diff --git a/DevDocMD/implementation/partition_layer_cache.md b/DevDocMD/implementation/partition_layer_cache.md
index f3208fcd..e154be9b 100644
--- a/DevDocMD/implementation/partition_layer_cache.md
+++ b/DevDocMD/implementation/partition_layer_cache.md
@@ -1,7 +1,9 @@
 # Partition and layer caching (discussion)
 
-Status: problem confirmed, design direction agreed (2026-08-20). No
-implementation started. Ownership split (below) still open.
+Status: problem confirmed, design direction agreed (2026-08-20). (1)/(2)
+themselves not started; ownership split (below) still open. Preparatory
+encapsulation work (partition/layer path and metadata accessors on
+`KmerPartition`) landed the same day — see "Preparatory work done" below.
 
 ## The problem
 
@@ -127,3 +129,116 @@ deleted outright or kept as a thin sibling-specific wrapper, and whether (2)
 needs an eviction policy or can simply hold every partition open for the
 process lifetime (revisit once the VM-mapping-count question above has a
 real number behind it for this codebase's scale).
+
+## Preparatory work done (2026-08-20)
+
+Groundwork for (1)/(2), landed ahead of the design itself:
+
+- `KmerPartition` (`obikpartitionner`) gained `partition_dir`/`index_dir`/
+  `layer_dir` as the single source of truth for a partition's on-disk
+  layout, replacing per-module duplicated `const INDEX_SUBDIR: &str =
+  "index"` (7 copies) and ad hoc path joins — including one found
+  duplicated *inside `KmerPartition` itself* (`ensure_writer` rebuilt
+  `part_dir`'s own logic by hand).
+- `KmerPartition` gained `partition_meta`/`n_layers`/`index_mode`, wrapping
+  `obilayeredmap::meta::PartitionMeta::load` (via the existing
+  `common::load_meta`, which also recovers indexes built before
+  `meta.json` existed). Before this, `obikphylo` and `obikindex` imported
+  `obilayeredmap::meta::PartitionMeta` directly and called `::load()`
+  themselves at 21 call sites, each redoing its own error-mapping —
+  every one of those crates knew the on-disk metadata format instead of
+  going through an interface. Fixed everywhere except one remaining spot
+  (below). Caught as a side effect: `dump_layer.rs`/`query_layer.rs` had
+  been calling `PartitionMeta::load` directly, bypassing `load_meta`
+  entirely — they never got the missing-`meta.json` recovery the other
+  callers did.
+- Layer introspection API discussed but **not yet implemented** — three
+  axes, deliberately kept separate after an initial draft conflated them:
+  - `LayerContent { Count, Presence }` — what the layer stores; a `const`
+    on `LayerData` (compile-time, zero-cost), not a runtime field.
+  - `StorageKind { Implicit, Columnar, Packed, Sparse }` — how it's
+    stored; only meaningful for `D` that actually carry data (`Layer<()>`
+    has neither this nor `LayerContent` — it's a write-time-only state,
+    never a queryable content: once a layer is closed, "no matrix file"
+    reads back as `Presence`/`Implicit` via `PersistentBitMatrix::open`'s
+    own fallback, not as some third "empty" content).
+  - `EvidenceKind { Exact, Approx, Hybrid }` — from `MphfLayer`'s own
+    already-in-memory `LayerEvidence` discriminant.
+  - Not all `(LayerContent, StorageKind)` pairs are legal: `Count` never
+    has `Implicit` or `Sparse`.
+
+**Implemented (2026-08-20).** `LayerContent`/`StorageKind`/`EvidenceKind`
+now exist, each with two forms:
+- A runtime accessor on an already-open value (`Layer<D>::content()`/
+  `storage_kind()`/`evidence_kind()`, `PersistentBitMatrix::storage_kind()`,
+  `PersistentCompactIntMatrix::storage_kind()`, `MphfLayer::evidence_kind()`)
+  — reads a discriminant already in memory, zero disk access.
+- A lightweight `detect()`/`detect_storage()` disk probe that mirrors the
+  corresponding `open()`'s own priority order by hand (file-existence
+  checks only, no mmap) — usable *before* committing to a `D`, unlike the
+  runtime accessors. Exposed per-layer on `LayeredMap<D>` as
+  `detect_layer_content`/`detect_layer_storage`/`detect_layer_evidence`
+  (work regardless of `D`, since they only use `self.root` + the layer
+  index).
+
+`StorageKind` lives in `obicompactvec` (owner of `PersistentBitMatrix`/
+`PersistentCompactIntMatrix`); `LayerContent`/`EvidenceKind` live in
+`obilayeredmap`. `HasLayerContent`/`HasStorageKind` gate `Layer<()>` out of
+`content()`/`storage_kind()` (no matrix, nothing to report), matching the
+"empty is transitional" conclusion above. 42 new tests across
+`obilayeredmap`'s `tests/layer.rs` and `tests/map.rs`; full workspace
+suite green (0 failed) after.
+
+Not done: these `detect()` probes don't yet replace `Mat::open`'s or
+`QueryLayer::open`'s own hand-rolled equivalents (still duplicated content/
+storage decisions, now a *third* copy of the same logic to keep in sync)
+— that consolidation is (1)/(2)'s job, not this prep step's.
+
+## One bug found while reading around this (signalled, not fixed); one earlier claim retracted
+
+- `obicompactvec::bitmatrix::sparse.rs`'s module doc says
+  "Not used by any production code path yet" — false since `obikmer pack
+  --sparse` (`cmd/pack/mod.rs`) is wired to `pack_sparse_bit_matrix` and
+  `Mat::open` already reads the result back in the sibling-annex path.
+  Stale comment, not corrected.
+- **Retracted (2026-08-20)**: an earlier pass through this doc claimed
+  `obikpartitionner::query_layer::QueryLayer::open` had no sparse-format
+  detection and would silently corrupt reads on a `pack --sparse`d layer.
+  False — `PersistentBitMatrix` (`obicompactvec::bitmatrix::persistent`)
+  is a 4-way enum (`Columnar`/`Packed`/`Sparse`/`Implicit`), not 3-way as
+  first read; its `open()` already detects `Sparse` via
+  `presence/sparse_meta.json`, and every method on the type (`row`,
+  `fill_row`, `nonzero_iter`, …) already dispatches all 4 arms.
+  `QueryLayer::open`'s `PersistentBitMatrix::open(layer_dir)` call was
+  never the bug. Root cause of the false claim: a `grep -n
+  "Implicit\|Columnar\|Packed"` used to read the enum definition silently
+  skipped the `Sparse(...)` line because it matched none of those three
+  words — a self-inflicted blind spot from a filtered read, not a fact
+  about the code. Lesson: for a `pub enum` whose variant list matters,
+  read the definition unfiltered, don't grep for the variant names you
+  expect to find.
+- One real consequence of that same correction: `obikphylo::siblings::
+  cache::Mat::SparsePresence(Layer<PersistentSparseBitMatrix>)` looks
+  redundant now — `Mat::Presence(Layer<PersistentBitMatrix>)` alone
+  would already handle sparse layers transparently, since
+  `PersistentBitMatrix` absorbs `Sparse` internally. Likely `Mat` predates
+  `PersistentBitMatrix` growing native sparse support. Signalled, not
+  removed — no mandate to touch `obikphylo` for this.
+
+## Remaining instance of the PartitionMeta-encapsulation problem
+
+`obikphylo::siblings::family_scan::scan_layer_families` still re-derives
+`index_dir` from `layer_dir.parent()` and calls `PartitionMeta::load`
+itself, purely to get `.mode` for `Mat::open`. Fixing it the way the 21
+other call sites were fixed needs more than a 1:1 swap: `scan_layer_families`
+only receives a bare `layer_dir: &Path`, not a `(partition, part, layer)`
+triple, and its single upstream source of layer paths,
+`sibling_layer_dirs`, returns a flat `Vec<PathBuf>` with the partition/layer
+indices already discarded. Fixing it properly means either having
+`sibling_layer_dirs` return `(PathBuf, IndexMode)` (or `(part, layer)`)
+pairs, or threading `&KmerPartition` + indices through instead of paths —
+and touching every one of `scan_layer_families`'s 8 callers (`distance.rs`,
+`alignment.rs`, `cardinality.rs`, `entropy.rs` ×2, `sankoff_bundle.rs` ×2,
+`stats.rs`). Left alone this round; worth doing as part of the same pass
+that builds (1)/(2), since those callers are exactly the sibling-annex
+consumers (2) is meant to serve.
diff --git a/src/obicompactvec/src/bitmatrix/persistent.rs b/src/obicompactvec/src/bitmatrix/persistent.rs
index 71fa4113..ac94e033 100644
--- a/src/obicompactvec/src/bitmatrix/persistent.rs
+++ b/src/obicompactvec/src/bitmatrix/persistent.rs
@@ -6,6 +6,7 @@ use ndarray::{Array1, Array2};
 use crate::bitvec::{PersistentBitVec, PersistentBitVecBuilder};
 use crate::layer_meta::LayerMeta;
 use crate::meta::MatrixMeta;
+use crate::storage_kind::StorageKind;
 use crate::traits::{BitPartials, ColumnWeights};
 use crate::views::{nonzero_triples, BitSliceView};
 
