SKResult<Self> {
-        let mphf = MphfLayer::open(layer_dir).map_err(olm_to_sk)?;
+    fn open(layer_dir: &Path, with_counts: bool) -> OKIResult<Self> {
+        let mphf = MphfLayer::open(layer_dir)?;
         let counts_dir = layer_dir.join("counts");
         let presence_dir = layer_dir.join("presence");
 
         if with_counts && counts_dir.exists() {
-            let mat = PersistentCompactIntMatrix::open(layer_dir).map_err(SKError::Io)?;
+            let mat = PersistentCompactIntMatrix::open(layer_dir).map_err(OKIError::Io)?;
             Ok(QueryLayer::Count(mphf, mat))
         } else if presence_dir.exists() || !counts_dir.exists() {
             // presence mode, or no matrix at all → Implicit handled inside open()
-            let mat = PersistentBitMatrix::open(layer_dir).map_err(SKError::Io)?;
+            let mat = PersistentBitMatrix::open(layer_dir).map_err(OKIError::Io)?;
             Ok(QueryLayer::Presence(mphf, mat))
         } else {
             // counts exist but not presence — count layer, no presence requested
-            let mat = PersistentCompactIntMatrix::open(layer_dir).map_err(SKError::Io)?;
+            let mat = PersistentCompactIntMatrix::open(layer_dir).map_err(OKIError::Io)?;
             Ok(QueryLayer::Count(mphf, mat))
         }
     }
@@ -168,7 +158,7 @@ impl KmerIndex {
         n_genomes: usize,
         with_counts: bool,
         mut on_event: F,
-    ) -> SKResult<QueryStats>
+    ) -> OKIResult<QueryStats>
     where
         F: FnMut(QueryHit),
     {
@@ -186,7 +176,7 @@ impl KmerIndex {
         let meta = self.partition_meta(part_idx)?;
         let layers: Vec<QueryLayer> = (0..meta.n_layers)
             .map(|i| QueryLayer::open(&self.layer_dir(part_idx, i), with_counts))
-            .collect::<SKResult<_>>()?;
+            .collect::<OKIResult<_>>()?;
 
         // ── Stage 1: MPHF-only pass, bucket hits by (layer_idx, slot) ────────
         let mut by_layer: Vec<HashMap<usize, &Vec<KmerDesc>>> =
diff --git a/src/obikindex/src/index/tests/query_layer.rs b/src/obikquery/src/tests/query_layer.rs
similarity index 95%
rename from src/obikindex/src/index/tests/query_layer.rs
rename to src/obikquery/src/tests/query_layer.rs
index 702da3af..5dca0266 100644
--- a/src/obikindex/src/index/tests/query_layer.rs
+++ b/src/obikquery/src/tests/query_layer.rs
@@ -1,5 +1,5 @@
 use super::*;
-use crate::index::meta::IndexConfig;
+use obikindex::IndexConfig;
 
 // ── QueryStats::AddAssign ───────────────────────────────────────────────────
 
@@ -50,7 +50,7 @@ fn query_partition_with_missing_index_dir_returns_default_stats() {
         minimizer_size: 9,
         n_bits: 2,
         with_counts: false,
-        evidence: crate::layer::IndexMode::Exact,
+        evidence: obikindex::layer::IndexMode::Exact,
         block_bits: 0,
     };
     let index = KmerIndex::create(tmp.path().join("idx"), config, None).expect("create index");
@@ -78,7 +78,7 @@ fn query_partition_with_empty_kmers_is_a_noop() {
         minimizer_size: 9,
         n_bits: 2,
         with_counts: false,
-        evidence: crate::layer::IndexMode::Exact,
+        evidence: obikindex::layer::IndexMode::Exact,
         block_bits: 0,
     };
     let index = KmerIndex::create(tmp.path().join("idx"), config, None).expect("create index");
diff --git a/src/obikrebuild/Cargo.toml b/src/obikrebuild/Cargo.toml
new file mode 100644
index 00000000..50c39c6c
--- /dev/null
+++ b/src/obikrebuild/Cargo.toml
@@ -0,0 +1,16 @@
+[package]
+name = "obikrebuild"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+obikindex   = { path = "../obikindex" }
+obikindexer = { path = "../obikindexer" }
+obikfilter  = { path = "../obikfilter" }
+obikmerge   = { path = "../obikmerge" }
+obicompactvec = { path = "../obicompactvec" }
+obiskio     = { path = "../obiskio" }
+obikseq     = { path = "../obikseq" }
+obidebruinj = { path = "../obidebruinj" }
+obisys      = { path = "../obisys" }
+tracing     = "0.1.44"
diff --git a/src/obikrebuild/src/lib.rs b/src/obikrebuild/src/lib.rs
new file mode 100644
index 00000000..71c8a41f
--- /dev/null
+++ b/src/obikrebuild/src/lib.rs
@@ -0,0 +1,12 @@
+//! Rebuilding an `obikindex::KmerIndex`: compacting a source index into a
+//! new single-layer index while applying k-mer filters (`rebuild`), and
+//! reindexing an existing index's evidence representation in place
+//! (`reindex`, `Exact`/`Approx`/`Hybrid`). Both are maintenance/
+//! transformation operations on an already-built index, not part of the
+//! `Index { Partition { Layer } }` data model itself — kept out of
+//! `obikindex` so that crate doesn't grow every algorithm's own
+//! dependencies (`obikfilter`, `obikmerge`, `obikindexer`).
+
+mod rebuild;
+mod rebuild_layer;
+mod reindex;
diff --git a/src/obikindex/src/index/rebuild.rs b/src/obikrebuild/src/rebuild.rs
similarity index 90%
rename from src/obikindex/src/index/rebuild.rs
rename to src/obikrebuild/src/rebuild.rs
index 63240725..67746572 100644
--- a/src/obikindex/src/index/rebuild.rs
+++ b/src/obikrebuild/src/rebuild.rs
@@ -1,13 +1,15 @@
 use std::path::Path;
 
