pathway_memory: consensus-designed sidecar + hot-swap learning loop

10-probe N=3 consensus (kimi-k2:1t / gpt-oss:120b / qwen3.5:latest /
deepseek-v3.1:671b / qwen3-coder:480b / mistral-large-3:675b /
qwen3.5:397b + 2 stability re-probes; 2 openrouter probes 429'd) locked
the design across three rounds. Full JSON responses in
data/_kb/consensus_reducer_design_{mocq3akn,mocq6pi1,mocqatik}.json.

What it does

Preserves FULL backtrack context per reviewed file (ladder attempts +
latencies + reject reasons, KB chunks with provenance + cosine + rank,
observer signals, context7 bridge hits, sub-pipeline calls, audit
consensus) and indexes them by narrow fingerprint for hot-swap of
proven review pathways.

When scrum reviews a file:
  1. narrow fingerprint = task_class + file_prefix + signal_class
  2. query_hot_swap checks pathway memory for a match that passes
     probation (≥3 replays @ ≥80% success) + audit gate + similarity
     (≥0.90 cosine on normalized-metadata-token embedding)
  3. if hot-swap eligible, recommended model tried first in the ladder
  4. replay outcome reported back, updating the pathway's success_rate
  5. pathways below 0.80 after ≥3 replays retire permanently (sticky)
  6. full PathwayTrace always inserted at end of review — hot-swap
     grows with use, it doesn't bootstrap from nothing

Gate design is load-bearing:
  - narrow fingerprint (6 of 8 consensus models converged on the same
    3-field composition; lock) — enables generalization within crate
  - probation ≥3 replays — binomial tail at 80% is ~5%, below is noise
  - success rate ≥0.80 — mistral + qwen3-coder independently proposed
    this exact threshold across two rounds
  - similarity ≥0.90 — middle of the 0.85/0.95 consensus spread
  - bootstrap: null audit_consensus ALLOWED (auditor → pathway update
    not wired yet; probation + success_rate gates alone enforce safety
    during bootstrap; explicit audit FAIL still blocks)
  - retirement is sticky — prevents oscillation on noise

Files

  + crates/vectord/src/pathway_memory.rs  (new, 600 lines + 18 tests)
    PathwayTrace, LadderAttempt, KbChunkRef, ObserverSignal, BridgeHit,
    SubPipelineCall, AuditConsensus, HotSwapCandidate, PathwayMemory,
    PathwayMemoryStats. 18/18 tests green.
    Cosine + 32-bucket L2-normalized embedding; mirror of TS impl.
  M crates/vectord/src/lib.rs
    pub mod pathway_memory;
  M crates/vectord/src/service.rs
    VectorState grows pathway_memory field;
    4 HTTP handlers (/pathway/insert, /pathway/query,
    /pathway/record_replay, /pathway/stats).
  M crates/gateway/src/main.rs
    Construct PathwayMemory + load from storage on boot,
    wire into VectorState.
  M tests/real-world/scrum_master_pipeline.ts
    Byte-matching TS bucket-hash (verified same bucket indices as
    Rust); pre-ladder hot-swap query; ladder reorder on hit;
    per-attempt latency capture; post-accept trace insert
    (fire-and-forget); replay outcome recording;
    observer /event emits pathway_hot_swap_hit, pathway_similarity,
    rungs_saved per review for the VCP UI.
  M ui/server.ts
    /data/pathway_stats aggregates /vectors/pathway/stats +
    scrum_reviews.jsonl window for the value metric.
  M ui/ui.js
    Three new metric cards:
      · pathway reuse rate (activity: is it firing?)
      · avg rungs saved (value: is it earning its keep?)
      · pathways tracked (stability: retirement = learning)

What's not in this commit (queued)

  - auditor → pathway audit_consensus update wire (explicit audit-fail
    block activates when this lands)
  - bridge_hits + sub_pipeline_calls population from context7 / LLM
    Team extract results (fields wired, callers not yet)
  - replay log (PathwayReplayOutcome {matched_id, succeeded, ts}) as
    a separate jsonl for forensic audit of why specific replays failed

Why > summarization

Summaries discard the causal chain. With this, auditor can verify
citation provenance, applier can distinguish lucky from learned paths,
and the matrix indexing actually stores end-to-end pathways instead of
just RAG chunks — which is what J meant by "why aren't we using it
for everything."

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -6898,6 +6898,7 @@ dependencies = [
  "storaged",
  "tokio",
  "tracing",
+ "truth",
  "url",
 ]
 

crates/gateway/src/main.rs

@@ -93,6 +93,12 @@ async fn main() {
     // operators call POST /vectors/playbook_memory/rebuild to populate.
     let pbm = vectord::playbook_memory::PlaybookMemory::new(store.clone());
     let _ = pbm.load_from_storage().await;
+    // Pathway memory — consensus-designed sidecar for full-context
+    // backtracking + hot-swap of successful review pathways. Same
+    // load-on-boot pattern as playbook_memory: empty state is fine,
+    // operators start populating via scrum_master_pipeline.ts.
+    let pwm = vectord::pathway_memory::PathwayMemory::new(store.clone());
+    let _ = pwm.load_from_storage().await;
 
     // Phase 16.2: spawn the autotune agent. When config.agent.enabled=false
     // this returns a handle that drops triggers silently — no surprise load.
@@ -178,6 +184,7 @@ async fn main() {
                 bucket_registry.clone(), index_reg.clone(),
             ),
             playbook_memory: pbm,
+            pathway_memory: pwm,
             embed_semaphore: std::sync::Arc::new(tokio::sync::Semaphore::new(1)),
         }))
         .nest("/workspaces", queryd::workspace_service::router(workspace_mgr))

crates/vectord/src/lib.rs

@@ -8,6 +8,7 @@ pub mod hnsw;
 pub mod index_registry;
 pub mod jobs;
 pub mod playbook_memory;
+pub mod pathway_memory;
 pub mod doc_drift;
 pub mod promotion;
 pub mod refresh;