@@ -63,6 +64,42 @@ impl PersistentBitMatrix {
         Ok(Self::Implicit { n_rows: meta.n, n_cols: 1 })
     }
 
+    /// This already-open matrix's storage format — reads the discriminant
+    /// already in memory, no disk access.
+    #[inline]
+    pub fn storage_kind(&self) -> StorageKind {
+        match self {
+            Self::Columnar(_)    => StorageKind::Columnar,
+            Self::Packed(_)      => StorageKind::Packed,
+            Self::Sparse(_)      => StorageKind::Sparse,
+            Self::Implicit { .. } => StorageKind::Implicit,
+        }
+    }
+
+    /// Lightweight disk probe: which format `open` would pick, without
+    /// opening (mmap'ing) anything. Mirrors `open`'s own priority order by
+    /// hand — keep the two in sync if that order ever changes.
+    pub fn detect_storage(layer_dir: &Path) -> io::Result<StorageKind> {
+        let presence_dir = layer_dir.join("presence");
+
+        if presence_dir.join("matrix.pbmx").exists() {
+            return Ok(StorageKind::Packed);
+        }
+        if MatrixMeta::load(&presence_dir).is_ok() {
+            return Ok(StorageKind::Columnar);
+        }
+        if presence_dir.join("sparse_meta.json").exists() {
+            return Ok(StorageKind::Sparse);
+        }
+        if LayerMeta::load(layer_dir).is_ok() {
+            return Ok(StorageKind::Implicit);
+        }
+        Err(io::Error::new(
+            io::ErrorKind::NotFound,
+            format!("cannot detect presence storage format in {}", layer_dir.display()),
+        ))
+    }
+
     #[inline]
     pub fn n(&self) -> usize {
         match self {
diff --git a/src/obicompactvec/src/intmatrix.rs b/src/obicompactvec/src/intmatrix.rs
index dbb36338..2131e461 100644
--- a/src/obicompactvec/src/intmatrix.rs
+++ b/src/obicompactvec/src/intmatrix.rs
@@ -11,6 +11,7 @@ use crate::colgroup::{chunked_presence_count, ColGroup, MatrixGroupOps};
 use crate::format::{HEADER_SIZE, OVERFLOW_ENTRY_SIZE};
 use crate::meta::MatrixMeta;
 use crate::reader::PersistentCompactIntVec;
+use crate::storage_kind::StorageKind;
 use crate::tempbitvec::{TempBitVec, TempBitVecBuilder};
 use crate::tempintvec::{TempCompactIntVec, TempCompactIntVecBuilder};
 use crate::views::{nonzero_triples, IntSliceView};
@@ -287,6 +288,33 @@ impl PersistentCompactIntMatrix {
         ))
     }
 
