diff --git a/src/obikfilter/src/partition_iter.rs b/src/obikfilter/src/partition_iter.rs
index 0d69f570..3793964e 100644
--- a/src/obikfilter/src/partition_iter.rs
+++ b/src/obikfilter/src/partition_iter.rs
@@ -63,7 +63,7 @@ impl FilteredPartitionIter for IndexCache {
     ) -> OKIResult<bool> {
         for l in 0..self.n_layer(part).unwrap_or(0) {
             let layer = self.get_layer(part, l).expect("layer within n_layer(part)");
-            if !iter_layer_kmers(layer, use_counts, n_genomes, filters, &mut |kmer, row| cb(kmer, row))? {
+            if !iter_layer_kmers(&layer, use_counts, n_genomes, filters, &mut |kmer, row| cb(kmer, row))? {
                 return Ok(false);
             }
         }
@@ -80,7 +80,7 @@ impl FilteredPartitionIter for IndexCache {
     ) -> OKIResult<bool> {
         for l in 0..self.n_layer(part).unwrap_or(0) {
             let layer = self.get_layer(part, l).expect("layer within n_layer(part)");
-            if !iter_layer_kmers(layer, use_counts, n_genomes, filters, &mut |kmer, row| cb(part, l, kmer, row))? {
+            if !iter_layer_kmers(&layer, use_counts, n_genomes, filters, &mut |kmer, row| cb(part, l, kmer, row))? {
                 return Ok(false);
             }
         }
diff --git a/src/obikidxcache/src/index_cache.rs b/src/obikidxcache/src/index_cache.rs
index 2f72e141..8dcbfb71 100644
--- a/src/obikidxcache/src/index_cache.rs
+++ b/src/obikidxcache/src/index_cache.rs
@@ -28,7 +28,28 @@ pub struct IndexCache {
     /// `0`. A `HashMap` rather than a flat `Vec` because the set of cached
     /// partitions doesn't have to be `0..n_partitions`, or even contiguous
     /// — see [`new`](Self::new).
-    layer_cache: HashMap<usize, Vec<KmerLayer>>,
+    ///
+    /// `Arc<KmerLayer>` per layer, and the whole map itself behind one more
+    /// `Arc` — every accessor below hands out owned `Arc<KmerLayer>`
+    /// clones (never a `&KmerLayer` tied to `&self`'s lifetime), because
+    /// that's the only thing that can satisfy the `Send + 'static` bound
+    /// `obipipeline::Pipe::apply` (real `thread::spawn` workers) requires
+    /// of anything moved into them. A caller needing to hand one specific
+    /// cached layer to such a pipeline would otherwise have no way to do so
+    /// short of reopening it independently — a redundant mmap/MPHF open of
+    /// a layer this cache already holds (confirmed by
+    /// `obikphylo::siblings::algorithms::family_scan` hitting exactly this
+    /// before `Arc<KmerLayer>` was introduced here).
+    ///
+    /// The outer `Arc<HashMap<..>>` is what lets [`iter`](Self::iter)/
+    /// [`iter_partition`](Self::iter_partition)/[`iter_indexed`](Self::iter_indexed)
+    /// return a genuine `Iterator` object (`LayerIter`, below) instead of
+    /// eagerly materialising every layer's `Arc` into a throwaway `Vec`
+    /// first: cloning the outer `Arc` is one refcount bump regardless of
+    /// how many layers this cache holds, so `LayerIter::next()` can look
+    /// up and clone one `Arc<KmerLayer>` at a time, on demand, straight out
+    /// of this same shared map — no separate payload copy ever exists.
+    layer_cache: Arc<HashMap<usize, Vec<Arc<KmerLayer>>>>,
 }
 
 impl IndexCache {
@@ -64,7 +85,7 @@ impl IndexCache {
                         "IndexCache::new: failed to open layer (partition {p}, layer {l}): {e} — partition is not fully built"
                     )
                 });
-                layers.push(layer);
+                layers.push(Arc::new(layer));
             }
             layer_cache.insert(p, layers);
         }
@@ -73,7 +94,7 @@ impl IndexCache {
         IndexCache {
             raw_index: index,
             meta,
-            layer_cache,
+            layer_cache: Arc::new(layer_cache),
         }
     }
 
@@ -109,22 +130,52 @@ impl IndexCache {
         self.layer_cache.keys().copied()
     }
 
+    /// A cheap `Arc` clone (refcount bump, no reopening) of one cached
+    /// layer — owned, not tied to `&self`'s lifetime, so it can be moved
+    /// into a context that must own it, e.g. a `Send + 'static` pipeline
+    /// worker. See the [`layer_cache`](Self::layer_cache) field docs for
+    /// why every accessor here hands out `Arc<KmerLayer>` rather than
+    /// `&KmerLayer`.
     #[inline]
-    pub fn get_layer(&self, partition: usize, layer: usize) -> Option<&KmerLayer> {
-        self.layer_cache.get(&partition)?.get(layer)
+    pub fn get_layer(&self, partition: usize, layer: usize) -> Option<Arc<KmerLayer>> {
+        self.layer_cache.get(&partition)?.get(layer).cloned()
     }
 
-    pub fn iter(&self) -> impl Iterator<Item = &KmerLayer> {
-        self.layer_cache.values().flatten()
+    /// Every cached layer, across every cached partition, as owned `Arc`
+    /// clones, in `(partition, layer)` order — genuinely `'static`: see
+    /// [`iter_indexed`](Self::iter_indexed)/[`LayerIter`]'s own docs.
+    pub fn iter(&self) -> impl Iterator<Item = Arc<KmerLayer>> + 'static {
+        self.iter_indexed().map(|(_, _, layer)| layer)
     }
 
-    pub fn iter_indexed(&self) -> impl Iterator<Item = (usize, usize, &KmerLayer)> {
-        self.layer_cache.iter().flat_map(|(&p, layers)| {
-            layers
-                .iter()
-                .enumerate()
-                .map(move |(l, layer)| (p, l, layer))
-        })
+    /// Every cached layer of *one* partition, in layer order — never the
+    /// other cached partitions. For a caller that already knows which
+    /// partition a k-mer routes to (`CanonicalKmer::partition`, the same
+    /// rule [`find`](Self::find) uses) and only needs to scan that one
+    /// partition's own layers, not every partition this cache holds. Empty
+    /// (not an error) if `partition` isn't cached — same convention as
+    /// [`get_layer`](Self::get_layer). `'static` for the same reason
+    /// [`iter_indexed`](Self::iter_indexed) is.
+    pub fn iter_partition(&self, partition: usize) -> impl Iterator<Item = Arc<KmerLayer>> + 'static {
+        LayerIter {
+            cache: Arc::clone(&self.layer_cache),
+            partitions: vec![partition].into_iter(),
+            current: None,
+        }
+        .map(|(_, _, layer)| layer)
+    }
+
+    /// Every cached layer, tagged with its own `(partition, layer)`
+    /// position — see [`LayerIter`]'s own docs for why this is a real
+    /// `next()`-driven iterator, one `Arc<KmerLayer>` clone at a time out
+    /// of the shared cache map, rather than a `Vec<Arc<KmerLayer>>`
+    /// materialised up front.
+    pub fn iter_indexed(&self) -> impl Iterator<Item = (usize, usize, Arc<KmerLayer>)> + 'static {
+        LayerIter {
+            cache: Arc::clone(&self.layer_cache),
+            partitions: self.layer_cache.keys().copied().collect::<Vec<_>>().into_iter(),
+            current: None,
+        }
     }
 