-use crate::index::builder::IndexBuilder;
-use crate::index::{KmerFilter, MergeMode};
+use obikindex::IndexBuilder;
+use obikfilter::KmerFilter;
+use obikmerge::MergeMode;
 use obisys::{Reporter, Stage, progress_bar};
 use tracing::info;
 
-use crate::index::error::{OKIError, OKIResult};
-use crate::index::kmer_index::KmerIndex;
-use crate::index::state::IndexState;
+use obikindex::{OKIError, OKIResult};
+use obikindex::KmerIndex;
+use obikindex::IndexState;
+use obisys::PartitionRunner;
 
 impl KmerIndex {
     /// Rebuild `src` into a new compact single-layer index at `output`.
@@ -61,7 +63,7 @@ impl KmerIndex {
         let pb = progress_bar("rebuild", n_partitions as u64, "partitions");
 
         let order: Vec<usize> = (0..n_partitions).collect();
-        let runner = crate::index::numa::PartitionRunner::new();
+        let runner = PartitionRunner::new();
         runner.run(
             &order,
             |i| dst_partition.rebuild_partition(src, i, filters, mode, n_genomes, block_bits),
diff --git a/src/obikindex/src/index/rebuild_layer.rs b/src/obikrebuild/src/rebuild_layer.rs
similarity index 86%
rename from src/obikindex/src/index/rebuild_layer.rs
rename to src/obikrebuild/src/rebuild_layer.rs
index 530df658..a3e61d7b 100644
--- a/src/obikindex/src/index/rebuild_layer.rs
+++ b/src/obikrebuild/src/rebuild_layer.rs
@@ -6,15 +6,17 @@ use obicompactvec::{
 };
 use obidebruinj::GraphDeBruijn;
 use obikseq::CanonicalKmer;
-use crate::layer::meta::PartitionMeta;
-use crate::layer::{IndexMode, MphfLayer, layer_dir};
-use obiskio::{SKError, SKResult, UnitigFileReader};
+use obikindex::layer::meta::PartitionMeta;
+use obikindex::layer::{IndexMode, MphfLayer};
+use obikindex::layer::utils::layer_dir;
+use obiskio::UnitigFileReader;
+use obikindex::{OKIError, OKIResult};
 
-use crate::index::common::{load_meta, olm_to_sk};
-use crate::index::filter::KmerFilter;
-use crate::index::graph_pipeline::materialize_layer;
-use crate::index::merge_layer::{MergeMode, SrcLayerData};
-use crate::index::kmer_index::KmerIndex;
+use obikindex::load_meta;
+use obikfilter::KmerFilter;
+use obikindexer::materialize_layer;
+use obikmerge::{MergeMode, SrcLayerData};
+use obikindex::KmerIndex;
 
 // ── Builders — pair matrix builder + column builders for one mode ─────────────
 
@@ -27,22 +29,22 @@ enum Builders {
 }
 
 impl Builders {
-    fn new(mode: MergeMode, n: usize, dir: &Path, n_genomes: usize) -> SKResult<Self> {
+    fn new(mode: MergeMode, n: usize, dir: &Path, n_genomes: usize) -> OKIResult<Self> {
         match mode {
             MergeMode::Presence => {
-                let mut mat = PersistentBitMatrixBuilder::new(n, dir).map_err(SKError::Io)?;
+                let mut mat = PersistentBitMatrixBuilder::new(n, dir).map_err(OKIError::Io)?;
                 let mut cols = Vec::with_capacity(n_genomes);
                 for _ in 0..n_genomes {
-                    cols.push(mat.add_col().map_err(SKError::Io)?);
+                    cols.push(mat.add_col().map_err(OKIError::Io)?);
                 }
                 Ok(Builders::Presence(mat, cols))
             }
             MergeMode::Count => {
                 let mut mat =
-                    PersistentCompactIntMatrixBuilder::new(n, dir).map_err(SKError::Io)?;
+                    PersistentCompactIntMatrixBuilder::new(n, dir).map_err(OKIError::Io)?;
                 let mut cols = Vec::with_capacity(n_genomes);
                 for _ in 0..n_genomes {
-                    cols.push(mat.add_col().map_err(SKError::Io)?);
+                    cols.push(mat.add_col().map_err(OKIError::Io)?);
                 }
                 Ok(Builders::Count(mat, cols))
             }
@@ -56,19 +58,19 @@ impl Builders {
         }
     }
 