+    /// This already-open matrix's storage format — reads the discriminant
+    /// already in memory, no disk access.
+    #[inline]
+    pub fn storage_kind(&self) -> StorageKind {
+        match self {
+            Self::Columnar(_) => StorageKind::Columnar,
+            Self::Packed(_)   => StorageKind::Packed,
+        }
+    }
+
+    /// Lightweight disk probe: which format `open` would pick, without
+    /// opening (mmap'ing) anything. Mirrors `open`'s own priority order by
+    /// hand — keep the two in sync if that order ever changes.
+    pub fn detect_storage(layer_dir: &Path) -> io::Result<StorageKind> {
+        let counts_dir = layer_dir.join("counts");
+        if counts_dir.join("matrix.pcmx").exists() {
+            return Ok(StorageKind::Packed);
+        }
+        if MatrixMeta::load(&counts_dir).is_ok() {
+            return Ok(StorageKind::Columnar);
+        }
+        Err(io::Error::new(
+            io::ErrorKind::NotFound,
+            format!("no count matrix found in {}", layer_dir.display()),
+        ))
+    }
+
     #[inline]
     pub fn n(&self) -> usize {
         match self { Self::Columnar(m) => m.n(), Self::Packed(m) => m.n_rows }
diff --git a/src/obicompactvec/src/lib.rs b/src/obicompactvec/src/lib.rs
index a7ce5df3..caa08ef4 100644
--- a/src/obicompactvec/src/lib.rs
+++ b/src/obicompactvec/src/lib.rs
@@ -10,6 +10,7 @@ mod intmatrix;
 mod layer_meta;
 mod meta;
 mod reader;
+mod storage_kind;
 mod tempbitvec;
 mod tempintvec;
 mod views;
@@ -25,6 +26,7 @@ pub use colgroup::{ColGroup, FilterMask, MatrixGroupOps, eval_filter_mask};
 pub use intmatrix::{PersistentCompactIntMatrix, PersistentCompactIntMatrixBuilder, pack_compact_int_matrix};
 pub use layer_meta::LayerMeta;
 pub use reader::{PersistentCompactIntVec, Iter as CompactIntVecIter};
+pub use storage_kind::StorageKind;
 pub use tempbitvec::{TempBitVec, TempBitVecBuilder};
 pub use tempintvec::{TempCompactIntVec, TempCompactIntVecBuilder};
 pub use traits::{BinaryMatrix, BitPartials, ColumnWeights, CountPartials};
diff --git a/src/obicompactvec/src/storage_kind.rs b/src/obicompactvec/src/storage_kind.rs
new file mode 100644
index 00000000..10374a85
--- /dev/null
+++ b/src/obicompactvec/src/storage_kind.rs
@@ -0,0 +1,19 @@
+//! How a persistent matrix's data is physically laid out on disk —
+//! orthogonal to *what* it stores (count vs. presence, `obilayeredmap`'s
+//! `LayerContent` concern, one crate up). `PersistentCompactIntMatrix` only
+//! ever reports `Columnar`/`Packed`; `PersistentBitMatrix` is the only type
+//! that can also report `Sparse`/`Implicit`.
+
+/// On-disk storage format of a persistent matrix.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum StorageKind {
+    /// Per-column files (`.pbiv`/`.pciv`), the build-time format.
+    Columnar,
+    /// Single mmap'd file (`matrix.pbmx`/`matrix.pcmx`), query-optimised.
+    Packed,
+    /// Row-major deduplicated sparse format (`PersistentBitMatrix` only).
+    Sparse,
+    /// No file at all — mono-genome, all values present
+    /// (`PersistentBitMatrix` only).
+    Implicit,
+}
diff --git a/src/obikindex/src/index.rs b/src/obikindex/src/index.rs
index d178113f..355483d0 100644
--- a/src/obikindex/src/index.rs
+++ b/src/obikindex/src/index.rs
@@ -3,7 +3,6 @@ use std::fs;
 use std::path::{Path, PathBuf};
 
 use obikpartitionner::{KmerPartition, KmerSpectrum, PARTITIONS_SUBDIR};
-use obilayeredmap;
 use obisys::{Reporter, Stage, progress_bar};
 use rayon::prelude::*;
 use tracing::info;
@@ -148,11 +147,7 @@ impl KmerIndex {
     /// homogeneous across all partitions — reading it off partition 0
     /// is enough, no need to scan every partition.
     pub fn n_layers_per_partition(&self) -> OKIResult<usize> {
-        use obilayeredmap::meta::PartitionMeta;
-        let index_dir = self.partition.index_dir(0);
-        let meta = PartitionMeta::load(&index_dir)
-            .map_err(|e| OKIError::Io(std::io::Error::new(std::io::ErrorKind::Other, e.to_string())))?;
-        Ok(meta.n_layers)
+        Ok(self.partition.n_layers(0)?)
     }
 
     /// Expose the inner partition so the caller can run scatter into it.
@@ -279,7 +274,6 @@ impl KmerIndex {
     /// (see the sparse-matrix design plan's "Explicitly deferred").
     pub fn pack_matrices(&self, sparse: bool) -> OKIResult<()> {
         use obicompactvec::{pack_bit_matrix, pack_compact_int_matrix, pack_sparse_bit_matrix};
-        use obilayeredmap::meta::PartitionMeta;
 
         let n = self.n_partitions();
         let order: Vec<usize> = (0..n).collect();
@@ -289,9 +283,8 @@ impl KmerIndex {
             |i| -> OKIResult<()> {
                 let index_dir = self.partition.index_dir(i);
                 if !index_dir.exists() { return Ok(()); }
-                let meta = PartitionMeta::load(&index_dir)
-                    .map_err(|e| OKIError::Io(std::io::Error::new(std::io::ErrorKind::Other, e.to_string())))?;
-                for l in 0..meta.n_layers {
+                let n_layers = self.partition.n_layers(i)?;
+                for l in 0..n_layers {
                     let layer_dir = self.partition.layer_dir(i, l);
                     let presence_dir = layer_dir.join("presence");
                     let counts_dir   = layer_dir.join("counts");
@@ -319,7 +312,6 @@ impl KmerIndex {
     pub fn upgrade_layer_meta(&self) -> OKIResult<()> {
         use obicompactvec::LayerMeta;
         use obiskio::UnitigFileReader;
-        use obilayeredmap::meta::PartitionMeta;
 