crates/vectord/src/pathway_memory.rs

@@ -0,0 +1,704 @@
+//! Pathway memory — full backtrack-able context for scrum/auditor reviews.
+//!
+//! Consensus-designed (10-probe N=3 ensemble, see
+//! `data/_kb/consensus_reducer_design_*.json`). The reducer emits a
+//! `PathwayTrace` sidecar alongside its legacy summary. Traces are
+//! fingerprinted narrowly (`task_class + file_prefix + signal_class`) for
+//! generalizing hot-swap, and embedded via normalized-metadata-token
+//! concatenation so the HNSW similarity search can discriminate between
+//! pathways that share a fingerprint but diverged in ladder/KB choices.
+//!
+//! The hot-swap decision requires four conditions in AND:
+//!   1. narrow fingerprint match
+//!   2. audit_consensus.pass == true
+//!   3. replay_count >= 3
+//!   4. replays_succeeded / replay_count >= 0.80
+//!   5. NOT retired
+//!   6. similarity(new, stored) >= 0.90
+//!
+//! Any replay reports its outcome via `record_replay_outcome`; pathways
+//! whose success rate drops below 0.80 after >=3 replays are marked
+//! retired and excluded from further hot-swap consideration. This is the
+//! self-correcting learning loop — a pathway that worked once but breaks
+//! under distribution shift removes itself automatically.
+
+use std::collections::HashMap;
+use std::sync::Arc;
+
+use chrono::{DateTime, Utc};
+use object_store::ObjectStore;
+use serde::{Deserialize, Serialize};
+use sha2::{Digest, Sha256};
+use storaged::ops;
+use tokio::sync::RwLock;
+
+const STATE_KEY: &str = "_pathway_memory/state.json";
+
+/// Outcome of one ladder rung attempt. Captured for every attempt,
+/// regardless of whether it was accepted — rejections are signal too.
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct LadderAttempt {
+    pub rung: u8,
+    pub model: String,
+    pub latency_ms: u64,
+    pub accepted: bool,
+    pub reject_reason: Option<String>,
+}
+
+/// Provenance of a RAG chunk retrieved for this review. The
+/// `cosine_score` is the similarity as returned by the index; `rank` is
+/// 0-indexed order in the top-K result list.
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct KbChunkRef {
+    pub source_doc: String,
+    pub chunk_id: String,
+    pub cosine_score: f32,
+    pub rank: u8,
+}
+
+/// Signal emitted by mcp-server/observer classifier.
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct ObserverSignal {
+    pub class: String,
+    pub priors: Vec<String>,
+    pub prior_iter_outcomes: Vec<String>,
+}
+
+/// Context7-bridge lookup snapshot.
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct BridgeHit {
+    pub library: String,
+    pub version: String,
+}
+
+/// Call to LLM Team (/api/run?mode=extract) or auditor N=3 consensus.
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct SubPipelineCall {
+    pub pipeline: String, // "llm_team_extract" / "audit_consensus" / etc.
+    pub result_summary: String,
+}
+
+/// N=3 independent consensus re-check result.
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct AuditConsensus {
+    pub pass: bool,
+    pub models: Vec<String>,
+    pub disagreements: u32,
+}
+
+/// Full backtrack-able context for one reviewed file. Lives alongside
+/// the reducer's summary — summary is what the reviewer LLM sees, this
+/// is what the auditor / future iterations / hot-swap use.
+#[derive(Clone, Debug, Serialize, Deserialize, PartialEq)]
+pub struct PathwayTrace {
+    pub pathway_id: String, // SHA256(task_class|file_prefix|signal_class)
+    pub task_class: String,
+    pub file_path: String,
+    pub signal_class: Option<String>,
+    pub created_at: DateTime<Utc>,
+
+    pub ladder_attempts: Vec<LadderAttempt>,
+    pub kb_chunks: Vec<KbChunkRef>,
+    pub observer_signals: Vec<ObserverSignal>,
+    pub bridge_hits: Vec<BridgeHit>,
+    pub sub_pipeline_calls: Vec<SubPipelineCall>,
+    pub audit_consensus: Option<AuditConsensus>,
+
+    pub reducer_summary: String,
+    pub final_verdict: String,
+
+    /// Normalized-metadata-token embedding. Dimension fixed per index
+    /// version (current: 32, sufficient to distinguish task/file/signal
+    /// combinations without requiring an external embedding model —
+    /// round-3 consensus said "small metadata tokens", not "full JSON").
+    pub pathway_vec: Vec<f32>,
+
+    /// Number of times this pathway has been replayed via hot-swap.
+    /// Replay only begins after first insert; initial insert itself is
+    /// NOT a replay. Probation of ≥3 replays is required before the
+    /// success-rate gate can fire.
+    pub replay_count: u32,
+    pub replays_succeeded: u32,
+    /// Marked true when replay_count >= 3 AND success_rate < 0.80.
+    /// Retired pathways are excluded from hot-swap forever. (If the
+    /// underlying file / task / signal characteristics genuinely change
+    /// such that a retired pathway would work again, a new PathwayTrace
+    /// with a fresh id will be inserted — retirement is per-id.)
+    pub retired: bool,
+}
+
+impl PathwayTrace {
+    /// Compute the narrow fingerprint id from task_class + file_prefix
+    /// + signal_class. `file_prefix` is the first path segment
+    /// ("crates/queryd", not "crates/queryd/src/service.rs") so that
+    /// related files in the same crate share pathways.
+    pub fn compute_id(task_class: &str, file_path: &str, signal_class: Option<&str>) -> String {
+        let prefix = file_prefix(file_path);
+        let sig = signal_class.unwrap_or("");
+        let mut hasher = Sha256::new();
+        hasher.update(task_class.as_bytes());
+        hasher.update(b"|");
+        hasher.update(prefix.as_bytes());
+        hasher.update(b"|");
+        hasher.update(sig.as_bytes());
+        format!("{:x}", hasher.finalize())
+    }
+
+    pub fn success_rate(&self) -> f32 {
+        if self.replay_count == 0 {
+            return 0.0;
+        }
+        self.replays_succeeded as f32 / self.replay_count as f32
+    }
+}
+
+/// First two path segments, so `crates/queryd/src/service.rs` →
+/// `crates/queryd`. This is intentional — similar files in the same
+/// crate often share task characteristics (e.g., all files in
+/// `crates/queryd/` are SQL-path Rust code), so fingerprinting on the
+/// crate-level prefix lets the hot-swap generalize across files within
+/// the crate. Exactly-matching file paths still match (same prefix).
+pub fn file_prefix(path: &str) -> String {
+    let parts: Vec<&str> = path.split('/').take(2).collect();
+    parts.join("/")
+}
+
+/// Build the pathway vector from trace metadata. Intentionally simple —
+/// deterministic bag-of-tokens hash into 32 buckets, normalized. Round-3
+/// consensus said "small metadata tokens, not full JSON." An external
+/// embedding model would work too but adds a dependency, failure mode,
+/// and drift risk the consensus flagged.
+pub fn build_pathway_vec(trace: &PathwayTrace) -> Vec<f32> {
+    let mut buckets = vec![0f32; 32];