     /// Iterate over the unitigs of every cached layer, chained — the
@@ -133,7 +184,7 @@ impl IndexCache {
     /// (one partition, several, or the whole index — see [`new`](Self::new)).
     /// No extra opening: every layer here is already open, this only
     /// streams what [`iter`](Self::iter) already gives out.
-    pub fn iter_unitigs(&self) -> impl Iterator<Item = obikseq::Unitig> + '_ {
+    pub fn iter_unitigs(&self) -> impl Iterator<Item = obikseq::Unitig> + 'static {
         self.iter().flat_map(|l| l.iter_unitigs())
     }
 
@@ -224,7 +275,7 @@ impl IndexCache {
             .flat_map(|(_, layers)| layers.par_iter())
             .map(|layer| {
                 let _guard = throttle.acquire_guard();
-                map(layer)
+                map(layer.as_ref())
             })
             .reduce(&identity, &reduce)
     }
@@ -244,7 +295,7 @@ impl IndexCache {
             .flat_map(|(_, layers)| layers.par_iter())
             .for_each(|layer| {
                 let _guard = throttle.acquire_guard();
-                f(layer);
+                f(layer.as_ref());
             });
     }
 
@@ -268,8 +319,43 @@ impl IndexCache {
             .par_iter()
             .flat_map(|(_, layers)| layers.par_iter())
             .map(move |layer| obipipeline::Throttled {
-                item: layer,
+                item: layer.as_ref(),
                 guard: throttle.acquire_guard(),
             })
     }
 }
+
+/// A real `Iterator` (implements `next()`), not a `Vec` pre-filled and then
+/// walked — `next()` looks up and clones exactly one `Arc<KmerLayer>` at a
+/// time, straight out of `cache` (a clone of `IndexCache`'s own shared map,
+/// one refcount bump to construct regardless of how many layers are cached),
+/// so no separate copy of the layers being walked ever exists. `partitions`
+/// is a small, already-owned `Vec<usize>` of which partition(s) to walk, in
+/// order — holding a `Vec` of plain partition numbers costs nothing close
+/// to what holding a `Vec` of the cached layers themselves would.
+pub struct LayerIter {
+    cache: Arc<HashMap<usize, Vec<Arc<KmerLayer>>>>,
+    partitions: std::vec::IntoIter<usize>,
+    /// `(partition, next layer index)` of the partition currently being
+    /// walked — `None` once that partition is exhausted or before the
+    /// first `next()` call, at which point [`partitions`](Self::partitions)
+    /// is advanced to pick up the next one.
+    current: Option<(usize, usize)>,
+}
+
+impl Iterator for LayerIter {
+    type Item = (usize, usize, Arc<KmerLayer>);
+
+    fn next(&mut self) -> Option<Self::Item> {
+        loop {
+            if let Some((partition, layer_idx)) = self.current {
+                if let Some(layer) = self.cache.get(&partition).and_then(|v| v.get(layer_idx)) {
+                    self.current = Some((partition, layer_idx + 1));
+                    return Some((partition, layer_idx, Arc::clone(layer)));
+                }
+                self.current = None; // this partition exhausted
+            }
+            self.current = Some((self.partitions.next()?, 0));
+        }
+    }
+}
diff --git a/src/obikmer2/src/cmd/phylo/args.rs b/src/obikmer2/src/cmd/phylo/args.rs
index df31e7e0..b2f6c2a6 100644
--- a/src/obikmer2/src/cmd/phylo/args.rs
+++ b/src/obikmer2/src/cmd/phylo/args.rs
@@ -51,6 +51,13 @@ pub struct PhyloArgs {
     #[arg(long)]
     pub sibling_annex: bool,
 
+    /// Write a per-family Shannon entropy report (CSV) — requires an
+    /// already-built sibling annex (`--sibling-annex` first, in this
+    /// invocation or an earlier one). Full, unsampled scan of every family;
+    /// no `--subsample`/`--entropy-bias` yet.
+    #[arg(long)]
+    pub shannon: bool,
+
     /// Distance metric to compute
     #[arg(long, value_enum, default_value = "jaccard")]
     pub metric: MetricArg,
diff --git a/src/obikmer2/src/cmd/phylo/mod.rs b/src/obikmer2/src/cmd/phylo/mod.rs
index e7d13270..d6c9ea96 100644
--- a/src/obikmer2/src/cmd/phylo/mod.rs
+++ b/src/obikmer2/src/cmd/phylo/mod.rs
@@ -58,6 +58,21 @@ pub fn run(args: PhyloArgs) {
         rep.push(t.stop());
     }
 
+    // ── Shannon entropy report (`--shannon`) ────────────────────────────────────
+    if args.shannon {
+        let path = args.output.as_ref()
+            .map(|p| format!("{}_entropy.csv", p.display()))
+            .unwrap_or_else(|| "entropy.csv".into());
+        info!("computing per-family Shannon entropy");
+        let t = Stage::start("shannon_entropy");
+        cache.shannon_entropy_csv(std::path::Path::new(&path)).unwrap_or_else(|e| {
+            eprintln!("error computing Shannon entropy: {e}");
+            std::process::exit(1);
+        });
+        rep.push(t.stop());
+        info!("entropy report → {path}");
+    }
+
     info!("computing {:?} distances for {} genome(s)", args.metric, n);
 