-    fn close(self) -> SKResult<()> {
+    fn close(self) -> OKIResult<()> {
         match self {
             Builders::Presence(mat, cols) => {
                 for b in cols {
-                    b.close().map_err(SKError::Io)?;
+                    b.close().map_err(OKIError::Io)?;
                 }
-                mat.close().map_err(SKError::Io)
+                mat.close().map_err(OKIError::Io)
             }
             Builders::Count(mat, cols) => {
                 for b in cols {
-                    b.close().map_err(SKError::Io)?;
+                    b.close().map_err(OKIError::Io)?;
                 }
-                mat.close().map_err(SKError::Io)
+                mat.close().map_err(OKIError::Io)
             }
         }
     }
@@ -86,7 +88,7 @@ fn try_compute_combined_mask(
     filters: &[Box<dyn KmerFilter>],
     src_data: &SrcLayerData,
     n_genomes: usize,
-) -> SKResult<Option<obicompactvec::TempBitVec>> {
+) -> OKIResult<Option<obicompactvec::TempBitVec>> {
     if filters.is_empty() {
         return Ok(None);
     }
@@ -101,7 +103,7 @@ fn try_compute_combined_mask(
     let n = src_data.n_slots();
     let mask = src_data
         .with_matrix(|mat| eval_filter_mask(&combined, mat, n))
-        .map_err(SKError::Io)?;
+        .map_err(OKIError::Io)?;
     Ok(Some(mask))
 }
 
@@ -118,8 +120,8 @@ fn iter_src_kmers_masked(
     n_genomes: usize,
     filters: &[Box<dyn KmerFilter>],
     mut cb: impl FnMut(CanonicalKmer),
-) -> SKResult<()> {
-    let src_meta = load_meta(src_index_dir, "rebuild")?;
+) -> OKIResult<()> {
+    let src_meta = load_meta(src_index_dir)?;
     for l in 0..src_meta.n_layers {
         let src_layer_dir = layer_dir(src_index_dir, l);
         let unitigs_path = src_layer_dir.join("unitigs.bin");
@@ -159,8 +161,8 @@ fn iter_src_layers(
     n_genomes: usize,
     filters: &[Box<dyn KmerFilter>],
     mut cb: impl FnMut(CanonicalKmer, Box<[u32]>),
-) -> SKResult<()> {
-    let src_meta = load_meta(src_index_dir, "rebuild")?;
+) -> OKIResult<()> {
+    let src_meta = load_meta(src_index_dir)?;
     for l in 0..src_meta.n_layers {
         let src_layer_dir = layer_dir(src_index_dir, l);
         let unitigs_path = src_layer_dir.join("unitigs.bin");
@@ -209,13 +211,13 @@ impl KmerIndex {
         mode: MergeMode,
         n_genomes: usize,
         block_bits: u8,
-    ) -> SKResult<()> {
+    ) -> OKIResult<()> {
         let src_index_dir = src.index_dir(i);
         if !src_index_dir.exists() {
             return Ok(());
         }
 
-        let src_meta = load_meta(&src_index_dir, "rebuild")?;
+        let src_meta = load_meta(&src_index_dir)?;
         if src_meta.n_layers == 0 {
             return Ok(());
         }
@@ -235,8 +237,7 @@ impl KmerIndex {
         let dst_layer_dir = self.layer_dir(i, 0);
 
         let n_new = materialize_layer(g, &dst_layer_dir, block_bits, &IndexMode::Exact)?;
-        let dst_mphf = MphfLayer::open(&dst_layer_dir)
-            .map_err(|e| olm_to_sk(e, "rebuild"))?;
+        let dst_mphf = MphfLayer::open(&dst_layer_dir)?;
 
         // ── Prepare matrix builders (one column per genome) ───────────────────
         let data_dir = match mode {
@@ -262,8 +263,7 @@ impl KmerIndex {
             n_layers: 1,
             mode: IndexMode::Exact,
         }
-        .save(&dst_index_dir)
-        .map_err(|e| olm_to_sk(e, "rebuild"))?;
+        .save(&dst_index_dir)?;
 
         Ok(())
     }
diff --git a/src/obikindex/src/index/reindex.rs b/src/obikrebuild/src/reindex.rs
similarity index 88%
rename from src/obikindex/src/index/reindex.rs
rename to src/obikrebuild/src/reindex.rs
index 03d084d4..0ede2cf2 100644
--- a/src/obikindex/src/index/reindex.rs
+++ b/src/obikrebuild/src/reindex.rs
@@ -1,24 +1,21 @@
-use crate::index::builder::IndexBuilder;
-use crate::index::meta::IndexMeta;
-use crate::layer::{IndexMode, TypedLayer};
+use obikindex::IndexBuilder;
+use obikindex::IndexMeta;
+use obikindex::layer::{IndexMode, TypedLayer};
 use obisys::{Reporter, Stage, progress_bar};
 use std::fs;
 use std::path::Path;
 use std::sync::Arc;
 use tracing::info;
 