+    let mut tokens: Vec<String> = Vec::new();
+    tokens.push(trace.task_class.clone());
+    tokens.push(trace.file_path.clone());
+    if let Some(s) = &trace.signal_class {
+        tokens.push(format!("signal:{s}"));
+    }
+    for a in &trace.ladder_attempts {
+        tokens.push(format!("rung:{}", a.rung));
+        tokens.push(format!("model:{}", a.model));
+        tokens.push(format!("accepted:{}", a.accepted));
+    }
+    for k in &trace.kb_chunks {
+        tokens.push(format!("kb:{}", k.source_doc));
+    }
+    for o in &trace.observer_signals {
+        tokens.push(format!("class:{}", o.class));
+    }
+    for b in &trace.bridge_hits {
+        tokens.push(format!("lib:{}", b.library));
+    }
+    for s in &trace.sub_pipeline_calls {
+        tokens.push(format!("pipeline:{}", s.pipeline));
+    }
+
+    for t in &tokens {
+        let mut h = Sha256::new();
+        h.update(t.as_bytes());
+        let d = h.finalize();
+        // Two bucket writes per token: use different byte windows to
+        // spread probability across buckets even when tokens share a
+        // common prefix.
+        let b1 = (d[0] as usize) % 32;
+        let b2 = (d[8] as usize) % 32;
+        buckets[b1] += 1.0;
+        buckets[b2] += 1.0;
+    }
+
+    // L2 normalize so cosine similarity becomes a dot product.
+    let norm: f32 = buckets.iter().map(|v| v * v).sum::<f32>().sqrt();
+    if norm > 0.0 {
+        for v in &mut buckets {
+            *v /= norm;
+        }
+    }
+    buckets
+}
+
+pub fn cosine(a: &[f32], b: &[f32]) -> f32 {
+    if a.len() != b.len() {
+        return 0.0;
+    }
+    a.iter().zip(b.iter()).map(|(x, y)| x * y).sum::<f32>()
+}
+
+#[derive(Default, Clone, Serialize, Deserialize)]
+struct PathwayMemoryState {
+    pathways: HashMap<String, Vec<PathwayTrace>>, // key = pathway_id (narrow fingerprint)
+    last_updated_at: i64,
+}
+
+#[derive(Clone)]
+pub struct PathwayMemory {
+    state: Arc<RwLock<PathwayMemoryState>>,
+    store: Arc<dyn ObjectStore>,
+}
+
+#[derive(Debug, Serialize)]
+pub struct HotSwapCandidate {
+    pub pathway_id: String,
+    pub similarity: f32,
+    pub replay_count: u32,
+    pub success_rate: f32,
+    pub recommended_rung: u8,
+    pub recommended_model: String,
+}
+
+impl PathwayMemory {
+    pub fn new(store: Arc<dyn ObjectStore>) -> Self {
+        Self {
+            state: Arc::new(RwLock::new(PathwayMemoryState::default())),
+            store,
+        }
+    }
+
+    pub async fn load_from_storage(&self) -> Result<usize, String> {
+        let data = match ops::get(&self.store, STATE_KEY).await {
+            Ok(d) => d,
+            Err(_) => return Ok(0),
+        };
+        let persisted: PathwayMemoryState = serde_json::from_slice(&data)
+            .map_err(|e| format!("parse pathway_memory state: {e}"))?;
+        let n: usize = persisted.pathways.values().map(|v| v.len()).sum();
+        *self.state.write().await = persisted;
+        tracing::info!("pathway_memory: loaded {n} traces from {STATE_KEY}");
+        Ok(n)
+    }
+
+    async fn persist(&self) -> Result<(), String> {
+        let snapshot = self.state.read().await.clone();
+        let bytes = serde_json::to_vec_pretty(&snapshot).map_err(|e| e.to_string())?;
+        ops::put(&self.store, STATE_KEY, bytes.into()).await
+    }
+
+    /// Insert a new pathway trace. Called by scrum_master_pipeline at
+    /// the end of each file's review. Computes the pathway_vec from
+    /// metadata if the caller didn't supply one. Appends to the bucket
+    /// for this pathway_id — multiple traces can share a fingerprint
+    /// (each represents one review of the same file/task/signal combo).
+    pub async fn insert(&self, mut trace: PathwayTrace) -> Result<(), String> {
+        if trace.pathway_vec.is_empty() {
+            trace.pathway_vec = build_pathway_vec(&trace);
+        }
+        let mut s = self.state.write().await;
+        s.pathways
+            .entry(trace.pathway_id.clone())
+            .or_default()
+            .push(trace);
+        s.last_updated_at = Utc::now().timestamp_millis();
+        drop(s);
+        self.persist().await
+    }
+
+    /// Query for a hot-swap candidate. Returns `None` if no eligible
+    /// pathway exists — caller should run the full ladder. Returns
+    /// `Some(cand)` if all gates pass — caller can short-circuit to
+    /// `cand.recommended_rung` / `cand.recommended_model`.
+    ///
+    /// Gates (all must hold):
+    ///  - narrow fingerprint match (same task/file_prefix/signal)
+    ///  - audit_consensus.pass == true on the stored trace
+    ///  - replay_count >= 3 (probation)
+    ///  - success_rate >= 0.80
+    ///  - NOT retired
+    ///  - similarity(query_vec, stored.pathway_vec) >= 0.90
+    pub async fn query_hot_swap(
+        &self,
+        task_class: &str,
+        file_path: &str,
+        signal_class: Option<&str>,
+        query_vec: &[f32],
+    ) -> Option<HotSwapCandidate> {
+        let id = PathwayTrace::compute_id(task_class, file_path, signal_class);
+        let s = self.state.read().await;
+        let candidates = s.pathways.get(&id)?;
+        let mut best: Option<(f32, &PathwayTrace)> = None;
+        for p in candidates {
+            if p.retired {
+                continue;
+            }
+            // audit_consensus gate: explicit FAIL blocks hot-swap. A null
+            // audit_consensus (auditor hasn't seen this pathway yet) is
+            // NOT a block — the success_rate gate below still requires
+            // ≥3 real-world replays at ≥80% success before a pathway
+            // becomes hot-swap eligible, so the learning loop itself
+            // provides the safety net during bootstrap. Once the auditor
+            // pipeline wires pathway audit updates, this gate tightens
+            // automatically: any explicit audit_consensus.pass == false
+            // here will skip the candidate.
+            if let Some(ac) = &p.audit_consensus {
+                if !ac.pass {
+                    continue;
+                }
+            }
+            if p.replay_count < 3 {
+                continue;
+            }
+            if p.success_rate() < 0.80 {
+                continue;
+            }
+            let sim = cosine(query_vec, &p.pathway_vec);
+            if sim < 0.90 {
+                continue;
+            }
+            if best.as_ref().map(|(b, _)| sim > *b).unwrap_or(true) {
+                best = Some((sim, p));
+            }
+        }
+        let (similarity, p) = best?;
+        // The "recommended" rung is the first accepted attempt in the
+        // stored pathway — that's the one the ladder converged on.
+        let accepted = p.ladder_attempts.iter().find(|a| a.accepted)?;
+        Some(HotSwapCandidate {
+            pathway_id: p.pathway_id.clone(),
+            similarity,
+            replay_count: p.replay_count,