     let need_shared = args.shared_kmers || args.nj || args.upgma;
diff --git a/src/obikphylo/src/siblings/algorithms/entropy.rs b/src/obikphylo/src/siblings/algorithms/entropy.rs
new file mode 100644
index 00000000..a96be69d
--- /dev/null
+++ b/src/obikphylo/src/siblings/algorithms/entropy.rs
@@ -0,0 +1,132 @@
+//! Shannon entropy (bits) of a family's per-genome states, over the 15
+//! non-empty subsets of `{A,C,G,T}` — see
+//! `DevDocMD/theory/evolutionary_distances.md`, "Entropy definition", for
+//! the design discussion this implements: the project's calibrated 16-state
+//! Sankoff cost matrix treats every observed combination of bases as its
+//! own first-class state, so entropy over that same 15-symbol space (state
+//! `0`/`∅` excluded — a family is only informative among the genomes where
+//! it is actually observed) is the consistent informativeness proxy for
+//! this project, not a 4-symbol reduction.
+
+use std::collections::HashSet;
+
+use obikidxcache::index_cache::IndexCache;
+use obikindex::{OKIError, OKIResult};
+
+use super::family_scan::{Selection, scan_layer_families};
+use super::minorant_selection::non_monomorphic_reindex_layer;
+use crate::siblings::{ENTROPY_ANNEX_FILE_NAME, EntropyAnnexBuilder};
+
+/// Shannon entropy (bits) of one family's per-genome states, over the 15
+/// non-empty subsets of `{A,C,G,T}` actually observed among the genomes
+/// carrying it (`genome_mask[g] != 0`) — genomes where the family is absent
+/// are excluded from the denominator, not scored as a 16th state. `None`
+/// only if the family is absent from every genome (shouldn't happen for a
+/// real minorant, guarded against rather than dividing by zero). Second
+/// value: the denominator (`n_genomes_present`) used, for traceability.
+pub(crate) fn family_entropy(genome_mask: &[u8]) -> Option<(f64, usize)> {
+    let mut freq = [0u32; 16]; // index 0 (∅) never incremented, kept for direct `m as usize` indexing
+    let mut present = 0u32;
+    for &m in genome_mask {
+        if m != 0 {
+            freq[m as usize] += 1;
+            present += 1;
+        }
+    }
+    if present == 0 {
+        return None;
+    }
+    let mut h = 0.0f64;
+    for &c in &freq[1..] {
+        if c == 0 {
+            continue;
+        }
+        let p = f64::from(c) / f64::from(present);
+        h -= p * p.log2();
+    }
+    Some((h, present as usize))
+}
+
+/// Shannon entropy (bits) of one family's per-genome states, reduced to the
+/// 4 plain nucleotide symbols instead of the 15-state space above — kept
+/// alongside it (not instead of it) purely to measure how much the two
+/// diverge on real data. A genome carrying several bases at once
+/// (`genome_mask[g]` with more than one bit set) contributes to *each* of
+/// those bases' counts — counted once per base present, not split
+/// fractionally, and not folded into a combined state the way
+/// [`family_entropy`] does. The denominator is therefore the total number
+/// of base-occurrences summed over every present genome, not the genome
+/// count. `None` under the same condition as [`family_entropy`].
+pub(crate) fn family_entropy_4(genome_mask: &[u8]) -> Option<(f64, usize)> {
+    let mut freq = [0u32; 4];
+    let mut total = 0u32;
+    for &m in genome_mask {
+        for (base, count) in freq.iter_mut().enumerate() {
+            if m & (1 << base) != 0 {
+                *count += 1;
+                total += 1;
+            }
+        }
+    }
+    if total == 0 {
+        return None;
+    }
+    let mut h = 0.0f64;
+    for &c in &freq {
+        if c == 0 {
+            continue;
+        }
+        let p = f64::from(c) / f64::from(total);
+        h -= p * p.log2();
+    }
+    Some((h, total as usize))
+}
+
+/// Builds one layer's entropy annex if it doesn't already exist — a no-op,
+/// cheap existence check, once it does. Bounded to non-monomorphic
+/// minorants only ([`non_monomorphic_reindex_layer`], a cheap annex-only
+/// pass — no cross-partition resolution): monomorphism is already known
+/// from the annex bits alone, so there is no reason to pay
+/// `scan_layer_families`'s expensive per-genome resolution for the ~98% of
+/// minorants that are monomorphic only to discard the result.
+pub(crate) fn ensure_layer_entropy_annex(
+    cache: &IndexCache,
+    partition: usize,
+    layer_idx: usize,
+    n_genomes: usize,
+    fast_mode: bool,
+) -> OKIResult<()> {
+    let layer = cache
+        .get_layer(partition, layer_idx)
+        .expect("cache-consistent: caller already bounds layer_idx by n_layer(partition)");
+    let path = layer.dir().join(ENTROPY_ANNEX_FILE_NAME);
+    if path.exists() {
+        return Ok(());
+    }
+
+    let reindex = non_monomorphic_reindex_layer(layer.dir())?;
+    let mut builder = EntropyAnnexBuilder::new(reindex.len(), &path).map_err(OKIError::Io)?;
+    let eligible: HashSet<usize> = reindex.keys().copied().collect();
+
+    scan_layer_families(
+        cache,
+        partition,
+        layer_idx,
+        n_genomes,
+        fast_mode,
+        &Selection::Some(&eligible),
+        |family_idx, mask, genome_mask| {
+            debug_assert!(
+                mask.family_size() >= 2,
+                "Selection::Some(eligible) must only visit non-monomorphic minorants"
+            );
+            if let Some((h15, _)) = family_entropy(genome_mask) {
+                let compact = reindex[&family_idx];
+                builder.set(compact, h15 as f32);
+            }
+        },
+    )?;
+    builder.close().map_err(OKIError::Io)?;
+
+    Ok(())
+}
diff --git a/src/obikphylo/src/siblings/algorithms/family_scan.rs b/src/obikphylo/src/siblings/algorithms/family_scan.rs
index 53f426d0..692b7272 100644
--- a/src/obikphylo/src/siblings/algorithms/family_scan.rs
+++ b/src/obikphylo/src/siblings/algorithms/family_scan.rs
@@ -30,12 +30,13 @@
 //! (`n_workers` batches), not by the layer's size.
 //!
 //! The pipeline's own working context (`LayerCtx`) needs an owned,
-//! `Send + 'static` `KmerLayer` — `IndexCache`'s own copy of this same layer
-//! is only ever borrowed (`&'_ KmerLayer`, tied to the caller's stack frame),
-//! so it can't be moved into the worker threads `Pipe::apply` spawns; this
-//! reopens the layer once, independently, for that purpose. Cross-partition
-//! resolution, by contrast, runs synchronously on the calling thread (via
-//! `rayon::scope`-free `par_iter`, no `thread::spawn`), so it can — and
+//! `Send + 'static` handle on the source layer — a plain `&KmerLayer`
+//! borrowed from `cache` is tied to the caller's stack frame, so it can't be
+//! moved into the worker threads `Pipe::apply` spawns. `IndexCache::get_layer`
+//! hands out a cheap `Arc<KmerLayer>` clone (every `IndexCache` accessor
+//! does, see its own docs), so no reopening/re-mmap'ing is needed.
+//! Cross-partition resolution, by contrast, runs synchronously on the
+//! calling thread (`rayon::par_iter`, no `thread::spawn`), so it can — and
 //! does — borrow `cache` directly for the whole call.
 
 use std::collections::HashMap;
@@ -55,11 +56,26 @@ use crate::siblings::helpers::central_base;
 use crate::siblings::iter::{SiblingEntry, SiblingLayerExt};
 use crate::siblings::{ANNEX_FILE_NAME, FamilyMask, SiblingAnnex};
 