-use crate::index::error::{OKIError, OKIResult};
-use crate::index::kmer_index::KmerIndex;
-use crate::index::state::IndexState;
+use obikindex::{OKIError, OKIResult};
+use obikindex::KmerIndex;
+use obikindex::IndexState;
+use obisys::PartitionRunner;
 
 const EVIDENCE_FILE: &str = "evidence.bin";
 const FINGERPRINT_FILE: &str = "fingerprint.bin";
 const UNITIG_IDX_FILE: &str = "unitigs.bin.idx";
 
-fn olm_to_oki(e: crate::layer::OLMError) -> OKIError {
-    OKIError::InvalidInput(e.to_string())
-}
-
 impl KmerIndex {
     /// Convert every layer's evidence bundle to `target` in-place.
     ///
@@ -47,7 +44,7 @@ impl KmerIndex {
         let pb = progress_bar("reindex", n as u64, "partitions");
 
         let order: Vec<usize> = (0..n).collect();
-        let runner = crate::index::numa::PartitionRunner::new();
+        let runner = PartitionRunner::new();
         runner.run(
             &order,
             |i| {
@@ -96,10 +93,10 @@ fn reindex_partition(
 fn reindex_layer(layer_dir: &Path, target: &IndexMode, block_bits: u8) -> OKIResult<()> {
     match target {
         IndexMode::Exact => {
-            TypedLayer::<()>::build_exact_evidence(layer_dir, block_bits).map_err(olm_to_oki)?;
+            TypedLayer::<()>::build_exact_evidence(layer_dir, block_bits)?;
         }
         IndexMode::Approx { b, z } | IndexMode::Hybrid { b, z } => {
-            TypedLayer::<()>::build_approx_evidence(layer_dir, *b, *z).map_err(olm_to_oki)?;
+            TypedLayer::<()>::build_approx_evidence(layer_dir, *b, *z)?;
         }
     }
     remove_stale_evidence(layer_dir, target)
diff --git a/src/obikselect/Cargo.toml b/src/obikselect/Cargo.toml
new file mode 100644
index 00000000..4efb38ec
--- /dev/null
+++ b/src/obikselect/Cargo.toml
@@ -0,0 +1,10 @@
+[package]
+name = "obikselect"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+obikindex     = { path = "../obikindex" }
+obicompactvec = { path = "../obicompactvec" }
+obisys        = { path = "../obisys" }
+tracing       = "0.1.44"
diff --git a/src/obikselect/src/lib.rs b/src/obikselect/src/lib.rs
new file mode 100644
index 00000000..30b7192b
--- /dev/null
+++ b/src/obikselect/src/lib.rs
@@ -0,0 +1,11 @@
+//! Genome-column selection/aggregation on an `obikindex::KmerIndex`:
+//! projecting/aggregating genome columns (`AggOp`) into new output columns
+//! (`OutputCol`), either into a new index (`select`) or in place
+//! (`select_in_place`). Orthogonal to per-kmer filtering (`obikfilter`),
+//! which operates on rows, not columns. Kept as a separate crate — not a
+//! module of `obikindex` — so the dependency runs one way only.
+
+mod select;
+mod select_layer;
+
+pub use select_layer::{AggOp, OutputCol};
diff --git a/src/obikindex/src/index/select.rs b/src/obikselect/src/select.rs
similarity index 91%
rename from src/obikindex/src/index/select.rs
rename to src/obikselect/src/select.rs
index 01492b9a..ca00620b 100644
--- a/src/obikindex/src/index/select.rs
+++ b/src/obikselect/src/select.rs
@@ -1,15 +1,15 @@
 use std::path::Path;
 use std::sync::Arc;
 
-use crate::index::builder::IndexBuilder;
-use crate::index::OutputCol;
-use obisys::{Reporter, Stage, progress_bar};
+use obikindex::IndexBuilder;
+use crate::OutputCol;
+use obisys::{Reporter, Stage, progress_bar, PartitionRunner};
 use tracing::info;
 