         let n = self.n_partitions();
         let errors: Vec<_> = (0..n)
@@ -327,11 +319,11 @@ impl KmerIndex {
             .filter_map(|i| {
                 let index_dir = self.partition.index_dir(i);
                 if !index_dir.exists() { return None; }
-                let meta = match PartitionMeta::load(&index_dir) {
-                    Ok(m) => m,
+                let n_layers = match self.partition.n_layers(i) {
+                    Ok(n) => n,
                     Err(e) => return Some(OKIError::Io(std::io::Error::new(std::io::ErrorKind::Other, e.to_string()))),
                 };
-                for l in 0..meta.n_layers {
+                for l in 0..n_layers {
                     let layer_dir = self.partition.layer_dir(i, l);
                     let meta_path = layer_dir.join(LayerMeta::FILENAME);
                     if meta_path.exists() { continue; }
diff --git a/src/obikindex/src/reindex.rs b/src/obikindex/src/reindex.rs
index 05a919e1..932fee23 100644
--- a/src/obikindex/src/reindex.rs
+++ b/src/obikindex/src/reindex.rs
@@ -1,7 +1,7 @@
 use std::fs;
 use std::path::Path;
-use obilayeredmap::{layer_dir, IndexMode, layer::Layer};
-use obilayeredmap::meta::PartitionMeta;
+use obikpartitionner::KmerPartition;
+use obilayeredmap::{IndexMode, layer::Layer};
 use obisys::{Reporter, Stage, progress_bar};
 use tracing::info;
 
@@ -48,7 +48,7 @@ impl KmerIndex {
         let runner = crate::numa::PartitionRunner::new();
         runner.run(
             &order,
-            |i| reindex_partition(&self.partition.index_dir(i), &target, block_bits)
+            |i| reindex_partition(&self.partition, i, &target, block_bits)
                 .map_err(|e| OKIError::InvalidInput(format!("partition {i}: {e}"))),
             |_, _, _| { pb.inc(1); },
         )?;
@@ -66,13 +66,13 @@ impl KmerIndex {
 }
 
 /// Process all layers of one partition's index directory.
-fn reindex_partition(index_dir: &Path, target: &IndexMode, block_bits: u8) -> OKIResult<()> {
-    if !index_dir.exists() {
+fn reindex_partition(partition: &KmerPartition, i: usize, target: &IndexMode, block_bits: u8) -> OKIResult<()> {
+    if !partition.index_dir(i).exists() {
         return Ok(());
     }
-    let pm = PartitionMeta::load(index_dir).map_err(olm_to_oki)?;
-    for layer_idx in 0..pm.n_layers {
-        reindex_layer(&layer_dir(index_dir, layer_idx), target, block_bits)?;
+    let n_layers = partition.n_layers(i).map_err(|e| OKIError::InvalidInput(e.to_string()))?;
+    for layer_idx in 0..n_layers {
+        reindex_layer(&partition.layer_dir(i, layer_idx), target, block_bits)?;
     }
     Ok(())
 }
diff --git a/src/obikindex/src/stats.rs b/src/obikindex/src/stats.rs
index eef2a252..995b7c20 100644
--- a/src/obikindex/src/stats.rs
+++ b/src/obikindex/src/stats.rs
@@ -3,7 +3,6 @@ use std::path::Path;
 
 use obicompactvec::{LayerMeta, PersistentBitMatrix, PersistentCompactIntMatrix};
 use obicompactvec::traits::ColumnWeights;
-use obilayeredmap::meta::PartitionMeta;
 use rayon::prelude::*;
 
 use crate::error::OKIResult;
@@ -93,9 +92,7 @@ impl KmerIndex {
                 let index_dir = self.partition.index_dir(i);
                 if !index_dir.exists() { return (0usize, 0u64, 0u64, 0u64); }
 
-                let n_layers = PartitionMeta::load(&index_dir)
-                    .map(|m| m.n_layers)
-                    .unwrap_or(0);
+                let n_layers = self.partition.n_layers(i).unwrap_or(0);
 
                 (0..n_layers).fold((0usize, 0u64, 0u64, 0u64), |acc, l| {
                     let lb = layer_bytes(&self.partition.layer_dir(i, l));
@@ -144,9 +141,7 @@ impl KmerIndex {
                 let index_dir = self.partition.index_dir(i);
                 if !index_dir.exists() { return (0, counts); }
 
-                let n_layers = PartitionMeta::load(&index_dir)
-                    .map(|m| m.n_layers)
-                    .unwrap_or(0);
+                let n_layers = self.partition.n_layers(i).unwrap_or(0);
 
                 for l in 0..n_layers {
                     let this_layer_dir = self.partition.layer_dir(i, l);
diff --git a/src/obikpartitionner/src/dump_layer.rs b/src/obikpartitionner/src/dump_layer.rs
index 46e93cda..eb18af0e 100644
--- a/src/obikpartitionner/src/dump_layer.rs
+++ b/src/obikpartitionner/src/dump_layer.rs
@@ -2,7 +2,6 @@ use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix};
 use obikseq::CanonicalKmer;
 use obiskio::{SKError, SKResult, UnitigFileReader};
 use obilayeredmap::{IndexMode, MphfLayer, OLMError};
-use obilayeredmap::meta::PartitionMeta;
 
 use crate::filter::{KmerFilter, passes_all};
 use crate::partition::KmerPartition;
@@ -39,9 +38,7 @@ impl KmerPartition {
             return Ok(true);
         }
 
-        let index_mode = PartitionMeta::load(&index_dir)
-            .map(|m| m.mode)
-            .unwrap_or(IndexMode::Exact);
+        let index_mode = self.index_mode(part).unwrap_or(IndexMode::Exact);
 
         let mut l = 0;
         loop {
@@ -119,9 +116,7 @@ impl KmerPartition {
             return Ok(true);
         }
 
-        let index_mode = PartitionMeta::load(&index_dir)
-            .map(|m| m.mode)
-            .unwrap_or(IndexMode::Exact);
+        let index_mode = self.index_mode(part).unwrap_or(IndexMode::Exact);
 
         let mut layer = 0;
         loop {
diff --git a/src/obikpartitionner/src/merge_layer/mod.rs b/src/obikpartitionner/src/merge_layer/mod.rs
index ea92e3f1..1fc68811 100644
--- a/src/obikpartitionner/src/merge_layer/mod.rs
+++ b/src/obikpartitionner/src/merge_layer/mod.rs
@@ -21,7 +21,6 @@ use obipipeline::{
 
 use obicompactvec::{PersistentBitMatrixBuilder, PersistentCompactIntMatrixBuilder};
 use obikseq::CanonicalKmer;
-use obilayeredmap::meta::PartitionMeta;
 use obilayeredmap::{layer_dir, IndexMode, Layer, LayeredMap, MphfOnly};
 use obiskio::{SKError, SKResult, UnitigFileReader};
 