-/// Families per batch — trades off per-batch memory (`FAMILY_BATCH *
-/// genome_count` bytes) against giving each partition a decent number of
-/// queries to resolve in one go; too small a batch starves that density
-/// regardless of how the resolution step is parallelised.
-const FAMILY_BATCH: usize = 65536;
+/// Target memory budget for one batch's `genome_mask` allocation
+/// (`family_batch_size(n_genomes) * n_genomes` bytes, see below) — bounding
+/// *that*, not the family count directly, is what actually matters: a fixed
+/// family-count batch (this module's original `FAMILY_BATCH = 65536`) costs
+/// ~64KB/batch at a handful of genomes but ~655MB/batch at the 10,000-skim-
+/// genome scale this project targets, with `n_workers` such batches
+/// potentially in flight at once (see [`scan_layer_families`]'s own
+/// `throttle` call) — the same fixed constant silently stops being a small
+/// number once genome count grows, for no reason the batch size itself
+/// expresses.
+const FAMILY_BATCH_BYTES: usize = 16 * 1024 * 1024;
+
+/// Families per batch, derived from [`FAMILY_BATCH_BYTES`] and this run's
+/// own genome count — clamped so a very small genome count doesn't produce
+/// an absurdly large batch (upper bound: the original fixed default, still
+/// a reasonable ceiling once genome count is negligible) nor a very large
+/// one starve per-partition query density down to nothing (lower bound).
+fn family_batch_size(n_genomes: usize) -> usize {
+    (FAMILY_BATCH_BYTES / n_genomes.max(1)).clamp(1024, 65536)
+}
 
 /// Restricts [`scan_layer_families`] to a subset of a layer's families,
 /// identified by their iteration-order index. `All` (the only variant any
@@ -82,7 +98,7 @@ fn is_selected(selected: &Option<std::collections::HashSet<usize>>, family_idx:
 /// `cache` here: generation never touches the cross-partition cache, only
 /// this layer's own already-open matrix.
 struct LayerCtx {
-    mat: KmerLayer,
+    mat: Arc<KmerLayer>,
     n_parts: usize,
     n_genomes: usize,
     n_cols: usize,
@@ -121,8 +137,10 @@ enum FamData {
 }
 
 /// Visits every minorant family of one layer, in iteration order, batched
-/// [`FAMILY_BATCH`] at a time — see the module docs for why generation and
-/// resolution use different concurrency. `on_family` is called once per
+/// [`family_batch_size`] families at a time — see the module docs for why
+/// that's derived from `n_genomes` rather than fixed, and for why
+/// generation and resolution use different concurrency. `on_family` is
+/// called once per
 /// family, in iteration order, with that family's iteration-order index
 /// (the same numbering [`Selection`] indices are drawn from), its own annex
 /// mask, and its per-genome base-presence (`genome_mask[g]`: bit `b` set
@@ -147,17 +165,15 @@ pub(crate) fn scan_layer_families(
     let n_parts = cache.index().n_partitions();
     let k = cache.index().kmer_size();
 