+            success_rate: p.success_rate(),
+            recommended_rung: accepted.rung,
+            recommended_model: accepted.model.clone(),
+        })
+    }
+
+    /// Record the outcome of a hot-swap replay. Increments replay_count
+    /// unconditionally; increments replays_succeeded iff succeeded;
+    /// retires the pathway if replay_count >= 3 and success_rate falls
+    /// below 0.80. Mistral's learning loop in code.
+    pub async fn record_replay_outcome(
+        &self,
+        pathway_id: &str,
+        succeeded: bool,
+    ) -> Result<(), String> {
+        let mut s = self.state.write().await;
+        // Find the specific pathway across the bucket that matches by
+        // full id (the bucket key is already the narrow id, but in case
+        // of future multi-trace-per-id we take the most recent).
+        let bucket = s
+            .pathways
+            .iter_mut()
+            .find(|(k, _)| k.as_str() == pathway_id)
+            .map(|(_, v)| v)
+            .ok_or_else(|| format!("pathway {pathway_id} not found"))?;
+        let p = bucket
+            .last_mut()
+            .ok_or_else(|| format!("pathway {pathway_id} has empty bucket"))?;
+        p.replay_count = p.replay_count.saturating_add(1);
+        if succeeded {
+            p.replays_succeeded = p.replays_succeeded.saturating_add(1);
+        }
+        if p.replay_count >= 3 && p.success_rate() < 0.80 {
+            p.retired = true;
+        }
+        s.last_updated_at = Utc::now().timestamp_millis();
+        drop(s);
+        self.persist().await
+    }
+
+    pub async fn stats(&self) -> PathwayMemoryStats {
+        let s = self.state.read().await;
+        let mut total = 0usize;
+        let mut retired = 0usize;
+        let mut with_audit_pass = 0usize;
+        let mut total_replays = 0u64;
+        let mut successful_replays = 0u64;
+        for bucket in s.pathways.values() {
+            for p in bucket {
+                total += 1;
+                if p.retired {
+                    retired += 1;
+                }
+                if p.audit_consensus.as_ref().map(|a| a.pass).unwrap_or(false) {
+                    with_audit_pass += 1;
+                }
+                total_replays += p.replay_count as u64;
+                successful_replays += p.replays_succeeded as u64;
+            }
+        }
+        PathwayMemoryStats {
+            total_pathways: total,
+            retired,
+            with_audit_pass,
+            total_replays,
+            successful_replays,
+            reuse_rate: if total == 0 {
+                0.0
+            } else {
+                total_replays as f32 / total as f32
+            },
+            replay_success_rate: if total_replays == 0 {
+                0.0
+            } else {
+                successful_replays as f32 / total_replays as f32
+            },
+        }
+    }
+}
+
+#[derive(Debug, Serialize, Deserialize)]
+pub struct PathwayMemoryStats {
+    pub total_pathways: usize,
+    pub retired: usize,
+    pub with_audit_pass: usize,
+    pub total_replays: u64,
+    pub successful_replays: u64,
+    pub reuse_rate: f32,          // total_replays / total_pathways
+    pub replay_success_rate: f32, // successful_replays / total_replays
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+    use object_store::memory::InMemory;
+
+    fn mk_store() -> Arc<dyn ObjectStore> {
+        Arc::new(InMemory::new())
+    }
+
+    fn mk_trace(id_tag: &str, audit_pass: bool, replays: u32, succ: u32) -> PathwayTrace {
+        let pathway_id =
+            PathwayTrace::compute_id("scrum_review", &format!("crates/{id_tag}/src/x.rs"), Some("CONVERGING"));
+        let attempts = vec![LadderAttempt {
+            rung: 2,
+            model: "qwen3-coder:480b".into(),
+            latency_ms: 1000,
+            accepted: true,
+            reject_reason: None,
+        }];
+        let mut trace = PathwayTrace {
+            pathway_id,
+            task_class: "scrum_review".into(),
+            file_path: format!("crates/{id_tag}/src/x.rs"),
+            signal_class: Some("CONVERGING".into()),
+            created_at: Utc::now(),
+            ladder_attempts: attempts,
+            kb_chunks: vec![KbChunkRef {
+                source_doc: "PRD.md".into(),
+                chunk_id: "c1".into(),
+                cosine_score: 0.88,
+                rank: 0,
+            }],
+            observer_signals: vec![],
+            bridge_hits: vec![],
+            sub_pipeline_calls: vec![],
+            audit_consensus: Some(AuditConsensus {
+                pass: audit_pass,
+                models: vec!["qwen3-coder:480b".into(), "gpt-oss:120b".into(), "kimi-k2:1t".into()],
+                disagreements: 0,
+            }),
+            reducer_summary: "ok".into(),
+            final_verdict: "accepted".into(),
+            pathway_vec: vec![],
+            replay_count: replays,
+            replays_succeeded: succ,
+            retired: false,
+        };
+        trace.pathway_vec = build_pathway_vec(&trace);
+        trace
+    }
+
+    #[test]
+    fn file_prefix_takes_first_two_segments() {
+        assert_eq!(file_prefix("crates/queryd/src/service.rs"), "crates/queryd");
+        assert_eq!(file_prefix("crates/gateway"), "crates/gateway");
+        assert_eq!(file_prefix("README.md"), "README.md");
+        assert_eq!(file_prefix(""), "");
+    }
+
+    #[test]
+    fn compute_id_is_deterministic() {
+        let a = PathwayTrace::compute_id("scrum", "crates/queryd/src/x.rs", Some("LOOPING"));
+        let b = PathwayTrace::compute_id("scrum", "crates/queryd/src/x.rs", Some("LOOPING"));
+        assert_eq!(a, b);
+    }
+
+    #[test]
+    fn compute_id_generalizes_across_same_prefix() {
+        // Same prefix + task + signal → same id. That IS the narrow
+        // generalization — it's what lets hot-swap fire for different
+        // files in the same crate that share the task/signal profile.
+        let a = PathwayTrace::compute_id("scrum", "crates/queryd/src/a.rs", Some("L"));
+        let b = PathwayTrace::compute_id("scrum", "crates/queryd/src/b.rs", Some("L"));
+        assert_eq!(a, b);
+    }
+
+    #[test]
+    fn compute_id_differs_on_signal_class() {
+        let a = PathwayTrace::compute_id("scrum", "crates/q/s", Some("CONVERGING"));
+        let b = PathwayTrace::compute_id("scrum", "crates/q/s", Some("LOOPING"));
+        assert_ne!(a, b);
+    }
+
+    #[test]
+    fn cosine_handles_mismatched_lengths() {
+        assert_eq!(cosine(&[1.0, 0.0], &[1.0]), 0.0);
+    }
+
+    #[test]
+    fn cosine_of_identical_normalized_is_one() {
+        let v = vec![0.6, 0.8];
+        let c = cosine(&v, &v);
+        assert!((c - 1.0).abs() < 1e-5);
+    }
+
+    #[test]
+    fn success_rate_is_zero_before_any_replay() {
+        let t = mk_trace("a", true, 0, 0);
+        assert_eq!(t.success_rate(), 0.0);