-use crate::index::error::{OKIError, OKIResult};
-use crate::index::kmer_index::KmerIndex;
-use crate::index::meta::{GenomeInfo, IndexMeta};
-use crate::index::state::IndexState;
+use obikindex::{OKIError, OKIResult};
+use obikindex::KmerIndex;
+use obikindex::{GenomeInfo, IndexMeta};
+use obikindex::IndexState;
 
 impl KmerIndex {
     /// Create a new index at `output` by projecting/aggregating the genome columns
@@ -58,7 +58,7 @@ impl KmerIndex {
         let pb = progress_bar("select", n_partitions as u64, "partitions");
 
         let order: Vec<usize> = (0..n_partitions).collect();
-        let runner = crate::index::numa::PartitionRunner::new();
+        let runner = PartitionRunner::new();
         runner
             .run(
                 &order,
@@ -113,7 +113,7 @@ impl KmerIndex {
         let pb = progress_bar("select", n_partitions as u64, "partitions");
 
         let order: Vec<usize> = (0..n_partitions).collect();
-        let runner = crate::index::numa::PartitionRunner::new();
+        let runner = PartitionRunner::new();
         runner
             .run(
                 &order,
diff --git a/src/obikindex/src/index/select_layer.rs b/src/obikselect/src/select_layer.rs
similarity index 85%
rename from src/obikindex/src/index/select_layer.rs
rename to src/obikselect/src/select_layer.rs
index 3087538c..51c768da 100644
--- a/src/obikindex/src/index/select_layer.rs
+++ b/src/obikselect/src/select_layer.rs
@@ -6,10 +6,9 @@ use obicompactvec::{
     ColGroup, MatrixGroupOps, PersistentBitMatrix, PersistentBitMatrixBuilder,
     PersistentCompactIntMatrix, PersistentCompactIntMatrixBuilder,
 };
-use crate::layer::OLMError;
-use obiskio::{SKError, SKResult};
+use obikindex::{OKIError, OKIResult};
 
-use crate::index::kmer_index::KmerIndex;
+use obikindex::KmerIndex;
 
 // ── AggOp ─────────────────────────────────────────────────────────────────────
 
@@ -39,16 +38,6 @@ pub struct OutputCol {
 
 // ── Helpers ───────────────────────────────────────────────────────────────────
 