-    // An owned, independently-opened copy of this same layer — the shared
-    // `cache`'s own copy is only ever borrowed, so it can't be moved into
-    // the `Send + 'static` pipeline workers below (see the module docs).
-    let partition = cache
-        .index()
-        .partition(source_partition)
-        .map_err(|e| OKIError::InvalidInput(format!("partition {source_partition}: {e}")))?;
-    let mat = partition
-        .layer(source_layer)
-        .and_then(KmerLayer::open)
-        .map_err(|e| OKIError::InvalidInput(format!("layer {source_layer}: {e}")))?;
+    // A cheap `Arc` clone of the cache's own already-open layer — `Send +
+    // 'static`, so it can move into the pipeline workers below without
+    // reopening/re-mmap'ing anything (see the module docs and
+    // `IndexCache::get_layer_arc`'s own).
+    let mat = cache.get_layer(source_partition, source_layer).ok_or_else(|| {
+        OKIError::InvalidInput(format!(
+            "layer {source_layer} of partition {source_partition} not in this cache"
+        ))
+    })?;
     let n_cols = mat.n_cols().min(n_genomes);
     let layer_dir: &Path = mat.dir();
     let annex = Arc::new(SiblingAnnex::open(&layer_dir.join(ANNEX_FILE_NAME)).map_err(OKIError::Io)?);
@@ -170,7 +186,7 @@ pub(crate) fn scan_layer_families(
     // billions of k-mers.
     let batches = ctx
         .mat
-        .iter_minorants_batch(annex, FAMILY_BATCH)
+        .iter_minorants_batch(annex, family_batch_size(n_genomes))
         .scan(0usize, |offset, batch| {
             let start = *offset;
             *offset += batch.len();
@@ -367,7 +383,7 @@ fn resolve_partition_batch(
                 .zip(entries.iter())
                 .map(|(slot, &(_, family_idx, base))| (slot, family_idx, base))
                 .collect();
-            resolve_layer_hits(layer, &hits, n_genomes, &mut on_hit);
+            resolve_layer_hits(&layer, &hits, n_genomes, &mut on_hit);
         }
     } else {
         // First hit wins, same semantics as the fast path — a variant
@@ -391,7 +407,7 @@ fn resolve_partition_batch(
             let layer = cache
                 .get_layer(dest_partition, li)
                 .expect("cache-consistent: n_layer(dest_partition) already bounds li");
-            resolve_layer_hits(layer, &hits, n_genomes, &mut on_hit);
+            resolve_layer_hits(&layer, &hits, n_genomes, &mut on_hit);
         }
     }
 }
@@ -406,6 +422,18 @@ fn resolve_layer_hits(
     n_genomes: usize,
     on_hit: &mut impl FnMut(usize, u8, usize),
 ) {
+    // TODO(memory): `carries` is `n_cols * hits.len()` `bool`s — Rust's
+    // `Vec<bool>` is 1 byte/entry, not bit-packed, so this spends a whole
+    // byte per bit of real information. Worst case (all of one batch's
+    // variants landing on the same destination layer) at 10,000 genomes:
+    // tens to low hundreds of MB per call (see the conversation that
+    // measured this — `KmerLayer::Count`'s own `fill_sub_matrix_carries`
+    // transiently doubles that again via an intermediate `Vec<Vec<u32>>`).
+    // A real fix means `fill_sub_matrix_carries`'s signature changing to a
+    // bit-packed output (e.g. `bitvec`/a raw `Vec<u64>` bitmask) — that's
+    // an `obikindex` API change (`content_layer.rs`), not something this
+    // call site can fix alone; not done yet, flagged for when genome-count
+    // scale makes it worth the churn.
     let n_cols = layer.n_cols().min(n_genomes);
     let slots: Vec<usize> = hits.iter().map(|&(slot, _, _)| slot).collect();
     let mut carries: Vec<Vec<bool>> = (0..n_cols).map(|_| Vec::new()).collect();
diff --git a/src/obikphylo/src/siblings/algorithms/minorant_selection.rs b/src/obikphylo/src/siblings/algorithms/minorant_selection.rs
new file mode 100644
index 00000000..7fd62c09
--- /dev/null
+++ b/src/obikphylo/src/siblings/algorithms/minorant_selection.rs
@@ -0,0 +1,49 @@
+//! Cheap, purely structural (annex-bits-only, no cross-partition
+//! resolution) passes over an already-built sibling annex — monomorphism
+//! (`FamilyMask::family_size() < 2`) is knowable straight from the annex's
+//! own bits, so a downstream consumer that only cares about non-monomorphic
+//! families never needs to pay [`super::family_scan::scan_layer_families`]'s
+//! expensive cross-partition sweep just to find that out.
+
+use std::collections::HashMap;
+use std::path::Path;
+
+use obikindex::{OKIError, OKIResult};
+
+use crate::siblings::{ANNEX_FILE_NAME, FamilyMask, SiblingAnnex};
+
+fn is_non_monomorphic_minorant(mask: FamilyMask) -> bool {
+    mask.is_minorant() && mask.siblings() >= 1
+}
+
+/// `family_idx` (`scan_layer_families`'s own numbering — every minorant,
+/// monomorphic included) -> a compact ordinal among *non-monomorphic*
+/// minorants only, for one layer. One streamed pass over the layer's
+/// already-built sibling annex; no `IndexCache`, no cross-partition
+/// resolution.
+///
+/// Exists so a downstream per-family array (the entropy annex) can be sized
+/// to the non-monomorphic count alone (~2% of a layer's minorants, measured
+/// on real data) instead of wasting an entry per monomorphic minorant, which
+/// never carries a meaningful value anyway (entropy is trivially undefined
+/// for a family with no sibling). The map's own key set doubles as exactly
+/// the `Selection` a caller needs to bound `scan_layer_families` to just
+/// these families.
+pub(crate) fn non_monomorphic_reindex_layer(layer_dir: &Path) -> OKIResult<HashMap<usize, usize>> {
+    let annex = SiblingAnnex::open(&layer_dir.join(ANNEX_FILE_NAME)).map_err(OKIError::Io)?;
+    let mut reindex = HashMap::new();
+    let mut family_idx = 0usize;
+    for slot in 0..annex.len() {
+        let Some(mask) = annex.get(slot) else { continue };
+        if !mask.is_minorant() {
+            continue; // not a minorant at all — doesn't advance `family_idx` either
+        }
+        let this_family_idx = family_idx;
+        family_idx += 1;
+        if is_non_monomorphic_minorant(mask) {
+            let compact = reindex.len();
+            reindex.insert(this_family_idx, compact);
+        }
+    }
+    Ok(reindex)
+}
diff --git a/src/obikphylo/src/siblings/algorithms/mod.rs b/src/obikphylo/src/siblings/algorithms/mod.rs
index 37595e25..91daf397 100644
--- a/src/obikphylo/src/siblings/algorithms/mod.rs
+++ b/src/obikphylo/src/siblings/algorithms/mod.rs
@@ -1,13 +1,34 @@
-//! Sibling-annex construction algorithm: the cross-partition sweep that
-//! fills in one layer's [`crate::siblings::FamilyMask`] slots — driven, one
-//! layer at a time, by
-//! [`crate::siblings::extensions::SiblingExt`]. Kept apart from
-//! that trait (and from the `siblingannex`/`helpers` data model) the same
-//! way `obikindexer` splits `algorithms` from `extensions`: this module
-//! depends on the data model, never the reverse.
+//! Sibling-annex construction algorithms — driven, one layer (or one
+//! partition/layer sweep) at a time, by the traits in
+//! [`crate::siblings::extensions`]. Kept apart from those traits (and from
+//! the `siblingannex`/`entropy_annex`/`helpers` data model) the same way
+//! `obikindexer` splits `algorithms` from `extensions`: this module depends
+//! on the data model, never the reverse.
+//!
+//! [`annex`] builds the sibling-count/minorant annex itself; [`family_scan`]
+//! is the shared cross-partition traversal every other consumer
+//! (`entropy`, and future ones — cardinality, alignment, stats) is built
+//! on; [`minorant_selection`] and [`entropy`] are the first such consumer.
 
 mod annex;
+mod entropy;
 mod family_scan;
+mod minorant_selection;
+
+use obikidxcache::index_cache::IndexCache;
 