+    }
+
+    #[test]
+    fn success_rate_ratio() {
+        let t = mk_trace("a", true, 4, 3);
+        assert!((t.success_rate() - 0.75).abs() < 1e-5);
+    }
+
+    #[tokio::test]
+    async fn insert_and_stats_roundtrip() {
+        let mem = PathwayMemory::new(mk_store());
+        mem.insert(mk_trace("a", true, 0, 0)).await.unwrap();
+        let stats = mem.stats().await;
+        assert_eq!(stats.total_pathways, 1);
+        assert_eq!(stats.retired, 0);
+        assert_eq!(stats.with_audit_pass, 1);
+    }
+
+    #[tokio::test]
+    async fn hot_swap_rejects_when_probation_not_met() {
+        // Probation: replay_count must be >= 3 before success-rate gate
+        // can fire. A fresh pathway with 0 replays must NEVER hot-swap
+        // even if its similarity is 1.0 and audit passes.
+        let mem = PathwayMemory::new(mk_store());
+        let trace = mk_trace("a", true, 0, 0);
+        let qvec = trace.pathway_vec.clone();
+        mem.insert(trace).await.unwrap();
+        let got = mem
+            .query_hot_swap("scrum_review", "crates/a/src/x.rs", Some("CONVERGING"), &qvec)
+            .await;
+        assert!(got.is_none(), "fresh pathway must not hot-swap");
+    }
+
+    #[tokio::test]
+    async fn hot_swap_rejects_when_audit_explicitly_fails() {
+        let mem = PathwayMemory::new(mk_store());
+        let trace = mk_trace("a", false, 5, 5); // audit FAILED explicitly
+        let qvec = trace.pathway_vec.clone();
+        mem.insert(trace).await.unwrap();
+        let got = mem
+            .query_hot_swap("scrum_review", "crates/a/src/x.rs", Some("CONVERGING"), &qvec)
+            .await;
+        assert!(got.is_none(), "pathway with explicit audit FAIL must not hot-swap");
+    }
+
+    #[tokio::test]
+    async fn hot_swap_accepts_unaudited_pathway_for_bootstrap() {
+        // v1 bootstrap: auditor doesn't update pathway audit_consensus
+        // until Phase N+1 wires it. Until then, null audit_consensus
+        // must NOT block hot-swap — the success_rate + probation gates
+        // alone prove safety. Once auditor wires up, explicit audit
+        // failures will re-introduce the block (see previous test).
+        let mem = PathwayMemory::new(mk_store());
+        let mut trace = mk_trace("a", true, 5, 5);
+        trace.audit_consensus = None; // bootstrap path
+        trace.pathway_vec = build_pathway_vec(&trace);
+        let qvec = trace.pathway_vec.clone();
+        mem.insert(trace).await.unwrap();
+        let got = mem
+            .query_hot_swap("scrum_review", "crates/a/src/x.rs", Some("CONVERGING"), &qvec)
+            .await;
+        assert!(got.is_some(), "unaudited pathway with good replay history must hot-swap");
+    }
+
+    #[tokio::test]
+    async fn hot_swap_rejects_when_success_rate_below_80pct() {
+        // 10 replays, 7 succeeded = 70% — below the 0.80 threshold.
+        let mem = PathwayMemory::new(mk_store());
+        let trace = mk_trace("a", true, 10, 7);
+        let qvec = trace.pathway_vec.clone();
+        mem.insert(trace).await.unwrap();
+        let got = mem
+            .query_hot_swap("scrum_review", "crates/a/src/x.rs", Some("CONVERGING"), &qvec)
+            .await;
+        assert!(got.is_none());
+    }
+
+    #[tokio::test]
+    async fn hot_swap_accepts_when_all_gates_pass() {
+        let mem = PathwayMemory::new(mk_store());
+        let trace = mk_trace("a", true, 5, 5); // 100% success after 5 replays
+        let qvec = trace.pathway_vec.clone();
+        mem.insert(trace).await.unwrap();
+        let got = mem
+            .query_hot_swap("scrum_review", "crates/a/src/x.rs", Some("CONVERGING"), &qvec)
+            .await;
+        let cand = got.expect("should hot-swap");
+        assert!(cand.similarity >= 0.90);
+        assert_eq!(cand.recommended_rung, 2);
+        assert_eq!(cand.recommended_model, "qwen3-coder:480b");
+    }
+
+    #[tokio::test]
+    async fn record_replay_retires_pathway_on_failure_pattern() {
+        let mem = PathwayMemory::new(mk_store());
+        let trace = mk_trace("a", true, 0, 0);
+        let pid = trace.pathway_id.clone();
+        mem.insert(trace).await.unwrap();
+        // Three replays, all fail → success_rate = 0.0 → retired.
+        mem.record_replay_outcome(&pid, false).await.unwrap();
+        mem.record_replay_outcome(&pid, false).await.unwrap();
+        mem.record_replay_outcome(&pid, false).await.unwrap();
+        let stats = mem.stats().await;
+        assert_eq!(stats.retired, 1, "3 failures after insert must retire");
+    }
+
+    #[tokio::test]
+    async fn record_replay_does_not_retire_before_probation() {
+        let mem = PathwayMemory::new(mk_store());
+        let trace = mk_trace("a", true, 0, 0);
+        let pid = trace.pathway_id.clone();
+        mem.insert(trace).await.unwrap();
+        // Two replays (below probation of 3), both fail. Should NOT
+        // retire yet — probation requires minimum 3 data points.
+        mem.record_replay_outcome(&pid, false).await.unwrap();
+        mem.record_replay_outcome(&pid, false).await.unwrap();
+        let stats = mem.stats().await;
+        assert_eq!(stats.retired, 0, "only 2 replays → below probation floor");
+    }
+
+    #[tokio::test]
+    async fn retired_pathway_never_hot_swaps_again() {
+        let mem = PathwayMemory::new(mk_store());
+        let trace = mk_trace("a", true, 0, 0);
+        let pid = trace.pathway_id.clone();
+        let qvec = trace.pathway_vec.clone();
+        mem.insert(trace).await.unwrap();
+        for _ in 0..3 {
+            mem.record_replay_outcome(&pid, false).await.unwrap();
+        }
+        // Now record 10 successes to push success_rate well above 0.80.
+        // Pathway is still retired — retirement is sticky by design, to
+        // prevent oscillation on noise.
+        for _ in 0..10 {
+            mem.record_replay_outcome(&pid, true).await.unwrap();
+        }
+        let got = mem
+            .query_hot_swap("scrum_review", "crates/a/src/x.rs", Some("CONVERGING"), &qvec)
+            .await;
+        assert!(got.is_none(), "retirement must be sticky");
+    }
+
+    #[tokio::test]
+    async fn pathway_vec_differs_for_different_models() {
+        // Two pathways with same fingerprint but different ladder
+        // models should have different embeddings so the similarity
+        // gate can discriminate. This is what enables narrow fingerprint
+        // + similarity-vec to cluster correctly.
+        let a = mk_trace("a", true, 5, 5);
+        let mut b = a.clone();
+        b.ladder_attempts[0].model = "kimi-k2:1t".into();
+        b.pathway_vec = build_pathway_vec(&b);
+        let sim = cosine(&a.pathway_vec, &b.pathway_vec);
+        assert!(sim < 1.0, "different models → different embeddings");
+        assert!(sim > 0.5, "shared fingerprint → embeddings still related");
+    }
+}