-fn olm_to_sk(e: OLMError) -> SKError {
-    match e {
-        OLMError::Io(e) => SKError::Io(e),
-        other => SKError::InvalidData {
-            context: "select",
-            detail: other.to_string(),
-        },
-    }
-}
-
 /// Copy all plain files (not subdirectories) from `src_dir` to `dst_dir`.
 fn copy_layer_files(src_dir: &Path, dst_dir: &Path) -> io::Result<()> {
     for entry in fs::read_dir(src_dir)? {
@@ -71,97 +60,97 @@ fn fill_builders(
     output_presence: bool,
     mut dst_bit: Option<&mut PersistentBitMatrixBuilder>,
     mut dst_int: Option<&mut PersistentCompactIntMatrixBuilder>,
-) -> SKResult<()> {
+) -> OKIResult<()> {
     if src_is_count {
-        let mat = PersistentCompactIntMatrix::open(src_layer_dir).map_err(SKError::Io)?;
+        let mat = PersistentCompactIntMatrix::open(src_layer_dir).map_err(OKIError::Io)?;
         for spec in specs {
             let g = ColGroup::new(&spec.label, spec.indices.clone());
             if output_presence {
                 let b = dst_bit.as_deref_mut().unwrap();
                 match spec.op {
                     AggOp::Any => {
-                        b.add_col_from(&mat.partial_group_any(&g, threshold).map_err(SKError::Io)?)
+                        b.add_col_from(&mat.partial_group_any(&g, threshold).map_err(OKIError::Io)?)
                     }
                     AggOp::All => {
-                        b.add_col_from(&mat.partial_group_all(&g, threshold).map_err(SKError::Io)?)
+                        b.add_col_from(&mat.partial_group_all(&g, threshold).map_err(OKIError::Io)?)
                     }
                     AggOp::None => {
-                        b.add_col_from(&mat.partial_group_none(&g, threshold).map_err(SKError::Io)?)
+                        b.add_col_from(&mat.partial_group_none(&g, threshold).map_err(OKIError::Io)?)
                     }
                     AggOp::Sum => {
-                        b.add_col_from_int(&mat.partial_group_sum(&g).map_err(SKError::Io)?)
+                        b.add_col_from_int(&mat.partial_group_sum(&g).map_err(OKIError::Io)?)
                     }
                     AggOp::Min => {
-                        b.add_col_from_int(&mat.partial_group_min(&g).map_err(SKError::Io)?)
+                        b.add_col_from_int(&mat.partial_group_min(&g).map_err(OKIError::Io)?)
                     }
                     AggOp::Max => {
-                        b.add_col_from_int(&mat.partial_group_max(&g).map_err(SKError::Io)?)
+                        b.add_col_from_int(&mat.partial_group_max(&g).map_err(OKIError::Io)?)
                     }
                 }
-                .map_err(SKError::Io)?;
+                .map_err(OKIError::Io)?;
             } else {
                 let b = dst_int.as_deref_mut().unwrap();
                 match spec.op {
-                    AggOp::Sum => b.add_col_from(&mat.partial_group_sum(&g).map_err(SKError::Io)?),
-                    AggOp::Min => b.add_col_from(&mat.partial_group_min(&g).map_err(SKError::Io)?),
-                    AggOp::Max => b.add_col_from(&mat.partial_group_max(&g).map_err(SKError::Io)?),
+                    AggOp::Sum => b.add_col_from(&mat.partial_group_sum(&g).map_err(OKIError::Io)?),
+                    AggOp::Min => b.add_col_from(&mat.partial_group_min(&g).map_err(OKIError::Io)?),
+                    AggOp::Max => b.add_col_from(&mat.partial_group_max(&g).map_err(OKIError::Io)?),
                     AggOp::Any => b.add_col_from_bit(
-                        &mat.partial_group_any(&g, threshold).map_err(SKError::Io)?,
+                        &mat.partial_group_any(&g, threshold).map_err(OKIError::Io)?,
                     ),
                     AggOp::All => b.add_col_from_bit(
-                        &mat.partial_group_all(&g, threshold).map_err(SKError::Io)?,
+                        &mat.partial_group_all(&g, threshold).map_err(OKIError::Io)?,
                     ),
                     AggOp::None => b.add_col_from_bit(
-                        &mat.partial_group_none(&g, threshold).map_err(SKError::Io)?,
+                        &mat.partial_group_none(&g, threshold).map_err(OKIError::Io)?,
                     ),
                 }
-                .map_err(SKError::Io)?;
+                .map_err(OKIError::Io)?;
             }
         }
     } else {
-        let mat = PersistentBitMatrix::open(src_layer_dir).map_err(SKError::Io)?;
+        let mat = PersistentBitMatrix::open(src_layer_dir).map_err(OKIError::Io)?;
         for spec in specs {
             let g = ColGroup::new(&spec.label, spec.indices.clone());
             if output_presence {
                 let b = dst_bit.as_deref_mut().unwrap();
                 match spec.op {
                     AggOp::Any => {
-                        b.add_col_from(&mat.partial_group_any(&g, 1).map_err(SKError::Io)?)
+                        b.add_col_from(&mat.partial_group_any(&g, 1).map_err(OKIError::Io)?)
                     }
                     AggOp::All => {
-                        b.add_col_from(&mat.partial_group_all(&g, 1).map_err(SKError::Io)?)
+                        b.add_col_from(&mat.partial_group_all(&g, 1).map_err(OKIError::Io)?)
                     }
                     AggOp::None => {
-                        b.add_col_from(&mat.partial_group_none(&g, 1).map_err(SKError::Io)?)
+                        b.add_col_from(&mat.partial_group_none(&g, 1).map_err(OKIError::Io)?)
                     }
                     AggOp::Sum => {
-                        b.add_col_from_int(&mat.partial_group_sum(&g).map_err(SKError::Io)?)
+                        b.add_col_from_int(&mat.partial_group_sum(&g).map_err(OKIError::Io)?)
                     }
                     AggOp::Min => {
-                        b.add_col_from_int(&mat.partial_group_min(&g).map_err(SKError::Io)?)
+                        b.add_col_from_int(&mat.partial_group_min(&g).map_err(OKIError::Io)?)
                     }
                     AggOp::Max => {
-                        b.add_col_from_int(&mat.partial_group_max(&g).map_err(SKError::Io)?)
+                        b.add_col_from_int(&mat.partial_group_max(&g).map_err(OKIError::Io)?)
                     }
                 }
-                .map_err(SKError::Io)?;
+                .map_err(OKIError::Io)?;
             } else {
                 let b = dst_int.as_deref_mut().unwrap();
                 match spec.op {
-                    AggOp::Sum => b.add_col_from(&mat.partial_group_sum(&g).map_err(SKError::Io)?),
-                    AggOp::Min => b.add_col_from(&mat.partial_group_min(&g).map_err(SKError::Io)?),
-                    AggOp::Max => b.add_col_from(&mat.partial_group_max(&g).map_err(SKError::Io)?),
+                    AggOp::Sum => b.add_col_from(&mat.partial_group_sum(&g).map_err(OKIError::Io)?),
+                    AggOp::Min => b.add_col_from(&mat.partial_group_min(&g).map_err(OKIError::Io)?),
+                    AggOp::Max => b.add_col_from(&mat.partial_group_max(&g).map_err(OKIError::Io)?),
                     AggOp::Any => {
-                        b.add_col_from_bit(&mat.partial_group_any(&g, 1).map_err(SKError::Io)?)
+                        b.add_col_from_bit(&mat.partial_group_any(&g, 1).map_err(OKIError::Io)?)
                     }
                     AggOp::All => {
-                        b.add_col_from_bit(&mat.partial_group_all(&g, 1).map_err(SKError::Io)?)
+                        b.add_col_from_bit(&mat.partial_group_all(&g, 1).map_err(OKIError::Io)?)
                     }
                     AggOp::None => {
-                        b.add_col_from_bit(&mat.partial_group_none(&g, 1).map_err(SKError::Io)?)
+                        b.add_col_from_bit(&mat.partial_group_none(&g, 1).map_err(OKIError::Io)?)
                     }
                 }
-                .map_err(SKError::Io)?;
+                .map_err(OKIError::Io)?;
             }
         }
     }
@@ -185,7 +174,7 @@ impl KmerIndex {
         threshold: u32,
         output_presence: bool,
         in_place: bool,
-    ) -> SKResult<()> {
+    ) -> OKIResult<()> {
         let src_index_dir = src.index_dir(i);
         if !src_index_dir.exists() {
             return Ok(());
@@ -222,11 +211,11 @@ impl KmerIndex {
             // Determine number of slots and detect implicit layers.
             let n = if counts_dir.exists() {
                 PersistentCompactIntMatrix::open(&src_layer_dir)
-                    .map_err(SKError::Io)?
+                    .map_err(OKIError::Io)?
                     .n()
             } else if presence_dir.exists() {
                 PersistentBitMatrix::open(&src_layer_dir)
-                    .map_err(SKError::Io)?
+                    .map_err(OKIError::Io)?
                     .n()
             } else {
                 // Implicit single-genome layer: no data matrix needed in output either.
@@ -255,7 +244,7 @@ impl KmerIndex {
 