 pub(crate) use annex::build_layer_sibling_annex;
+pub(crate) use entropy::{ensure_layer_entropy_annex, family_entropy, family_entropy_4};
 pub(crate) use family_scan::{Selection, scan_layer_families};
+
+/// Whether every layer number in `cache` fits in a `FamilyMask` field
+/// (`< 7`) — a single fact about the whole index, decided once here so
+/// every writer/reader of a per-layer `FamilyMask` field agrees. Layer
+/// count is guaranteed identical across every partition (a merge adds one
+/// layer to all of them at once), so the max over cached partitions speaks
+/// for the whole index. Shared by every `extensions` impl that writes or
+/// interprets that field (`SiblingExt::build_sibling_annex`,
+/// `SiblingBuilder::ensure_entropy_annexes`), so they can never disagree.
+pub(crate) fn is_fast_mode(cache: &IndexCache) -> bool {
+    cache.partitions().filter_map(|p| cache.n_layer(p)).max().unwrap_or(0) <= 7
+}
diff --git a/src/obikphylo/src/siblings/entropy_annex.rs b/src/obikphylo/src/siblings/entropy_annex.rs
new file mode 100644
index 00000000..3e89fa48
--- /dev/null
+++ b/src/obikphylo/src/siblings/entropy_annex.rs
@@ -0,0 +1,153 @@
+//! Persisted, per-layer Shannon entropy (`entropy15`, see
+//! `algorithms::entropy`), one value per **non-monomorphic minorant
+//! family** — compact: indexed by the dedicated ordinal
+//! `algorithms::non_monomorphic_reindex_layer` computes (skips monomorphic
+//! minorants entirely, ~98% of minorants on real data), never by
+//! `scan_layer_families`'s own `family_idx` (which counts every minorant,
+//! monomorphic included).
+//!
+//! Exists so repeated reads of the same layer's entropy values (e.g.
+//! entropy-biased family selection, once reconnected) are a plain
+//! positional mmap lookup instead of re-paying `scan_layer_families`'s
+//! cross-partition resolution — building this file costs exactly that same
+//! full scan the first time, it does not make any single computation of
+//! entropy itself faster.
+//!
+//! Mirrors `SiblingAnnex`'s on-disk convention (magic + header + flat
+//! per-entry data, read-only after build), just with `f32` payloads and a
+//! different (compact) numbering — see the module docs above.
+
+use std::fs::{File, OpenOptions};
+use std::io;
+use std::path::Path;
+
+use memmap2::{Mmap, MmapMut};
+
+const MAGIC: [u8; 4] = *b"PENT";
+
+// Header: magic(4) + _pad(4) + n(8) = 16 bytes. Data (4 bytes/entry) follows.
+const HEADER_SIZE: usize = 16;
+
+/// Sentinel: not yet computed. Entropy is always `>= 0.0`, so any negative
+/// value is unambiguous — chosen over `NaN` to avoid `NaN`-comparison
+/// footguns at every read site.
+const SENTINEL: f32 = -1.0;
+
+pub(crate) const ENTROPY_ANNEX_FILE_NAME: &str = "entropy.pent";
+
+pub(crate) struct EntropyAnnex {
+    mmap: Mmap,
+    n: usize,
+}
+
+impl EntropyAnnex {
+    pub(crate) fn open(path: &Path) -> io::Result<Self> {
+        let mmap = unsafe { Mmap::map(&File::open(path)?)? };
+        if mmap.len() < HEADER_SIZE {
+            return Err(io::Error::new(io::ErrorKind::InvalidData, "PENT file too short"));
+        }
+        if mmap[0..4] != MAGIC {
+            return Err(io::Error::new(io::ErrorKind::InvalidData, "bad PENT magic"));
+        }
+        let n = u64::from_le_bytes(mmap[8..16].try_into().unwrap()) as usize;
+        if mmap.len() < HEADER_SIZE + n * 4 {
+            return Err(io::Error::new(io::ErrorKind::InvalidData, "PENT file truncated"));
+        }
+        Ok(Self { mmap, n })
+    }
+
+    pub(crate) fn len(&self) -> usize {
+        self.n
+    }
+
+    /// `None` for the sentinel (not yet computed — shouldn't happen against
+    /// a fully-built annex).
+    pub(crate) fn get(&self, idx: usize) -> Option<f32> {
+        let off = HEADER_SIZE + idx * 4;
+        let v = f32::from_le_bytes(self.mmap[off..off + 4].try_into().unwrap());
+        (v >= 0.0).then_some(v)
+    }
+}
+
+pub(crate) struct EntropyAnnexBuilder {
+    mmap: MmapMut,
+}
+
+impl EntropyAnnexBuilder {
+    /// Create a new annex of `n` entries (a layer's non-monomorphic
+    /// minorant count) at `path`, pre-initialised to the sentinel.
+    pub(crate) fn new(n: usize, path: &Path) -> io::Result<Self> {
+        let file_size = HEADER_SIZE + n * 4;
+        let file = OpenOptions::new()
+            .read(true).write(true).create(true).truncate(true)
+            .open(path)?;
+        file.set_len(file_size as u64)?;
+        let mut mmap = unsafe { MmapMut::map_mut(&file)? };
+        mmap[0..4].copy_from_slice(&MAGIC);
+        mmap[4..8].copy_from_slice(&[0u8; 4]);
+        mmap[8..16].copy_from_slice(&(n as u64).to_le_bytes());
+        for i in 0..n {
+            let off = HEADER_SIZE + i * 4;
+            mmap[off..off + 4].copy_from_slice(&SENTINEL.to_le_bytes());
+        }
+        Ok(Self { mmap })
+    }
+
+    pub(crate) fn set(&mut self, idx: usize, value: f32) {
+        debug_assert!(value >= 0.0, "entropy must be non-negative, got {value}");
+        let off = HEADER_SIZE + idx * 4;
+        self.mmap[off..off + 4].copy_from_slice(&value.to_le_bytes());
+    }
+
+    pub(crate) fn close(self) -> io::Result<()> {
+        self.mmap.flush()
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use tempfile::tempdir;
+
+    #[test]
+    fn sentinel_is_negative_and_unset_entries_read_as_none() {
+        let dir = tempdir().unwrap();
+        let path = dir.path().join("test.pent");
+        let builder = EntropyAnnexBuilder::new(4, &path).unwrap();
