diff --git a/examples/connectome-fly/src/analysis/mod.rs b/examples/connectome-fly/src/analysis/mod.rs
index 2c547859..75dc76b3 100644
--- a/examples/connectome-fly/src/analysis/mod.rs
+++ b/examples/connectome-fly/src/analysis/mod.rs
@@ -17,6 +17,7 @@
 pub mod gpu;
 pub mod motif;
 pub mod partition;
+pub mod rate_encoder;
 pub mod structural;
 pub mod types;
 
@@ -95,6 +96,19 @@ impl Analysis {
             &self.cfg, &self.sdpa, &self.w_q, &self.w_k, &self.w_v, conn, spikes, k,
         )
     }
+
+    /// Same as [`Self::retrieve_motifs`] but uses the rate-histogram
+    /// encoder (see `analysis::rate_encoder`) instead of SDPA. Exposed
+    /// for the ADR-154 §17 item 10 encoder-vs-substrate A/B
+    /// diagnostic; prefer the SDPA path for production use.
+    pub fn retrieve_motifs_rate(
+        &self,
+        conn: &Connectome,
+        spikes: &[Spike],
+        k: usize,
+    ) -> (MotifIndex, Vec<MotifHit>) {
+        rate_encoder::rate_histogram_retrieve_motifs(&self.cfg, conn, spikes, k)
+    }
 }
 
 #[cfg(test)]
diff --git a/examples/connectome-fly/src/analysis/motif.rs b/examples/connectome-fly/src/analysis/motif.rs
index 2b56a291..961ae2c9 100644
--- a/examples/connectome-fly/src/analysis/motif.rs
+++ b/examples/connectome-fly/src/analysis/motif.rs
@@ -56,12 +56,12 @@ pub(crate) fn retrieve_motifs(
     (index, hits)
 }
 
-struct WindowMeta {
-    spike_count: u32,
-    dominant_class_idx: u8,
+pub(super) struct WindowMeta {
+    pub(super) spike_count: u32,
+    pub(super) dominant_class_idx: u8,
 }
 
-fn build_raster(
+pub(super) fn build_raster(
     conn: &Connectome,
     spikes: &[Spike],
     t_start: f32,
diff --git a/examples/connectome-fly/src/analysis/rate_encoder.rs b/examples/connectome-fly/src/analysis/rate_encoder.rs
new file mode 100644
index 00000000..788a5647
--- /dev/null
+++ b/examples/connectome-fly/src/analysis/rate_encoder.rs
@@ -0,0 +1,151 @@
+//! Rate-histogram motif encoder — alternative to the SDPA path in
+//! `motif.rs`. Designed as a *controlled A/B baseline* for the AC-2
+//! encoder-vs-substrate diagnosis in ADR-154 §17 item 10.
+//!
+//! Design intent:
+//!
+//! - The shipped SDPA + deterministic-low-rank-projection encoder is
+//!   protocol-blind on the expanded 8-protocol labeled corpus
+//!   (precision@5 ≈ random). Three remediations plateau at ≤ 0.60.
+//!   The ADR calls for pinning the bottleneck: encoder, substrate, or
+//!   labels.
+//! - This module implements the *encoder* axis: a trivial
+//!   row-major flatten of the normalised raster produced by
+//!   `motif::build_raster`. No projection, no attention, no additional
+//!   normalisation. Every bin of every class is preserved verbatim.
+//! - If this cheap baseline scores *higher* than SDPA on the same
+//!   8-protocol labeled corpus, SDPA is actively hurting. If it scores
+//!   the same or lower, the substrate — not the encoder — is the
+//!   bottleneck.
+//!
+//! The encoder is deterministic (no RNG, no state) and uses exactly
+//! one allocation (the output vector).
+
+use crate::connectome::Connectome;
+use crate::lif::Spike;
+
+use super::motif::build_raster;
+use super::types::{AnalysisConfig, MotifHit, MotifIndex, MotifWindow};
+
+/// Flatten a raster `[n_rows][n_cols]` into a row-major `Vec<f32>`.
+///
+/// The output length is `n_rows * n_cols` and `out[r * n_cols + c] ==
+/// raster[r][c]`. No normalisation beyond what the caller already
+/// applied — we preserve the row-normalised form emitted by
+/// `motif::build_raster` verbatim so the A/B comparison isolates "what
+/// does SDPA add beyond the raster itself".
+///
+/// Empty rasters return an empty vector.