crates/vectord/src/service.rs

@@ -13,7 +13,7 @@ use std::sync::Arc;
 use aibridge::client::{AiClient, EmbedRequest, GenerateRequest};
 use catalogd::registry::Registry as CatalogRegistry;
 use storaged::registry::BucketRegistry;
-use crate::{agent, autotune, chunker, embedding_cache, harness, hnsw, index_registry, jobs, lance_backend, playbook_memory, promotion, rag, refresh, search, store, supervisor, trial};
+use crate::{agent, autotune, chunker, embedding_cache, harness, hnsw, index_registry, jobs, lance_backend, pathway_memory, playbook_memory, promotion, rag, refresh, search, store, supervisor, trial};
 use tokio::sync::Semaphore;
 
 #[derive(Clone)]
@@ -55,6 +55,11 @@ pub struct VectorState {
     /// and, when `use_playbook_memory` is set on /vectors/hybrid, boosts
     /// workers that were actually filled in semantically-similar past ops.
     pub playbook_memory: playbook_memory::PlaybookMemory,
+    /// Pathway memory — consensus-designed sidecar for full-context
+    /// backtracking + hot-swap of successful review pathways. See
+    /// crates/vectord/src/pathway_memory.rs for the design rationale
+    /// (10-probe N=3 ensemble, locked 2026-04-24).
+    pub pathway_memory: pathway_memory::PathwayMemory,
     /// Serializes embed calls from seed_playbook_memory to avoid
     /// concurrent socket collisions with the Python sidecar.
     pub embed_semaphore: Arc<Semaphore>,
@@ -137,6 +142,15 @@ pub fn router(state: VectorState) -> Router {
         // Phase 45 slice 3 — doc drift detection + human re-admission.
         .route("/playbook_memory/doc_drift/check/{id}", post(check_doc_drift))
         .route("/playbook_memory/doc_drift/resolve/{id}", post(resolve_doc_drift))
+        // Pathway memory — consensus-designed sidecar (2026-04-24).
+        // scrum_master_pipeline POSTs /pathway/insert at the end of each
+        // review, calls /pathway/query before running the ladder for a
+        // potential hot-swap, and posts /pathway/record_replay after a
+        // hot-swap succeeds or fails.
+        .route("/pathway/insert", post(pathway_insert))
+        .route("/pathway/query", post(pathway_query))
+        .route("/pathway/record_replay", post(pathway_record_replay))
+        .route("/pathway/stats", get(pathway_stats))
         .with_state(state)
 }
 
@@ -2833,6 +2847,73 @@ async fn lance_build_scalar_index(
     }
 }
 
+// ─── Pathway memory handlers ──────────────────────────────────────────
+//
+// Thin wrappers around pathway_memory::PathwayMemory. HTTP surface is
+// deliberately small — four endpoints cover the full lifecycle:
+// insert at end-of-review, query before running the ladder,
+// record_replay after a hot-swap, and stats for the VCP UI.
+
+#[derive(Deserialize)]
+struct PathwayQueryRequest {
+    task_class: String,
+    file_path: String,
+    signal_class: Option<String>,
+    query_vec: Vec<f32>,
+}
+
+async fn pathway_insert(
+    State(state): State<VectorState>,
+    Json(trace): Json<pathway_memory::PathwayTrace>,
+) -> impl IntoResponse {
+    match state.pathway_memory.insert(trace).await {
+        Ok(()) => Ok(Json(json!({"ok": true}))),
+        Err(e) => Err((StatusCode::INTERNAL_SERVER_ERROR, e)),
+    }
+}
+
+async fn pathway_query(
+    State(state): State<VectorState>,
+    Json(req): Json<PathwayQueryRequest>,
+) -> impl IntoResponse {
+    let cand = state
+        .pathway_memory
+        .query_hot_swap(
+            &req.task_class,
+            &req.file_path,
+            req.signal_class.as_deref(),
+            &req.query_vec,
+        )
+        .await;
+    // 200 with null candidate means "no hot-swap"; this is a normal
+    // path, not an error — callers should proceed with the full ladder.
+    Json(json!({ "candidate": cand }))
+}
+
+#[derive(Deserialize)]
+struct PathwayReplayRequest {
+    pathway_id: String,
+    succeeded: bool,
+}
+
+async fn pathway_record_replay(
+    State(state): State<VectorState>,
+    Json(req): Json<PathwayReplayRequest>,
+) -> impl IntoResponse {
+    match state
+        .pathway_memory
+        .record_replay_outcome(&req.pathway_id, req.succeeded)
+        .await
+    {
+        Ok(()) => Ok(Json(json!({"ok": true}))),
+        Err(e) => Err((StatusCode::NOT_FOUND, e)),
+    }
+}
+
+async fn pathway_stats(State(state): State<VectorState>) -> impl IntoResponse {
+    Json(state.pathway_memory.stats().await)
+}
+
 #[cfg(test)]
 mod extractor_tests {
     use super::*;

tests/real-world/scrum_master_pipeline.ts

@@ -157,6 +157,158 @@ async function embedBatch(texts: string[]): Promise<number[][]> {
   return (await r.json() as any).embeddings;
 }
 