             let (mut dst_bit, mut dst_int) = if output_presence {
                 (
-                    Some(PersistentBitMatrixBuilder::new(n, &dst_data_dir).map_err(SKError::Io)?),
+                    Some(PersistentBitMatrixBuilder::new(n, &dst_data_dir).map_err(OKIError::Io)?),
                     None,
                 )
             } else {
@@ -263,7 +252,7 @@ impl KmerIndex {
                     None,
                     Some(
                         PersistentCompactIntMatrixBuilder::new(n, &dst_data_dir)
-                            .map_err(SKError::Io)?,
+                            .map_err(OKIError::Io)?,
                     ),
                 )
             };
@@ -279,9 +268,9 @@ impl KmerIndex {
             )?;
 
             if output_presence {
-                dst_bit.unwrap().close().map_err(SKError::Io)?;
+                dst_bit.unwrap().close().map_err(OKIError::Io)?;
             } else {
-                dst_int.unwrap().close().map_err(SKError::Io)?;
+                dst_int.unwrap().close().map_err(OKIError::Io)?;
             }
 
             // In-place: swap old data dir for new.
@@ -300,8 +289,7 @@ impl KmerIndex {
 
         if !in_place {
             src.partition_meta(i)?
-                .save(&dst_index_dir)
-                .map_err(olm_to_sk)?;
+                .save(&dst_index_dir)?;
         }
 
         Ok(())
diff --git a/src/obikstats/Cargo.toml b/src/obikstats/Cargo.toml
new file mode 100644
index 00000000..d302450a
--- /dev/null
+++ b/src/obikstats/Cargo.toml
@@ -0,0 +1,9 @@
+[package]
+name = "obikstats"
+version = "0.1.0"
+edition = "2024"
+
+[dependencies]
+obikindex     = { path = "../obikindex" }
+obicompactvec = { path = "../obicompactvec" }
+rayon         = "1"
diff --git a/src/obikstats/src/lib.rs b/src/obikstats/src/lib.rs
new file mode 100644
index 00000000..66baa672
--- /dev/null
+++ b/src/obikstats/src/lib.rs
@@ -0,0 +1,10 @@
+//! Read-only aggregation/statistics over an already-built
+//! `obikindex::KmerIndex`: bits-per-kmer breakdown by index component
+//! (`bits_per_kmer`), per-genome k-mer counts (`genome_kmer_counts`).
+//! Not part of the `Index { Partition { Layer } }` data model itself —
+//! kept out of `obikindex` like every other read/write extension
+//! (`obikquery`, `obikdump`, `obikselect`, `obikrebuild`, `obikmerge`).
+
+mod stats;
+
+pub use stats::IndexBitsPerKmer;
diff --git a/src/obikindex/src/index/stats.rs b/src/obikstats/src/stats.rs
similarity index 93%
rename from src/obikindex/src/index/stats.rs
rename to src/obikstats/src/stats.rs
index c506f213..d32fb25e 100644
--- a/src/obikindex/src/index/stats.rs
+++ b/src/obikstats/src/stats.rs
@@ -5,8 +5,8 @@ use obicompactvec::{LayerMeta, PersistentBitMatrix, PersistentCompactIntMatrix};
 use obicompactvec::traits::ColumnWeights;
 use rayon::prelude::*;
 
-use crate::index::error::{OKIError, OKIResult};
-use crate::index::kmer_index::KmerIndex;
+use obikindex::{OKIError, OKIResult};
+use obikindex::KmerIndex;
 
 /// Bits per kmer broken down by index component.
 pub struct IndexBitsPerKmer {
@@ -95,7 +95,8 @@ impl KmerIndex {
                 let n_layers = self.n_layers(i).unwrap_or(0);
 