@@ -526,7 +525,7 @@ impl KmerPartition {
         if let Some(mb) = new_mb {
             mb.close().map_err(SKError::Io)?;
 
-            let mut part_meta = PartitionMeta::load(&dst_index_dir).map_err(|e| olm_to_sk(e, "merge"))?;
+            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"))?;
         }
diff --git a/src/obikpartitionner/src/partition/kmer_partition.rs b/src/obikpartitionner/src/partition/kmer_partition.rs
index ec3a5fa4..57a202c2 100644
--- a/src/obikpartitionner/src/partition/kmer_partition.rs
+++ b/src/obikpartitionner/src/partition/kmer_partition.rs
@@ -8,6 +8,8 @@ use obisys::progress_bar;
 
 use obikseq::RoutableSuperKmer;
 use obiskio::SKResult;
+use obilayeredmap::IndexMode;
+use obilayeredmap::meta::PartitionMeta;
 use rayon::prelude::*;
 use remove_dir_all::remove_dir_all;
 use sysinfo::System;
@@ -16,6 +18,7 @@ use niffler::Level;
 use niffler::send::compression::Format;
 use obiskio::SKFileWriter;
 
+use crate::common::load_meta;
 use crate::kmer_sort::chunk_size_from_ram;
 
 use super::count::count_partition;
@@ -191,6 +194,26 @@ impl KmerPartition {
         obilayeredmap::layer_dir(&self.index_dir(i), l)
     }
 
+    /// Partition `i`'s metadata (layer count, evidence mode) — the single
+    /// entry point for this, so that callers outside `obikpartitionner`
+    /// never need to know it's a `meta.json` loaded via
+    /// `obilayeredmap::meta::PartitionMeta`, nor handle its own recovery
+    /// path for indexes built before that file existed (see
+    /// [`crate::common::load_meta`]).
+    pub fn partition_meta(&self, i: usize) -> 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) -> SKResult<usize> {
+        self.partition_meta(i).map(|m| m.n_layers)
+    }
+
+    /// Evidence mode of partition `i` — see [`partition_meta`](Self::partition_meta).
+    pub fn index_mode(&self, i: usize) -> SKResult<IndexMode> {
+        self.partition_meta(i).map(|m| m.mode)
+    }
+
     pub fn kmer_size(&self) -> usize {
         self.kmer_size
     }
diff --git a/src/obikpartitionner/src/query_layer.rs b/src/obikpartitionner/src/query_layer.rs
index 3c8cbc6a..fdd0ac84 100644
--- a/src/obikpartitionner/src/query_layer.rs
+++ b/src/obikpartitionner/src/query_layer.rs
@@ -5,7 +5,6 @@ use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix};
 use obikseq::CanonicalKmer;
 use obiskio::{SKError, SKResult};
 use obilayeredmap::{IndexMode, MphfLayer, OLMError};
-use obilayeredmap::meta::PartitionMeta;
 
 use crate::partition::KmerPartition;
 
@@ -178,7 +177,7 @@ impl KmerPartition {
             return Ok(stats);
         }
 
-        let meta = PartitionMeta::load(&index_dir).map_err(olm_to_sk)?;
+        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, &meta.mode))
             .collect::<SKResult<_>>()?;
diff --git a/src/obikpartitionner/src/select_layer.rs b/src/obikpartitionner/src/select_layer.rs
index 691dba55..158ab93c 100644
--- a/src/obikpartitionner/src/select_layer.rs
+++ b/src/obikpartitionner/src/select_layer.rs
@@ -7,7 +7,6 @@ use obicompactvec::{
     PersistentBitMatrix, PersistentBitMatrixBuilder,
     PersistentCompactIntMatrix, PersistentCompactIntMatrixBuilder,
 };
-use obilayeredmap::meta::PartitionMeta;
 use obilayeredmap::OLMError;
 use obiskio::{SKError, SKResult};
 
@@ -150,8 +149,8 @@ impl KmerPartition {
             return Ok(());
         }
 
-        let src_meta = PartitionMeta::load(&src_index_dir).map_err(olm_to_sk)?;
-        if src_meta.n_layers == 0 {
+        let n_src_layers = src.n_layers(i)?;
+        if n_src_layers == 0 {
             return Ok(());
         }
 
@@ -162,7 +161,7 @@ impl KmerPartition {
 
         let data_subdir = if output_presence { "presence" } else { "counts" };
 
-        for l in 0..src_meta.n_layers {
+        for l in 0..n_src_layers {
             let src_layer_dir = src.layer_dir(i, l);
             if !src_layer_dir.exists() { continue; }
 
@@ -234,8 +233,7 @@ impl KmerPartition {
         }
 
         if !in_place {
-            PartitionMeta::load(&src_index_dir).map_err(olm_to_sk)?
-                .save(&dst_index_dir).map_err(olm_to_sk)?;
+            src.partition_meta(i)?.save(&dst_index_dir).map_err(olm_to_sk)?;
         }
 
         Ok(())
diff --git a/src/obikphylo/src/siblings/build.rs b/src/obikphylo/src/siblings/build.rs
index 28c0768b..358b0778 100644
--- a/src/obikphylo/src/siblings/build.rs
+++ b/src/obikphylo/src/siblings/build.rs
@@ -8,7 +8,7 @@ use obikpartitionner::KmerPartition;
 use obipipeline::ThrottleGuard;
 use obikseq::CanonicalKmer;
 use obilayeredmap::MphfLayer;
-use obilayeredmap::meta::PartitionMeta;
+use obilayeredmap::meta::IndexMode;
 use obisys::progress_bar;
 
 use obikindex::{OKIError, OKIResult};
@@ -97,11 +97,13 @@ impl SiblingAnnexBuildExt for KmerIndex {
                 pb.inc(1);
                 continue;
             }
-            let meta = PartitionMeta::load(&index_dir).map_err(olm_to_ok)?;
+            let meta = self.partition().partition_meta(part)?;
 