+pub fn rate_histogram_encode(raster: &[Vec<f32>]) -> Vec<f32> {
+    if raster.is_empty() {
+        return Vec::new();
+    }
+    let n_cols = raster[0].len();
+    // Guard against ragged rasters (shouldn't occur from build_raster but
+    // we validate anyway — the public API surface treats this as input).
+    for row in raster.iter() {
+        debug_assert_eq!(
+            row.len(),
+            n_cols,
+            "rate_histogram_encode: ragged raster (row len differs from first)"
+        );
+    }
+    let n_rows = raster.len();
+    let mut out = Vec::with_capacity(n_rows * n_cols);
+    for row in raster {
+        // Explicit raw-bin-count copy; `extend_from_slice` compiles to a
+        // `memcpy` on contiguous data. No projection, no attention.
+        out.extend_from_slice(row);
+    }
+    out
+}
+
+/// Build motif embeddings over sliding windows using the rate-histogram
+/// encoder and index them. Mirrors `motif::retrieve_motifs` so the two
+/// paths can be swapped at call sites without other changes. Returns
+/// the index plus the top-k repeated motifs.
+///
+/// The sliding-window schedule, the in-memory kNN index, and the
+/// dominant-class accounting are *identical* to the SDPA path — the
+/// only difference is the per-window embedding function. This is the
+/// A/B invariant the diagnostic test relies on.
+pub fn rate_histogram_retrieve_motifs(
+    cfg: &AnalysisConfig,
+    conn: &Connectome,
+    spikes: &[Spike],
+    k: usize,
+) -> (MotifIndex, Vec<MotifHit>) {
+    let mut index = MotifIndex::new(cfg.index_capacity);
+    if spikes.is_empty() {
+        return (index, Vec::new());
+    }
+    let t_end = spikes.last().map(|s| s.t_ms).unwrap_or(0.0);
+    let win = cfg.motif_window_ms;
+    let bins = cfg.motif_bins;
+    let step = win / 2.0;
+    let mut t = 0.0;
+    while t + win <= t_end + step {
+        let (raster, meta) = build_raster(conn, spikes, t, win, bins);
+        if meta.spike_count == 0 {
+            t += step;
+            continue;
+        }
+        let vec = rate_histogram_encode(&raster);
+        index.insert(
+            vec,
+            MotifWindow {
+                t_center_ms: t + win * 0.5,
+                spike_count: meta.spike_count,
+                dominant_class_idx: meta.dominant_class_idx,
+            },
+        );
+        t += step;
+    }
+    let hits = index.top_k(k);
+    (index, hits)
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn encode_empty_raster_returns_empty_vec() {
+        let raster: Vec<Vec<f32>> = Vec::new();
+        let v = rate_histogram_encode(&raster);
+        assert!(v.is_empty());
+    }
+
+    #[test]
+    fn encode_is_row_major_and_preserves_values() {
+        // 3 rows × 4 cols — pick distinct values so we can spot
+        // row-vs-column-major mistakes.
+        let raster: Vec<Vec<f32>> = vec![
+            vec![1.0, 2.0, 3.0, 4.0],
+            vec![5.0, 6.0, 7.0, 8.0],
+            vec![9.0, 10.0, 11.0, 12.0],
+        ];
+        let flat = rate_histogram_encode(&raster);
+        assert_eq!(flat.len(), 12);
+        for r in 0..3 {
+            for c in 0..4 {
+                assert_eq!(flat[r * 4 + c], raster[r][c]);
+            }
+        }
+    }
+
+    #[test]
+    fn encode_is_deterministic_across_runs() {
+        let raster: Vec<Vec<f32>> = vec![
+            vec![0.1, 0.2, 0.3],
+            vec![0.4, 0.5, 0.6],
+            vec![0.7, 0.8, 0.9],
+        ];
+        let a = rate_histogram_encode(&raster);
+        let b = rate_histogram_encode(&raster);
+        assert_eq!(a.len(), b.len());
+        for (x, y) in a.iter().zip(b.iter()) {
+            assert_eq!(x.to_bits(), y.to_bits(), "bit-level determinism required");
+        }
+    }
+}
diff --git a/examples/connectome-fly/tests/ac_2_encoder_comparison.rs b/examples/connectome-fly/tests/ac_2_encoder_comparison.rs
new file mode 100644
index 00000000..0e79270b
--- /dev/null
+++ b/examples/connectome-fly/tests/ac_2_encoder_comparison.rs
@@ -0,0 +1,349 @@
+#![allow(clippy::needless_range_loop)]
+//! ADR-154 §17 item 10 follow-up — encoder-vs-substrate diagnostic.
+//!