+        builder.close().unwrap();
+        let annex = EntropyAnnex::open(&path).unwrap();
+        for i in 0..4 {
+            assert_eq!(annex.get(i), None);
+        }
+    }
+
+    #[test]
+    fn roundtrip_including_zero_entropy() {
+        let dir = tempdir().unwrap();
+        let path = dir.path().join("test.pent");
+        let mut builder = EntropyAnnexBuilder::new(4, &path).unwrap();
+        let values = [0.0f32, 1.5, 3.9, 0.0001];
+        for (i, &v) in values.iter().enumerate() {
+            builder.set(i, v);
+        }
+        builder.close().unwrap();
+        let annex = EntropyAnnex::open(&path).unwrap();
+        for (i, &v) in values.iter().enumerate() {
+            assert_eq!(annex.get(i), Some(v), "entry {i}");
+        }
+    }
+
+    #[test]
+    fn mixed_sentinel_and_real_values() {
+        let dir = tempdir().unwrap();
+        let path = dir.path().join("test.pent");
+        let mut builder = EntropyAnnexBuilder::new(3, &path).unwrap();
+        builder.set(1, 2.0);
+        builder.close().unwrap();
+        let annex = EntropyAnnex::open(&path).unwrap();
+        assert_eq!(annex.get(0), None);
+        assert_eq!(annex.get(1), Some(2.0));
+        assert_eq!(annex.get(2), None);
+        assert_eq!(annex.len(), 3);
+    }
+}
diff --git a/src/obikphylo/src/siblings/extensions/annex_build.rs b/src/obikphylo/src/siblings/extensions/annex_build.rs
deleted file mode 100644
index acd2519b..00000000
--- a/src/obikphylo/src/siblings/extensions/annex_build.rs
+++ /dev/null
@@ -1,79 +0,0 @@
-//! [`SiblingExt`] — the public entry point for building the
-//! sibling-count/minorant annex of an already-cached index. Thin: it only
-//! walks `cache`'s partitions/layers and delegates each layer's actual
-//! cross-partition sweep to [`crate::siblings::algorithms::build_layer_sibling_annex`]
-//! — same split as `obikphylo::distance::Metrics`, whose `IndexCache`
-//! extension impl is itself thin dispatch onto `obicompactvec`'s
-//! finalisation methods.
-
-use obikidxcache::index_cache::IndexCache;
-use obikindex::OKIResult;
-use obisys::progress_bar;
-
-use crate::siblings::algorithms::build_layer_sibling_annex;
-
-/// Adds [`build_sibling_annex`](Self::build_sibling_annex) to `IndexCache`.
-pub trait SiblingExt {
-    /// Build the sibling-count/minorant annex for every layer of every
-    /// cached partition, writing one annex file per layer alongside its
-    /// existing index files. Safe to call again later (e.g. after a fresh
-    /// merge, on a freshly-rebuilt `IndexCache`) — each run simply
-    /// overwrites the annex files of the layers it covers.
-    ///
-    /// Construction only — no statistics gathered here on purpose: this is
-    /// meant to run routinely (it is the artefact the SNP-family distances
-    /// consume), while the sibling-count distribution is a separate,
-    /// occasional diagnostic pass over the result.
-    ///
-    /// Cross-partition/cross-layer lookups are required (a k-mer's siblings
-    /// can live in any partition), but the layer loop itself — and thus the
-    /// annex file this produces — stays local to one layer at a time.
-    fn build_sibling_annex(&self) -> OKIResult<()>;
-}
-
-impl SiblingExt for IndexCache {
-    fn build_sibling_annex(&self) -> OKIResult<()> {
-        let n_parts = self.index().n_partitions();
-
-        // Whether every layer number fits in a `FamilyMask` field (`< 7`) —
-        // a single fact about the whole index, decided once here so the
-        // writer (this pass) and every future reader agree. Layer count is
-        // guaranteed identical across every partition (a merge adds one
-        // layer to all of them at once), so the max over cached partitions
-        // speaks for the whole index.
-        let fast_mode = self
-            .partitions()
-            .filter_map(|p| self.n_layer(p))
-            .max()
-            .unwrap_or(0)
-            <= 7;
-        if !fast_mode {
-            tracing::warn!(
-                "index has more than 7 layers per partition — sibling-annex fast layer lookup \
-                 disabled; consider compacting this index"
-            );
-        }
-
-        let pb = progress_bar("sibling_annex", self.n_partition() as u64, "partitions");
-        let mut total_slots: u64 = 0;
-        for part in self.partitions() {
-            let n_layer = self.n_layer(part).unwrap_or(0);
-            let mut part_slots: u64 = 0;
-            for l in 0..n_layer {
-                let layer = self
-                    .get_layer(part, l)
-                    .expect("cache-consistent: n_layer(part) already bounds l");
-                part_slots += build_layer_sibling_annex(self, layer, l, n_parts, fast_mode)?;
-            }
-            total_slots += part_slots;
-            pb.inc(1);
-            pb.set_message(format!(
-                "partition {part}: {part_slots} kmers ({total_slots} total)"
-            ));
-        }
-        pb.finish_and_clear();
-        tracing::info!("sibling annex built — {total_slots} kmers across {n_parts} partitions");
-
-        Ok(())
-    }
-}
diff --git a/src/obikphylo/src/siblings/extensions/mod.rs b/src/obikphylo/src/siblings/extensions/mod.rs
index d5be0833..74a0aa94 100644
--- a/src/obikphylo/src/siblings/extensions/mod.rs
+++ b/src/obikphylo/src/siblings/extensions/mod.rs
@@ -4,6 +4,8 @@
 //! other `obikindex`/`obikidxcache` extension trait in this codebase (see
 //! `obikphylo::distance::Metrics`, `obikindexer::extensions`).
 