+// ─── Pathway memory (2026-04-24 consensus design) ───────────────────
+//
+// Mirrors vectord/src/pathway_memory.rs. The bucket-hash vector MUST
+// byte-match the Rust implementation so traces written from TypeScript
+// are searchable against the same embedding space. Verified by running
+// both implementations on the same input tokens and asserting matching
+// bucket indices.
+
+function filePrefix(path: string): string {
+  return path.split("/").slice(0, 2).join("/");
+}
+
+function computePathwayId(taskClass: string, filePath: string, signalClass: string | null): string {
+  const h = createHash("sha256");
+  h.update(taskClass);
+  h.update("|");
+  h.update(filePrefix(filePath));
+  h.update("|");
+  h.update(signalClass ?? "");
+  return h.digest("hex");
+}
+
+// 32-bucket L2-normalized token hash. Same algorithm as Rust.
+function buildPathwayVec(tokens: string[]): number[] {
+  const buckets = new Array(32).fill(0);
+  for (const t of tokens) {
+    const d = createHash("sha256").update(t, "utf8").digest();
+    const b1 = d[0] % 32;
+    const b2 = d[8] % 32;
+    buckets[b1] += 1;
+    buckets[b2] += 1;
+  }
+  let norm = 0;
+  for (const v of buckets) norm += v * v;
+  norm = Math.sqrt(norm);
+  if (norm > 0) for (let i = 0; i < buckets.length; i++) buckets[i] /= norm;
+  return buckets;
+}
+
+// Build the minimal query vector for a pre-ladder hot-swap check. We
+// don't yet know the ladder attempts or KB chunks — the query vec is
+// computed from what we CAN know up front: task/file/signal. This is
+// a weaker embedding than the one computed at trace-insert time, but
+// similarity still discriminates between task/file/signal combinations.
+function buildQueryVec(taskClass: string, filePath: string, signalClass: string | null): number[] {
+  const tokens = [taskClass, filePath];
+  if (signalClass) tokens.push(`signal:${signalClass}`);
+  return buildPathwayVec(tokens);
+}
+
+interface HotSwapCandidate {
+  pathway_id: string;
+  similarity: number;
+  replay_count: number;
+  success_rate: number;
+  recommended_rung: number;
+  recommended_model: string;
+}
+
+async function queryHotSwap(taskClass: string, filePath: string, signalClass: string | null): Promise<HotSwapCandidate | null> {
+  try {
+    const query_vec = buildQueryVec(taskClass, filePath, signalClass);
+    const r = await fetch(`${GATEWAY}/vectors/pathway/query`, {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: JSON.stringify({ task_class: taskClass, file_path: filePath, signal_class: signalClass, query_vec }),
+      signal: AbortSignal.timeout(5000),
+    });
+    if (!r.ok) return null;
+    const j = await r.json() as { candidate: HotSwapCandidate | null };
+    return j.candidate ?? null;
+  } catch {
+    // Pathway service unavailable → run full ladder. Hot-swap is
+    // always an optimization, never a correctness requirement.
+    return null;
+  }
+}
+
+interface LadderAttemptRec {
+  rung: number;
+  model: string;
+  latency_ms: number;
+  accepted: boolean;
+  reject_reason: string | null;
+}
+
+interface PathwayTracePayload {
+  pathway_id: string;
+  task_class: string;
+  file_path: string;
+  signal_class: string | null;
+  created_at: string;
+  ladder_attempts: LadderAttemptRec[];
+  kb_chunks: { source_doc: string; chunk_id: string; cosine_score: number; rank: number }[];
+  observer_signals: { class: string; priors: string[]; prior_iter_outcomes: string[] }[];
+  bridge_hits: { library: string; version: string }[];
+  sub_pipeline_calls: { pipeline: string; result_summary: string }[];
+  audit_consensus: { pass: boolean; models: string[]; disagreements: number } | null;
+  reducer_summary: string;
+  final_verdict: string;
+  pathway_vec: number[];
+  replay_count: number;
+  replays_succeeded: number;
+  retired: boolean;
+}
+
+async function writePathwayTrace(trace: PathwayTracePayload): Promise<void> {
+  try {
+    await fetch(`${GATEWAY}/vectors/pathway/insert`, {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: JSON.stringify(trace),
+      signal: AbortSignal.timeout(10000),
+    });
+  } catch {
+    // Fire-and-forget: scrum runs shouldn't fail if pathway insert fails.
+  }
+}
+
+async function recordPathwayReplay(pathwayId: string, succeeded: boolean): Promise<void> {
+  try {
+    await fetch(`${GATEWAY}/vectors/pathway/record_replay`, {
+      method: "POST",
+      headers: { "content-type": "application/json" },
+      body: JSON.stringify({ pathway_id: pathwayId, succeeded }),
+      signal: AbortSignal.timeout(5000),
+    });
+  } catch {
+    // Fire-and-forget. Not critical.
+  }
+}
+
+// Deterministic signal_class lookup from scrum_reviews.jsonl history.
+// First-time files get `null`. Files seen before get the signal class
+// the observer assigned on their most-recent review (if any). Keeps the
+// pathway fingerprint stable across iterations for LOOPING files.
+async function lookupSignalClass(filePath: string): Promise<string | null> {
+  try {
+    const { readFile } = await import("node:fs/promises");
+    const raw = await readFile(SCRUM_REVIEWS_JSONL, "utf8").catch(() => "");
+    if (!raw) return null;
+    const lines = raw.trim().split("\n").reverse();
+    for (const line of lines) {
+      try {
+        const r = JSON.parse(line);
+        if (r.file === filePath && r.signal_class) return r.signal_class;
+      } catch {}
+    }
+    return null;
+  } catch { return null; }
+}
+
 async function chat(opts: {
   provider: "ollama" | "ollama_cloud",
   model: string,
@@ -389,32 +541,63 @@ Respond with markdown. Be specific, not generic. Cite file-region + PRD-chunk-of
   let acceptedModel = "";
   let acceptedOn = 0;
 
-  for (let i = 0; i < MAX_ATTEMPTS; i++) {
-    const n = i + 1;
+  // Pathway hot-swap pre-check. If a proven pathway exists for this
+  // (task, file_prefix, signal) combo with ≥3 replays at ≥80% success,
+  // skip the ladder and try its winning rung first. On success we
+  // record a positive replay; on failure we fall through to the full
+  // ladder and record a negative replay. Fire-and-forget — pathway
+  // service unavailable → null candidate → business as usual.
+  const signalClass = await lookupSignalClass(rel);
+  const taskClass = "scrum_review";
+  const hotSwap = await queryHotSwap(taskClass, rel, signalClass);
+  let hotSwapOrderedIndices: number[] | null = null;
+  if (hotSwap) {
+    // Reorder the ladder to try the recommended model first. Rung
+    // indices are preserved in the output so the trace still reflects
+    // the true ladder position the model sits at.
+    const recommendedIdx = LADDER.findIndex(r => r.model === hotSwap.recommended_model);
+    if (recommendedIdx >= 0) {
+      log(`  🔥 hot-swap candidate: ${hotSwap.recommended_model} (rung ${hotSwap.recommended_rung}, sim=${hotSwap.similarity.toFixed(3)}, success_rate=${hotSwap.success_rate.toFixed(2)}, ${hotSwap.replay_count} replays)`);
+      hotSwapOrderedIndices = [recommendedIdx, ...LADDER.map((_, i) => i).filter(i => i !== recommendedIdx)];
+    }
+  }
+  const ladderOrder = hotSwapOrderedIndices ?? LADDER.map((_, i) => i);
+
+  // Collect attempts for the pathway trace sidecar.
+  const pathwayAttempts: LadderAttemptRec[] = [];
+
+  for (let step = 0; step < MAX_ATTEMPTS; step++) {
+    const i = ladderOrder[step];
+    const n = step + 1;
     const rung = LADDER[i];
     const learning = history.length > 0
       ? `\n\n═══ PRIOR ATTEMPTS FAILED. Specific issues to fix: ═══\n${history.map(h => `Attempt ${h.n} (${h.model}, ${h.chars} chars): ${h.status} — ${h.error ?? "thin/unstructured answer"}`).join("\n")}\n═══`
       : "";
 
     log(`  attempt ${n}/${MAX_ATTEMPTS}: ${rung.provider}::${rung.model}${learning ? " [w/ learning]" : ""}`);
+    const attemptStarted = Date.now();
     const r = await chat({
       provider: rung.provider,
       model: rung.model,
       prompt: baseTask + learning,
       max_tokens: 1500,
     });
+    const attemptMs = Date.now() - attemptStarted;
 
     if (r.error) {
       history.push({ n, model: rung.model, status: "error", chars: 0, error: r.error.slice(0, 180) });
+      pathwayAttempts.push({ rung: i + 1, model: rung.model, latency_ms: attemptMs, accepted: false, reject_reason: `error: ${r.error.slice(0, 100)}` });
       log(`    ✗ error: ${r.error.slice(0, 80)}`);
       continue;
     }
     if (!isAcceptable(r.content)) {
       history.push({ n, model: rung.model, status: "thin", chars: r.content.length, error: `thin/unstructured (${r.content.length} chars)` });
+      pathwayAttempts.push({ rung: i + 1, model: rung.model, latency_ms: attemptMs, accepted: false, reject_reason: `thin (${r.content.length} chars)` });
       log(`    ✗ thin/unstructured (${r.content.length} chars)`);
       continue;
     }
     history.push({ n, model: rung.model, status: "accepted", chars: r.content.length });
+    pathwayAttempts.push({ rung: i + 1, model: rung.model, latency_ms: attemptMs, accepted: true, reject_reason: null });
     accepted = r.content;
     acceptedModel = `${rung.provider}/${rung.model}`;
     acceptedOn = n;
@@ -422,6 +605,15 @@ Respond with markdown. Be specific, not generic. Cite file-region + PRD-chunk-of
     break;
   }
 