+//! The shipped SDPA + deterministic-low-rank-projection motif encoder
+//! was measured protocol-blind on this substrate: expanded-corpus AC-2
+//! at 8 protocols landed at `precision@5 = 0.117` (random = 0.125). The
+//! ADR names three axes to fix this — different encoder, different
+//! substrate, different labels — and asks that the cheapest axis
+//! (encoder) be investigated first with a controlled A/B.
+//!
+//! This test is that A/B. It runs the same 8-protocol labeled corpus
+//! through BOTH encoders and reports precision@5 side-by-side. The
+//! test is **publish-only**: it does not gate on absolute precision
+//! numbers. It fails only on non-deterministic output, malformed
+//! vectors, or an empty corpus — the AC-2 precision numbers go into
+//! the ADR §17 table, not into a regression gate.
+//!
+//! Interpretation rubric (fill in the commit message from the
+//! printed verdict):
+//!
+//! - rate > SDPA by a meaningful margin (≥ 0.05) → SDPA is actively
+//!   hurting on this substrate.
+//! - rate ≈ SDPA (within 0.05) → encoder is NOT the bottleneck; try
+//!   substrate or labels next.
+//! - rate < SDPA → rate histogram is actively worse; SDPA at least
+//!   preserves some protocol-specific signal.
+
+use connectome_fly::{
+    Analysis, AnalysisConfig, Connectome, ConnectomeConfig, Engine, EngineConfig, MotifIndex,
+    NeuronId, Observer, Spike, Stimulus,
+};
+
+// -----------------------------------------------------------------
+// 8-protocol corpus
+// -----------------------------------------------------------------
+
+/// One stimulus protocol in the 8-protocol labeled corpus.
+///
+/// Axes (ADR-154 §17 item 10 mirrors this):
+/// - `sensory_subset`  — 0 = first half of sensory neurons, 1 = second half.
+/// - `freq_hz`         — pulse-train rate.
+/// - `amplitude_pa`    — per-pulse charge.
+/// - `duration_ms`     — pulse-train window width.
+#[derive(Clone, Copy, Debug)]
+struct Protocol {
+    id: u8,
+    sensory_subset: u8,
+    freq_hz: f32,
+    amplitude_pa: f32,
+    duration_ms: f32,
+}
+
+/// Build an 8-protocol corpus spanning the four axes called out in
+/// ADR-154 §17 item 10. The eight points are an asymmetric partial
+/// factorial — not all 2⁴ combinations (the budget is 8 protocols) —
+/// chosen so every axis varies at least once against the P0 baseline.
+fn eight_protocols() -> [Protocol; 8] {
+    [
+        Protocol {
+            id: 0,
+            sensory_subset: 0,
+            freq_hz: 60.0,
+            amplitude_pa: 80.0,
+            duration_ms: 200.0,
+        },
+        Protocol {
+            id: 1,
+            sensory_subset: 0,
+            freq_hz: 60.0,
+            amplitude_pa: 80.0,
+            duration_ms: 300.0,
+        },
+        Protocol {
+            id: 2,
+            sensory_subset: 0,
+            freq_hz: 60.0,
+            amplitude_pa: 130.0,
+            duration_ms: 200.0,
+        },
+        Protocol {
+            id: 3,
+            sensory_subset: 0,
+            freq_hz: 120.0,
+            amplitude_pa: 80.0,
+            duration_ms: 200.0,
+        },
+        Protocol {
+            id: 4,
+            sensory_subset: 1,
+            freq_hz: 60.0,
+            amplitude_pa: 80.0,
+            duration_ms: 200.0,
+        },
+        Protocol {
+            id: 5,
+            sensory_subset: 1,
+            freq_hz: 120.0,
+            amplitude_pa: 130.0,
+            duration_ms: 200.0,
+        },
+        Protocol {
+            id: 6,
+            sensory_subset: 0,
+            freq_hz: 120.0,
+            amplitude_pa: 130.0,
+            duration_ms: 300.0,
+        },
+        Protocol {
+            id: 7,
+            sensory_subset: 1,
+            freq_hz: 60.0,
+            amplitude_pa: 130.0,
+            duration_ms: 300.0,
+        },
+    ]
+}
+
+/// Build the current-injection schedule for one protocol.
+fn stimulus_for(conn: &Connectome, p: &Protocol) -> Stimulus {
+    let sensory = conn.sensory_neurons();
+    let half = sensory.len() / 2;
+    let subset: Vec<NeuronId> = if p.sensory_subset == 0 {
+        sensory[..half].to_vec()
+    } else {
+        sensory[half..].to_vec()
+    };
+    Stimulus::pulse_train(&subset, 20.0, p.duration_ms, p.amplitude_pa, p.freq_hz)
+}
+
+/// Run one protocol through a fresh LIF engine and return all spikes
+/// plus the simulation end-time.