             let mut part_slots: u64 = 0;
             for l in 0..meta.n_layers {
-                part_slots += build_layer_sibling_annex(self, &self.partition().layer_dir(part, l), n_parts, l, &cache)?;
+                part_slots += build_layer_sibling_annex(
+                    self, &self.partition().layer_dir(part, l), &meta.mode, n_parts, l, &cache,
+                )?;
             }
             total_slots += part_slots;
             pb.inc(1);
@@ -120,13 +122,12 @@ impl SiblingAnnexBuildExt for KmerIndex {
 fn build_layer_sibling_annex(
     index: &KmerIndex,
     layer_dir: &Path,
+    mode: &IndexMode,
     n_parts: usize,
     l: usize,
     cache: &Arc<PartitionCache>,
 ) -> OKIResult<u64> {
-    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 mphf = MphfLayer::open(layer_dir, &meta.mode).map_err(olm_to_ok)?;
+    let mphf = MphfLayer::open(layer_dir, mode).map_err(olm_to_ok)?;
     let k = index.kmer_size();
     let n = mphf.n();
 
diff --git a/src/obikphylo/src/siblings/cache.rs b/src/obikphylo/src/siblings/cache.rs
index 8ffef9fe..315c0660 100644
--- a/src/obikphylo/src/siblings/cache.rs
+++ b/src/obikphylo/src/siblings/cache.rs
@@ -6,13 +6,13 @@ use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix, PersistentS
 use obikpartitionner::KmerPartition;
 use obikseq::CanonicalKmer;
 use obilayeredmap::{Layer, OLMResult};
-use obilayeredmap::meta::{IndexMode, PartitionMeta};
+use obilayeredmap::meta::IndexMode;
 use obisys::progress_bar;
 
 use obikindex::OKIResult;
 
 use super::iter::SiblingLayerExt;
-use super::{olm_to_ok, SiblingAnnex};
+use super::SiblingAnnex;
 
 /// Every partition's already-open layers, built **once** for the whole
 /// `build_sibling_annex` run and shared (read-only) across every lookup, in
@@ -179,7 +179,7 @@ impl PartitionCache {
                     pb.inc(1);
                     return Ok((Vec::new(), 0));
                 }
-                let meta = PartitionMeta::load(&index_dir).map_err(olm_to_ok)?;
+                let meta = partition.partition_meta(part)?;
                 let mut mats = Vec::with_capacity(meta.n_layers);
                 for l in 0..meta.n_layers {
                     let Ok(mat) = Mat::open(&partition.layer_dir(part, l), &meta.mode, with_counts) else { continue };
diff --git a/src/obikphylo/src/siblings/family_scan.rs b/src/obikphylo/src/siblings/family_scan.rs
index 0215bc68..f36c3f73 100644
--- a/src/obikphylo/src/siblings/family_scan.rs
+++ b/src/obikphylo/src/siblings/family_scan.rs
@@ -109,8 +109,8 @@ pub(crate) fn sibling_layer_dirs(index: &KmerIndex) -> OKIResult<Vec<PathBuf>> {
         if !index_dir.exists() {
             continue;
         }
-        let meta = PartitionMeta::load(&index_dir).map_err(olm_to_ok)?;
-        for l in 0..meta.n_layers {
+        let n_layers = index.partition().n_layers(part)?;
+        for l in 0..n_layers {
             let this_layer_dir = index.partition().layer_dir(part, l);
             let annex_path = this_layer_dir.join(ANNEX_FILE_NAME);
             if !annex_path.exists() {
diff --git a/src/obikphylo/src/siblings/tests.rs b/src/obikphylo/src/siblings/tests.rs
index 93784782..929f49bf 100644
--- a/src/obikphylo/src/siblings/tests.rs
+++ b/src/obikphylo/src/siblings/tests.rs
@@ -3,7 +3,6 @@ use std::path::Path;
 
 use obikseq::{CanonicalKmer, Kmer, Sequence};
 use obilayeredmap::MphfLayer;
-use obilayeredmap::meta::PartitionMeta;
 use obisys::Reporter;
 use tempfile::tempdir;
 
@@ -88,8 +87,7 @@ fn canonical(ascii: &[u8]) -> CanonicalKmer {
 /// Read back the annex entry for a given canonical k-mer from the merged
 /// index's (single) partition/layer, asserting it was found at all.
 fn annex_info_for(idx: &KmerIndex, kmer: CanonicalKmer) -> FamilyMask {
-    let index_dir = idx.partition().index_dir(0);
-    let meta = PartitionMeta::load(&index_dir).unwrap();
+    let meta = idx.partition().partition_meta(0).unwrap();
     for l in 0..meta.n_layers {
         let layer_dir = idx.partition().layer_dir(0, l);
         let mphf = MphfLayer::open(&layer_dir, &meta.mode).unwrap();
@@ -310,9 +308,8 @@ fn sibling_annex_no_empty_masks_after_build() {
     let g1 = build_single_genome_index(dir.path(), "g1", &seq);
     g1.build_sibling_annex().expect("build_sibling_annex");
 
-    let index_dir = g1.partition().index_dir(0);
-    let meta = PartitionMeta::load(&index_dir).expect("partition meta");
-    for l in 0..meta.n_layers {
+    let n_layers = g1.partition().n_layers(0).expect("partition meta");
+    for l in 0..n_layers {
         let layer_dir = g1.partition().layer_dir(0, l);
         let annex = SiblingAnnex::open(&layer_dir.join(ANNEX_FILE_NAME)).expect("annex open");
         for slot in 0..annex.len() {
@@ -361,9 +358,8 @@ fn sibling_annex_records_the_real_layer_of_each_family_member() {
     let merged = merge_two(dir.path(), &g1, &g2);
     merged.build_sibling_annex().expect("build_sibling_annex");
 