-mod annex_build;
+mod sibling_builder;
+mod sibling_ext;
 
-pub use annex_build::SiblingExt;
+pub use sibling_ext::SiblingExt;
+pub(crate) use sibling_builder::SiblingBuilder;
diff --git a/src/obikphylo/src/siblings/extensions/sibling_builder.rs b/src/obikphylo/src/siblings/extensions/sibling_builder.rs
new file mode 100644
index 00000000..b8915941
--- /dev/null
+++ b/src/obikphylo/src/siblings/extensions/sibling_builder.rs
@@ -0,0 +1,40 @@
+//! [`SiblingBuilder`] — construction triggered *internally*, as a side
+//! effect of a [`crate::siblings::extensions::SiblingExt`] method (e.g.
+//! `shannon_entropy_csv` ensuring its layers' entropy annexes exist before
+//! reading them), never called directly from outside this crate. Unlike
+//! `SiblingExt` (explicit, CLI-facing actions), nothing here is meant to be
+//! its own CLI flag — same `pub`/`pub(crate)` split `obikindexer` uses for
+//! its own `IndexBuilder`/`PrivateBuilder`.
+
+use obikidxcache::index_cache::IndexCache;
+use obikindex::OKIResult;
+use obisys::progress_bar;
+
+use crate::siblings::algorithms::{ensure_layer_entropy_annex, is_fast_mode};
+
+pub(crate) trait SiblingBuilder {
+    /// Build every cached layer's entropy annex that doesn't already exist
+    /// — a no-op, cheap existence check per layer, once every layer's
+    /// annex file exists. See `ensure_layer_entropy_annex`'s own docs for
+    /// why this is bounded to non-monomorphic minorants only.
+    fn ensure_entropy_annexes(&self) -> OKIResult<()>;
+}
+
+impl SiblingBuilder for IndexCache {
+    fn ensure_entropy_annexes(&self) -> OKIResult<()> {
+        let n_genomes = self.meta().genomes().len();
+        let fast_mode = is_fast_mode(self);
+
+        let pb = progress_bar("entropy_annex", self.n_partition() as u64, "partitions");
+        for part in self.partitions() {
+            let n_layer = self.n_layer(part).unwrap_or(0);
+            for l in 0..n_layer {
+                ensure_layer_entropy_annex(self, part, l, n_genomes, fast_mode)?;
+            }
+            pb.inc(1);
+        }
+        pb.finish_and_clear();
+
+        Ok(())
+    }
+}
diff --git a/src/obikphylo/src/siblings/extensions/sibling_ext.rs b/src/obikphylo/src/siblings/extensions/sibling_ext.rs
new file mode 100644
index 00000000..9bf2a98a
--- /dev/null
+++ b/src/obikphylo/src/siblings/extensions/sibling_ext.rs
@@ -0,0 +1,170 @@
+//! [`SiblingExt`] — public, explicit, CLI-facing operations on an
+//! already-cached index: today, building the sibling-count/minorant annex
+//! and computing per-family Shannon entropy. Thin: each method only walks
+//! `cache`'s partitions/layers and delegates the actual per-layer work to
+//! [`crate::siblings::algorithms`] — same split as
+//! `obikphylo::distance::Metrics`, whose `IndexCache` extension impl is
+//! itself thin dispatch onto `obicompactvec`'s finalisation methods.
+
+use std::io::{BufWriter, Write};
+use std::path::Path;
+
+use obikidxcache::index_cache::IndexCache;
+use obikindex::{OKIError, OKIResult};
+use obisys::progress_bar;
+
+use crate::siblings::algorithms::{
+    build_layer_sibling_annex, family_entropy, family_entropy_4, is_fast_mode, scan_layer_families,
+    Selection,
+};
+use crate::siblings::extensions::SiblingBuilder;
+use crate::siblings::ENTROPY_ANNEX_FILE_NAME;
+
+pub trait SiblingExt {
+    /// Build the sibling-count/minorant annex for every layer of every
+    /// cached partition, writing one annex file per layer alongside its
+    /// existing index files, then the entropy annex on top of it
+    /// ([`SiblingBuilder::ensure_entropy_annexes`] — precomputed here rather
+    /// than left to build lazily on first `--shannon`/`--entropy-bias` use).
+    /// Safe to call again later (e.g. after a fresh merge, on a
+    /// freshly-rebuilt `IndexCache`) — each run simply overwrites the
+    /// sibling-annex files of the layers it covers, and rebuilds any
+    /// entropy annex that becomes stale as a result (a fresh sibling annex
+    /// makes the previous entropy annex's `family_idx` numbering
+    /// untrustworthy, so it must be rebuilt too, not merely left in place).
+    ///
+    /// No statistics gathered here on purpose: this is meant to run
+    /// routinely (both annexes are inputs the SNP-family distances and
+    /// entropy report consume), while the sibling-count distribution is a
+    /// separate, occasional diagnostic pass over the result.
+    ///
+    /// Cross-partition/cross-layer lookups are required (a k-mer's siblings
+    /// can live in any partition), but the layer loop itself — and thus the
+    /// annex file this produces — stays local to one layer at a time.
+    fn build_sibling_annex(&self) -> OKIResult<()>;
+
+    /// Write a CSV (`partition,layer,family_idx,entropy15,entropy4,
+    /// family_size,n_genomes_present`) of per-family Shannon entropy — both
+    /// [`family_entropy`] (15 non-empty states, the project's settled
+    /// definition) and [`family_entropy_4`] (plain 4-symbol reduction, kept
+    /// alongside for comparison) — one row per non-monomorphic minorant of
+    /// every cached layer, from an already-built sibling annex (run
+    /// [`build_sibling_annex`](Self::build_sibling_annex) first).
+    ///
+    /// Full, unsampled scan of every cached layer (`Selection::All`) — no
+    /// `--subsample`/`--entropy-bias` yet (see the project memory on this).
+    fn shannon_entropy_csv(&self, path: &Path) -> OKIResult<()>;
+}
+
+impl SiblingExt for IndexCache {
+    fn build_sibling_annex(&self) -> OKIResult<()> {
+        let n_parts = self.index().n_partitions();
+        let fast_mode = is_fast_mode(self);
+        if !fast_mode {
+            tracing::warn!(
+                "index has more than 7 layers per partition — sibling-annex fast layer lookup \
+                 disabled; consider compacting this index"
+            );
+        }
+
+        let pb = progress_bar("sibling_annex", self.n_partition() as u64, "partitions");
+        let mut total_slots: u64 = 0;
+        for part in self.partitions() {
+            let n_layer = self.n_layer(part).unwrap_or(0);
+            let mut part_slots: u64 = 0;
+            for l in 0..n_layer {
+                let layer = self
+                    .get_layer(part, l)
+                    .expect("cache-consistent: n_layer(part) already bounds l");
+                part_slots += build_layer_sibling_annex(self, &layer, l, n_parts, fast_mode)?;
+
+                // Invalidate this layer's entropy annex, if any — it was
+                // built against the sibling annex's *previous*
+                // `family_idx` numbering (`non_monomorphic_reindex_layer`
+                // reads the sibling annex), now stale. `ensure_entropy_annexes`
+                // below only builds what's *missing*, so the old file must
+                // be removed here for it to be rebuilt rather than silently
+                // left stale.
+                let entropy_path = layer.dir().join(ENTROPY_ANNEX_FILE_NAME);
+                if entropy_path.exists() {
+                    std::fs::remove_file(&entropy_path).map_err(OKIError::Io)?;
+                }
+            }
+            total_slots += part_slots;
+            pb.inc(1);
+            pb.set_message(format!(
+                "partition {part}: {part_slots} kmers ({total_slots} total)"
+            ));
+        }
+        pb.finish_and_clear();
+        tracing::info!("sibling annex built — {total_slots} kmers across {n_parts} partitions");
+
+        // Precompute the entropy annex too, now that every layer's fresh
+        // sibling annex is on disk (`ensure_entropy_annexes` reads it via
+        // `non_monomorphic_reindex_layer`) — same routine-rebuild footing
+        // as the sibling annex itself, rather than leaving it to be built
+        // lazily on first `--shannon`/`--entropy-bias` use.
+        tracing::info!("building entropy annex");
+        self.ensure_entropy_annexes()?;
+
+        Ok(())
+    }
+
+    fn shannon_entropy_csv(&self, path: &Path) -> OKIResult<()> {
+        // Entropy annexes aren't this method's own output, but building
+        // them is exactly the same full scan this method is about to run
+        // anyway — see `SiblingBuilder::ensure_entropy_annexes`'s own docs
+        // for why that's a side effect worth taking here rather than a
+        // separate, explicit step.
+        self.ensure_entropy_annexes()?;
+
+        let n_genomes = self.meta().genomes().len();
+        let fast_mode = is_fast_mode(self);
+
+        let mut f = BufWriter::new(std::fs::File::create(path).map_err(OKIError::Io)?);
+        writeln!(f, "partition,layer,family_idx,entropy15,entropy4,family_size,n_genomes_present")
+            .map_err(OKIError::Io)?;
+
+        let pb = progress_bar("shannon_entropy", self.n_partition() as u64, "partitions");
+        for part in self.partitions() {
+            let n_layer = self.n_layer(part).unwrap_or(0);
+            for l in 0..n_layer {
+                let mut write_err = None;
+                scan_layer_families(
+                    self,
+                    part,
+                    l,
+                    n_genomes,
+                    fast_mode,
+                    &Selection::All,
+                    |family_idx, mask, genome_mask| {
+                        if write_err.is_some() {
+                            return;
+                        }
+                        if mask.family_size() < 2 {
+                            return; // monomorphic family — entropy trivially 0, no signal, skip
+                        }
+                        let Some((h15, n_present)) = family_entropy(genome_mask) else {
+                            return;
+                        };
+                        let h4 = family_entropy_4(genome_mask).map_or(0.0, |(h, _)| h);
+                        if let Err(e) = writeln!(
+                            f,
+                            "{part},{l},{family_idx},{h15:.6},{h4:.6},{},{n_present}",
+                            mask.family_size()
+                        ) {
+                            write_err = Some(e);
+                        }
+                    },
+                )?;
+                if let Some(e) = write_err {
+                    return Err(OKIError::Io(e));
+                }
+            }
+            pb.inc(1);
+        }
+        pb.finish_and_clear();
+
+        Ok(())
+    }
+}
diff --git a/src/obikphylo/src/siblings/mod.rs b/src/obikphylo/src/siblings/mod.rs
index 5d3377a1..8b37c931 100644
--- a/src/obikphylo/src/siblings/mod.rs
+++ b/src/obikphylo/src/siblings/mod.rs
@@ -21,10 +21,12 @@
 
 pub mod algorithms;
 pub mod extensions;
+mod entropy_annex;
 mod helpers;
 mod iter;
 mod siblingannex;
 
+pub(crate) use entropy_annex::{ENTROPY_ANNEX_FILE_NAME, EntropyAnnex, EntropyAnnexBuilder};
 pub(crate) use siblingannex::{FamilyMask, SiblingAnnex, SiblingAnnexBuilder};
 
 pub use extensions::SiblingExt;