+fn run_protocol(conn: &Connectome, p: &Protocol) -> (f32, Vec<Spike>) {
+    let stim = stimulus_for(conn, p);
+    let mut eng = Engine::new(conn, EngineConfig::default());
+    let mut obs = Observer::new(conn.num_neurons());
+    let t_end = 20.0 + p.duration_ms + 80.0;
+    eng.run_with(&stim, &mut obs, t_end);
+    (t_end, obs.spikes().to_vec())
+}
+
+/// One labeled motif vector: the encoder output plus the protocol id
+/// it was produced under.
+#[derive(Clone, Debug)]
+struct LabeledVec {
+    vector: Vec<f32>,
+    protocol_id: u8,
+}
+
+/// Run all 8 protocols and collect labeled motif vectors from the
+/// given encoder (SDPA via `retrieve_motifs`, rate histogram via
+/// `retrieve_motifs_rate`). `encoder_fn` takes `(connectome, spikes)`
+/// and returns the populated motif index; the caller decides which
+/// `Analysis` method to call.
+fn collect_labeled_vectors<F>(
+    conn: &Connectome,
+    protocols: &[Protocol],
+    mut encoder_fn: F,
+) -> Vec<LabeledVec>
+where
+    F: FnMut(&Connectome, &[Spike]) -> MotifIndex,
+{
+    let mut labeled: Vec<LabeledVec> = Vec::new();
+    for p in protocols {
+        let (_t_end, spikes) = run_protocol(conn, p);
+        if spikes.is_empty() {
+            continue;
+        }
+        let index = encoder_fn(conn, &spikes);
+        for v in index.vectors() {
+            labeled.push(LabeledVec {
+                vector: v.clone(),
+                protocol_id: p.id,
+            });
+        }
+    }
+    labeled
+}
+
+// -----------------------------------------------------------------
+// Precision@k
+// -----------------------------------------------------------------
+
+#[inline]
+fn l2(a: &[f32], b: &[f32]) -> f32 {
+    let mut s = 0.0_f32;
+    let n = a.len().min(b.len());
+    for i in 0..n {
+        let d = a[i] - b[i];
+        s += d * d;
+    }
+    s.sqrt()
+}
+
+/// Labeled precision@k: for each labeled vector, brute-force find its
+/// top-k nearest neighbours in the labeled corpus (excluding itself),
+/// count how many share its label. Returns the mean across the corpus.
+fn precision_at_k(corpus: &[LabeledVec], k: usize) -> f32 {
+    if corpus.len() < 2 || k == 0 {
+        return 0.0;
+    }
+    let mut total = 0.0_f32;
+    for (qi, q) in corpus.iter().enumerate() {
+        // Score every other vector; keep top-k smallest distances.
+        let mut pairs: Vec<(f32, u8)> = Vec::with_capacity(corpus.len() - 1);
+        for (ci, c) in corpus.iter().enumerate() {
+            if ci == qi {
+                continue;
+            }
+            pairs.push((l2(&q.vector, &c.vector), c.protocol_id));
+        }
+        pairs.sort_by(|a, b| a.0.partial_cmp(&b.0).unwrap_or(std::cmp::Ordering::Equal));
+        let take = k.min(pairs.len());
+        let hits = pairs[..take]
+            .iter()
+            .filter(|(_, lbl)| *lbl == q.protocol_id)
+            .count();
+        total += hits as f32 / take as f32;
+    }
+    total / corpus.len() as f32
+}
+
+// -----------------------------------------------------------------
+// Main A/B diagnostic
+// -----------------------------------------------------------------
+
+#[test]
+fn ac_2_encoder_comparison_sdpa_vs_rate_histogram() {
+    // Same connectome for both encoders — isolates the encoder as the
+    // only variable.
+    let conn = Connectome::generate(&ConnectomeConfig::default());
+    let protocols = eight_protocols();
+
+    // Motif-window config matches the expanded-corpus AC-2 test
+    // described in ADR-154 §17 item 10: 20 ms windows, 10 bins. The
+    // index is large enough to hold every window from all 8 protocols
+    // (≈ 8 × 20 = 160 at 200 ms, more at 300 ms).