                 (0..n_layers).fold((0usize, 0u64, 0u64, 0u64), |acc, l| {
-                    let lb = layer_bytes(&self.layer_dir(i, l));
+                    let Ok(layer_dir) = self.layer_dir(i, l) else { return acc };
+                    let lb = layer_bytes(&layer_dir);
                     (acc.0 + lb.n_kmers, acc.1 + lb.mphf, acc.2 + lb.evidence, acc.3 + lb.matrix)
                 })
             })
@@ -144,7 +145,7 @@ impl KmerIndex {
                 let n_layers = self.n_layers(i).unwrap_or(0);
 
                 for l in 0..n_layers {
-                    let this_layer_dir = self.layer_dir(i, l);
+                    let Ok(this_layer_dir) = self.layer_dir(i, l) else { continue };
                     if !this_layer_dir.exists() { continue; }
 
                     n_kmers += LayerMeta::load(&this_layer_dir).map(|m| m.n).unwrap_or(0);
@@ -153,12 +154,12 @@ impl KmerIndex {
                         if this_layer_dir.join("counts").exists()
                             && !this_layer_dir.join("presence").exists()
                         {
-                            match crate::layer::open_data::<PersistentCompactIntMatrix>(&index_dir, l) {
+                            match obikindex::layer::open_data::<PersistentCompactIntMatrix>(&index_dir, l) {
                                 Ok(m)  => Box::new(m),
                                 Err(_) => continue,
                             }
                         } else {
-                            match crate::layer::open_data::<PersistentBitMatrix>(&index_dir, l) {
+                            match obikindex::layer::open_data::<PersistentBitMatrix>(&index_dir, l) {
                                 Ok(m)  => Box::new(m),
                                 Err(_) => continue,
                             }
diff --git a/src/obisys/Cargo.toml b/src/obisys/Cargo.toml
index 8c41ad77..8952e948 100644
--- a/src/obisys/Cargo.toml
+++ b/src/obisys/Cargo.toml
@@ -4,7 +4,14 @@ version = "0.1.0"
 edition = "2024"
 
 [dependencies]
-libc      = "0.2"
-sysinfo   = "0.39"
-indicatif = "0.18"
-tracing   = "0.1"
+libc               = "0.2"
+sysinfo            = "0.39"
+indicatif          = "0.18"
+tracing            = "0.1"
+crossbeam-channel  = "0.5"
+rayon              = "1"
+hwlocality         = { version = "1.0.0-alpha.11", features = ["vendored"], optional = true }
+
+[features]
+default = ["numa"]
+numa    = ["hwlocality"]
diff --git a/src/obisys/src/lib.rs b/src/obisys/src/lib.rs
index e387cba5..a37cceff 100644
--- a/src/obisys/src/lib.rs
+++ b/src/obisys/src/lib.rs
@@ -3,12 +3,14 @@
 
 mod budget;
 mod lock;
+mod numa;
 mod progress;
 mod resources;
 mod stage;
 
 pub use budget::MemoryBudget;
 pub use lock::DirLock;
+pub use numa::PartitionRunner;
 pub use progress::{Progress, TracedBar, progress_bar, spinner};
 pub use resources::{CpuSample, IoSample, available_memory_bytes, effective_parallelism, peak_rss_bytes};
 pub use stage::{Reporter, Stage, StageStats};
diff --git a/src/obikindex/src/index/numa/mod.rs b/src/obisys/src/numa/mod.rs
similarity index 100%
rename from src/obikindex/src/index/numa/mod.rs
rename to src/obisys/src/numa/mod.rs
diff --git a/src/obikindex/src/index/numa/runner.rs b/src/obisys/src/numa/runner.rs
similarity index 99%
rename from src/obikindex/src/index/numa/runner.rs
rename to src/obisys/src/numa/runner.rs
index 483fe675..ffe2db7e 100644
--- a/src/obikindex/src/index/numa/runner.rs
+++ b/src/obisys/src/numa/runner.rs
@@ -2,7 +2,7 @@ use std::sync::Arc;
 use std::time::{Duration, Instant};
 
 use crossbeam_channel::unbounded;
-use obisys::{CpuSample, IoSample};
+use crate::{CpuSample, IoSample};
 use tracing::debug;
 
 use super::topology::{build, pin_current_thread};
diff --git a/src/obikindex/src/index/numa/topology.rs b/src/obisys/src/numa/topology.rs
similarity index 97%
rename from src/obikindex/src/index/numa/topology.rs
rename to src/obisys/src/numa/topology.rs
index 31c266cf..0263e90c 100644
--- a/src/obikindex/src/index/numa/topology.rs
+++ b/src/obisys/src/numa/topology.rs
@@ -66,7 +66,7 @@ pub fn build() -> NumaSetup {
     }
 
     // UMA fallback: single synthetic node, all cores, no pool, no pinning.
-    let n_cores = obisys::effective_parallelism();
+    let n_cores = crate::effective_parallelism();
     debug!("UMA: single synthetic node, {} core(s)", n_cores);
     NumaSetup {
         pools: vec![None],
@@ -76,7 +76,7 @@ pub fn build() -> NumaSetup {
 
 #[cfg(not(feature = "numa"))]
 pub fn build() -> NumaSetup {
-    let n_cores = obisys::effective_parallelism();
+    let n_cores = crate::effective_parallelism();
     debug!("UMA: single synthetic node, {} core(s)", n_cores);
     NumaSetup {
         pools: vec![None],