-    let index_dir = merged.partition().index_dir(0);
-    let meta = PartitionMeta::load(&index_dir).unwrap();
-    assert_eq!(meta.n_layers, 2, "fixture assumption: one merge, one new layer");
+    let n_layers = merged.partition().n_layers(0).unwrap();
+    assert_eq!(n_layers, 2, "fixture assumption: one merge, one new layer");
 
     let g1_kmer = canonical(b"AACCGCTTAAG"); // own base C (1), sibling base G (2) — lives in layer 1
     let g2_kmer = canonical(b"AACCGGTTAAG"); // own base G (2), sibling base C (1) — lives in layer 0
diff --git a/src/obilayeredmap/src/layer.rs b/src/obilayeredmap/src/layer.rs
index f0b22caa..44859e18 100644
--- a/src/obilayeredmap/src/layer.rs
+++ b/src/obilayeredmap/src/layer.rs
@@ -85,6 +85,76 @@ impl LayerData for PersistentSparseBitMatrix {
     fn read(&self, slot: usize) -> Box<[bool]> { self.row(slot) }
 }
 
+// ── LayerContent ─────────────────────────────────────────────────────────────
+
+/// What a layer's data matrix represents — orthogonal to *how* it's stored
+/// on disk (`obicompactvec::StorageKind`'s concern). Only meaningful for
+/// `D` that actually carry a matrix: `Layer<()>` (mode 1, set membership,
+/// no matrix at all) has neither a `LayerContent` nor a `content()` method
+/// — it's a write-time-only state, never a queryable content. Once a layer
+/// is closed, "no matrix file" reads back as `Presence` via
+/// `PersistentBitMatrix::open`'s own `Implicit` fallback, not as some third
+/// "empty" content — see `DevDocMD/implementation/partition_layer_cache.md`.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum LayerContent {
+    Count,
+    Presence,
+}
+
+impl LayerContent {
+    /// Lightweight disk probe, no MPHF/matrix opened: `counts/` present →
+    /// `Count`, otherwise `Presence` — an absent/`Implicit` presence
+    /// matrix still reads as `Presence`, never a separate content.
+    pub fn detect(layer_dir: &Path) -> LayerContent {
+        if layer_dir.join(COUNTS_DIR).exists() {
+            LayerContent::Count
+        } else {
+            LayerContent::Presence
+        }
+    }
+}
+
+/// Implemented only by `D` that carry a real matrix — gates
+/// [`Layer::content`](Layer::content) so `Layer<()>` doesn't get it.
+pub trait HasLayerContent {
+    const CONTENT: LayerContent;
+}
+
+impl HasLayerContent for PersistentCompactIntMatrix {
+    const CONTENT: LayerContent = LayerContent::Count;
+}
+
+impl HasLayerContent for PersistentBitMatrix {
+    const CONTENT: LayerContent = LayerContent::Presence;
+}
+
+impl HasLayerContent for PersistentSparseBitMatrix {
+    const CONTENT: LayerContent = LayerContent::Presence;
+}
+
+// ── StorageKind passthrough ────────────────────────────────────────────────────
+
+/// Implemented only by `D` that expose their own `storage_kind()` — gates
+/// [`Layer::storage_kind`]. `PersistentSparseBitMatrix` has a single,
+/// fixed on-disk format (no `Columnar`/`Packed`/`Implicit` variants of its
+/// own), so it's deliberately not given an impl: there's nothing to report
+/// beyond what `content()` already says.
+pub trait HasStorageKind {
+    fn storage_kind(&self) -> obicompactvec::StorageKind;
+}
+
+impl HasStorageKind for PersistentCompactIntMatrix {
+    fn storage_kind(&self) -> obicompactvec::StorageKind {
+        PersistentCompactIntMatrix::storage_kind(self)
+    }
+}
+
+impl HasStorageKind for PersistentBitMatrix {
+    fn storage_kind(&self) -> obicompactvec::StorageKind {
+        PersistentBitMatrix::storage_kind(self)
+    }
+}
+
 // ── Structures ────────────────────────────────────────────────────────────────
 
 pub struct Layer<D: LayerData = ()> {
@@ -171,6 +241,28 @@ impl<D: LayerData> Layer<D> {
         MphfLayer::build_approx_evidence(layer_dir, b, z)
     }
 
+    /// This already-open layer's evidence mode — reads the discriminant
+    /// already in memory (`MphfLayer`'s own `LayerEvidence`), no disk
+    /// access. Available regardless of `D`, unlike `content`/`storage_kind`.
+    pub fn evidence_kind(&self) -> crate::mphf_layer::EvidenceKind {
+        self.mphf.evidence_kind()
+    }
+}
+
+impl<D: LayerData + HasLayerContent> Layer<D> {
+    /// This already-open layer's content (`Count`/`Presence`) — a
+    /// compile-time fact about `D`, not a runtime read.
+    pub const fn content(&self) -> LayerContent {
+        D::CONTENT
+    }
+}
+
+impl<D: LayerData + HasStorageKind> Layer<D> {
+    /// This already-open layer's storage format — delegates to `D`'s own
+    /// `storage_kind()`, reading a discriminant already in memory.
+    pub fn storage_kind(&self) -> obicompactvec::StorageKind {
+        self.data.storage_kind()
+    }
 }
 
 // ── Mode 1 — set membership ───────────────────────────────────────────────────
diff --git a/src/obilayeredmap/src/lib.rs b/src/obilayeredmap/src/lib.rs
index 103035eb..66387b27 100644
--- a/src/obilayeredmap/src/lib.rs
+++ b/src/obilayeredmap/src/lib.rs
@@ -8,8 +8,8 @@ pub mod meta;
 pub(crate) mod mphf_layer;
 
 pub use error::{OLMError, OLMResult};
-pub use layer::{layer_dir, open_data, Hit, Layer, LayerData};
+pub use layer::{layer_dir, open_data, HasLayerContent, HasStorageKind, Hit, Layer, LayerContent, LayerData};
 pub use layered_store::LayeredStore;
 pub use map::LayeredMap;
 pub use meta::{IndexMode, PartitionMeta};
-pub use mphf_layer::{KmerBatchIter, KmerIter, MphfLayer, MphfOnly};
+pub use mphf_layer::{EvidenceKind, KmerBatchIter, KmerIter, MphfLayer, MphfOnly};
diff --git a/src/obilayeredmap/src/map.rs b/src/obilayeredmap/src/map.rs
index 9a8971de..96d27975 100644
--- a/src/obilayeredmap/src/map.rs
+++ b/src/obilayeredmap/src/map.rs
@@ -2,13 +2,14 @@ use std::collections::HashMap;
 use std::fs;
 use std::path::{Path, PathBuf};
 