+    let cfg = AnalysisConfig {
+        motif_window_ms: 20.0,
+        motif_bins: 10,
+        index_capacity: 1024,
+        ..AnalysisConfig::default()
+    };
+    let an = Analysis::new(cfg.clone());
+
+    // ---- SDPA path (shipped) ----
+    let sdpa_corpus = collect_labeled_vectors(&conn, &protocols, |c, sp| {
+        let (index, _hits) = an.retrieve_motifs(c, sp, 5);
+        index
+    });
+
+    // ---- Rate-histogram path (this commit) ----
+    let rate_corpus = collect_labeled_vectors(&conn, &protocols, |c, sp| {
+        let (index, _hits) = an.retrieve_motifs_rate(c, sp, 5);
+        index
+    });
+
+    // ---- Hard asserts: diagnostic sanity, NOT precision floor ----
+    assert!(
+        !sdpa_corpus.is_empty(),
+        "SDPA corpus is empty — LIF engine or SDPA path failed"
+    );
+    assert!(
+        !rate_corpus.is_empty(),
+        "rate-histogram corpus is empty — LIF engine or rate path failed"
+    );
+    // Both encoders see the same windows — they must produce the same
+    // count of labeled vectors. If this differs the A/B is invalid
+    // (one path is dropping or inserting windows the other isn't).
+    assert_eq!(
+        sdpa_corpus.len(),
+        rate_corpus.len(),
+        "corpus size mismatch ({} SDPA vs {} rate) — one encoder is \
+         filtering differently, invalidating the A/B",
+        sdpa_corpus.len(),
+        rate_corpus.len()
+    );
+    // Each protocol must be represented — otherwise the 1/8 random
+    // baseline is not the right floor.
+    let mut counts = [0_u32; 8];
+    for v in &sdpa_corpus {
+        counts[v.protocol_id as usize] += 1;
+    }
+    let distinct = counts.iter().filter(|c| **c > 0).count();
+    assert!(
+        distinct >= 2,
+        "corpus collapsed to {distinct} distinct protocols out of 8 \
+         — random baseline not comparable"
+    );
+
+    // Determinism check: re-run the rate path and confirm bit-identical
+    // vectors. (SDPA relies on an external crate whose internal ordering
+    // we don't gate here; the rate path has no RNG so MUST be exact.)
+    let rate_corpus_b = collect_labeled_vectors(&conn, &protocols, |c, sp| {
+        let (index, _hits) = an.retrieve_motifs_rate(c, sp, 5);
+        index
+    });
+    assert_eq!(rate_corpus.len(), rate_corpus_b.len());
+    for (a, b) in rate_corpus.iter().zip(rate_corpus_b.iter()) {
+        assert_eq!(a.vector.len(), b.vector.len(), "rate: vector length drift");
+        for (x, y) in a.vector.iter().zip(b.vector.iter()) {
+            assert_eq!(
+                x.to_bits(),
+                y.to_bits(),
+                "rate encoder is non-deterministic"
+            );
+        }
+    }
+
+    // Malformed-vector guard: every rate vector should have length
+    // 15 * motif_bins (15 classes in the connectome).
+    let expected_dim = 15 * cfg.motif_bins;
+    for v in &rate_corpus {
+        assert_eq!(
+            v.vector.len(),
+            expected_dim,
+            "rate vector has dim {} expected {expected_dim}",
+            v.vector.len()
+        );
+    }
+
+    // ---- Soft measurement: precision@5 ----
+    let k = 5;
+    let sdpa_p = precision_at_k(&sdpa_corpus, k);
+    let rate_p = precision_at_k(&rate_corpus, k);
+    let delta = rate_p - sdpa_p;
+    let random_baseline = 1.0 / 8.0;
+
+    // Verdict marker for the ADR §17 follow-up row.
+    let marker = if delta > 0.05 {
+        "PASS (rate > SDPA — SDPA is actively hurting)"
+    } else if delta < -0.05 {
+        "MISS (rate < SDPA — rate histogram is actively worse)"
+    } else {
+        "TIE (rate ≈ SDPA — encoder is NOT the bottleneck; try substrate or labels)"
+    };
+
+    eprintln!(
+        "ac-2-encoder-comparison:\n\
+           corpus_size           = {} windows\n\
+           distinct_protocols    = {}/8\n\
+           SDPA precision@{k}     = {sdpa_p:.3}\n\
+           rate precision@{k}     = {rate_p:.3}\n\
+           delta (rate - SDPA)   = {delta:+.3}\n\
+           random baseline (1/8) = {random_baseline:.3}\n\
+           verdict               = {marker}",
+        sdpa_corpus.len(),
+        distinct,
+    );
+}