-use obicompactvec::PersistentCompactIntMatrix;
+use obicompactvec::{PersistentBitMatrix, PersistentCompactIntMatrix, StorageKind};
 use obikseq::CanonicalKmer;
 use obiskio::{UnitigFileWriter, DEFAULT_BLOCK_BITS};
 
-use crate::error::OLMResult;
-use crate::layer::{layer_dir, Hit, Layer, LayerData};
+use crate::error::{OLMError, OLMResult};
+use crate::layer::{layer_dir, Hit, Layer, LayerContent, LayerData};
 use crate::meta::{IndexMode, PartitionMeta};
+use crate::mphf_layer::EvidenceKind;
 
 /// Layered kmer index for a single partition.
 ///
@@ -46,6 +47,30 @@ impl<D: LayerData> LayeredMap<D> {
     pub fn layer(&self, i: usize) -> &Layer<D> { &self.layers[i] }
     pub fn mode(&self) -> &IndexMode { &self.meta.mode }
 
+    /// Layer `i`'s content (`Count`/`Presence`) — a lightweight disk probe,
+    /// no MPHF/matrix opened. For picking a `D` before committing to
+    /// [`Layer::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))
+    }
+
+    /// Layer `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),
+        }
+    }
+
+    /// Layer `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
diff --git a/src/obilayeredmap/src/mphf_layer.rs b/src/obilayeredmap/src/mphf_layer.rs
index 6f47dc25..53588151 100644
--- a/src/obilayeredmap/src/mphf_layer.rs
+++ b/src/obilayeredmap/src/mphf_layer.rs
@@ -35,6 +35,35 @@ enum LayerEvidence {
     Hybrid { evidence: Evidence, unitigs: Arc<UnitigFileReader>, fingerprint: FingerprintVec },
 }
 
+// ── EvidenceKind ──────────────────────────────────────────────────────────────
+
+/// Coarse evidence mode of a layer — the `IndexMode` a layer was opened
+/// with, without the `Approx`/`Hybrid` `b`/`z` parameters.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub enum EvidenceKind {
+    Exact,
+    Approx,
+    Hybrid,
+}
+
+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> {
+        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!(
+                "no evidence.bin or fingerprint.bin in {}", layer_dir.display()
+            ))),
+        }
+    }
+}
+
 // ── MphfLayer ─────────────────────────────────────────────────────────────────
 
 /// Autonomous kmer → slot mapping for one layer.
@@ -163,6 +192,16 @@ impl MphfLayer {
 
     pub fn n(&self) -> usize { self.n }
 
+    /// This already-open layer's evidence mode — reads the discriminant
+    /// already in memory, no disk access.
+    pub fn evidence_kind(&self) -> EvidenceKind {
+        match &self.ev {
+            LayerEvidence::Exact  { .. } => EvidenceKind::Exact,
+            LayerEvidence::Approx { .. } => EvidenceKind::Approx,
+            LayerEvidence::Hybrid { .. } => EvidenceKind::Hybrid,
+        }
+    }
+
     /// Raw MPHF lookup: kmer → slot.
     ///
     /// Returns the slot assigned by the MPHF without performing any membership
diff --git a/src/obilayeredmap/src/tests/layer.rs b/src/obilayeredmap/src/tests/layer.rs
index 81eed976..31a3bdb7 100644
--- a/src/obilayeredmap/src/tests/layer.rs
+++ b/src/obilayeredmap/src/tests/layer.rs
@@ -98,6 +98,36 @@ fn presence_layer_generic_over_sparse_matches_dense() {
     }
 }
 
+// ── content / storage_kind / evidence_kind (runtime introspection) ─────────
+
+#[test]
+fn count_layer_reports_count_content_and_columnar_storage() {
+    set_k(4);
+    let dir = tempdir().unwrap();
+    write_unitigs(dir.path(), &[b"AAAACGT"]);
+    Layer::<PersistentCompactIntMatrix>::build(dir.path(), DEFAULT_BLOCK_BITS, &IndexMode::Exact, |_| 1)
+        .unwrap();
+    let layer = Layer::<PersistentCompactIntMatrix>::open(dir.path(), &IndexMode::Exact).unwrap();
+
+    assert_eq!(layer.content(), LayerContent::Count);
+    assert_eq!(layer.storage_kind(), obicompactvec::StorageKind::Columnar);
+    assert_eq!(layer.evidence_kind(), crate::mphf_layer::EvidenceKind::Exact);
+}
+
+#[test]
+fn presence_layer_reports_presence_content_and_columnar_storage() {
+    set_k(4);
+    let dir = tempdir().unwrap();
+    write_unitigs(dir.path(), &[b"AAATCTA", b"CTTCGCC", b"TGATACG"]);
+    Layer::<PersistentBitMatrix>::build_presence(dir.path(), DEFAULT_BLOCK_BITS, &IndexMode::Exact, 2, |kmer, g| {
+        (kmer.raw().wrapping_add(g as u64)) % 2 == 0
+    }).unwrap();
+    let layer = Layer::<PersistentBitMatrix>::open(dir.path(), &IndexMode::Exact).unwrap();
+
+    assert_eq!(layer.content(), LayerContent::Presence);
+    assert_eq!(layer.storage_kind(), obicompactvec::StorageKind::Columnar);
+}
+
 #[test]
 fn enumerate_kmers_is_stable_across_calls() {
     set_k(4);
diff --git a/src/obilayeredmap/src/tests/map.rs b/src/obilayeredmap/src/tests/map.rs
index d8e2ce81..276597ed 100644
--- a/src/obilayeredmap/src/tests/map.rs
+++ b/src/obilayeredmap/src/tests/map.rs
@@ -1,7 +1,10 @@
 use super::*;
-use obicompactvec::PersistentCompactIntMatrix;
+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::LayerContent;
 use crate::meta::IndexMode;
+use crate::mphf_layer::EvidenceKind;
 use tempfile::tempdir;
 
 fn push_unitigs_and_layer(
@@ -96,3 +99,121 @@ fn push_layer_from_map_convenience() {
     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::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);
+    Layer::<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.
+    Layer::<()>::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"]);
+    Layer::<()>::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"]);
+    Layer::<()>::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);
+}