+  // Hot-swap bookkeeping: if we tried the recommended model first,
+  // report whether it worked so the pathway's success_rate updates.
+  if (hotSwap) {
+    const replaySucceeded = acceptedModel.endsWith(`/${hotSwap.recommended_model}`);
+    log(`  pathway replay ${replaySucceeded ? "✓" : "✗"} (${hotSwap.pathway_id.slice(0, 12)}…)`);
+    // Fire and forget — don't await; observer can handle it.
+    recordPathwayReplay(hotSwap.pathway_id, replaySucceeded);
+  }
+
   const review: FileReview = {
     file: rel,
     file_bytes: content.length,
@@ -599,6 +791,54 @@ Respond with markdown. Be specific, not generic. Cite file-region + PRD-chunk-of
       console.error(`[scrum] failed to append scrum_reviews.jsonl: ${(e as Error).message}`);
     }
 
+    // Pathway trace sidecar (consensus-designed 2026-04-24). Captures
+    // FULL context (ladder attempts, KB chunks, observer signal, verdict)
+    // for similarity-based hot-swap in future iterations. First-review
+    // pathways start in probation (replay_count=0); they become
+    // hot-swappable only after ≥3 replays at ≥80% success.
+    try {
+      const pathwayTrace: PathwayTracePayload = {
+        pathway_id: computePathwayId(taskClass, rel, signalClass),
+        task_class: taskClass,
+        file_path: rel,
+        signal_class: signalClass,
+        created_at: row.reviewed_at,
+        ladder_attempts: pathwayAttempts,
+        kb_chunks: [
+          ...topPrd.map((c, idx) => ({
+            source_doc: "PRD.md", chunk_id: `prd@${c.offset}`, cosine_score: (c as any)._score ?? 0, rank: idx,
+          })),
+          ...topPlan.map((c, idx) => ({
+            source_doc: "cohesion_plan", chunk_id: `plan@${c.offset}`, cosine_score: (c as any)._score ?? 0, rank: idx,
+          })),
+        ],
+        observer_signals: signalClass ? [{ class: signalClass, priors: [], prior_iter_outcomes: [] }] : [],
+        bridge_hits: [],      // context7 not wired into scrum yet; empty for v1
+        sub_pipeline_calls: [], // LLM Team extract happens after this row; out of scope for v1
+        audit_consensus: null, // set by auditor's later N=3 pass, via /pathway/insert update
+        reducer_summary: accepted.slice(0, 4000),
+        final_verdict: verdict ?? "accepted",
+        // Vec built from the full attempts/chunks — richer than the
+        // query-time vector. The similarity gate will still discriminate
+        // between pathways with the same fingerprint but different
+        // ladder/KB profiles.
+        pathway_vec: buildPathwayVec([
+          taskClass,
+          rel,
+          ...(signalClass ? [`signal:${signalClass}`] : []),
+          ...pathwayAttempts.flatMap(a => [`rung:${a.rung}`, `model:${a.model}`, `accepted:${a.accepted}`]),
+          ...topPrd.map(c => `kb:PRD.md`),
+          ...topPlan.map(c => `kb:cohesion_plan`),
+        ]),
+        replay_count: 0,
+        replays_succeeded: 0,
+        retired: false,
+      };
+      writePathwayTrace(pathwayTrace); // fire-and-forget
+    } catch (e) {
+      console.error(`[scrum] pathway trace failed: ${(e as Error).message}`);
+    }
+
     // Close the scrum → observer loop (fix 2026-04-24). Architecture
     // audit surfaced: observer ring had 2000 ops, 1999 from Langfuse,
     // zero from scrum. Observer's analyzeErrors + PLAYBOOK_BUILDER loops
@@ -643,6 +883,20 @@ Respond with markdown. Be specific, not generic. Cite file-region + PRD-chunk-of
           critical_failures_count,
           verified_components_count,
           missing_components_count,
+          // Pathway fields: emitted on every review so the observer
+          // can build a full picture of hot-swap performance over time.
+          // `pathway_hot_swap_hit` flags whether the first-tried rung
+          // this review was a pathway recommendation vs the default
+          // ladder top. `rungs_saved` quantifies the compute we avoided
+          // when a hot-swap landed — this is the value metric the VCP
+          // UI surfaces ("avg_rungs_saved_per_commit").
+          pathway_hot_swap_hit: hotSwap !== null,
+          pathway_id: hotSwap?.pathway_id ?? null,
+          pathway_similarity: hotSwap?.similarity ?? null,
+          pathway_success_rate: hotSwap?.success_rate ?? null,
+          rungs_saved: hotSwap && acceptedModel.endsWith(`/${hotSwap.recommended_model}`)
+            ? Math.max(0, hotSwap.recommended_rung - 1)
+            : 0,
           ts: row.reviewed_at,
         }),
         signal: AbortSignal.timeout(3000),

ui/server.ts

@@ -349,6 +349,44 @@ Bun.serve({
     if (path === "/data/outcomes") return Response.json(await tailJsonl(`${KB}/outcomes.jsonl`, 30));
     if (path === "/data/audit_facts") return Response.json(await tailJsonl(`${KB}/audit_facts.jsonl`, 30));
 
+    // Pathway memory — consensus-designed sidecar (2026-04-24). Two
+    // exposed metrics: reuse_rate (activity — is it firing?) and
+    // avg_rungs_saved_per_commit (value — is it earning its keep?).
+    // Round-3 consensus (qwen3.5:397b) pointed out that activity
+    // without value tells us nothing; the UI needs both to judge the
+    // health of the hot-swap learning loop.
+    if (path === "/data/pathway_stats") {
+      try {
+        const r = await fetch("http://localhost:3100/vectors/pathway/stats", { signal: AbortSignal.timeout(3000) });
+        if (!r.ok) return Response.json({ error: `vectord ${r.status}`, stats: null });
+        const stats = await r.json();
+        // Tail recent scrum events to compute avg_rungs_saved_per_commit
+        // (a committed review = any row in scrum_reviews.jsonl; rungs_saved
+        // only populates when pathway memory fired AND the recommended
+        // model actually produced the accept).
+        const reviews = await tailJsonl(`${KB}/scrum_reviews.jsonl`, 200);
+        let totalCommits = 0;
+        let totalRungsSaved = 0;
+        let hotSwapHits = 0;
+        for (const r of reviews) {
+          totalCommits++;
+          if (r.pathway_hot_swap_hit) hotSwapHits++;
+          if (typeof r.rungs_saved === "number") totalRungsSaved += r.rungs_saved;
+        }
+        return Response.json({
+          stats,
+          scrum_window: {
+            reviews: totalCommits,
+            hot_swap_hits: hotSwapHits,
+            pathway_reuse_rate: totalCommits ? hotSwapHits / totalCommits : 0,
+            avg_rungs_saved_per_commit: totalCommits ? totalRungsSaved / totalCommits : 0,
+          },
+        });
+      } catch (e) {
+        return Response.json({ error: (e as Error).message, stats: null });
+      }
+    }
+
     if (path.startsWith("/data/file/")) {
       const relpath = decodeURIComponent(path.slice("/data/file/".length));
       return Response.json(await fileHistory(relpath));

ui/ui.js

@@ -589,6 +589,37 @@ function metricBox(label, big, kind, opts = {}) {
 function drawMetrics() {
   const grid = document.getElementById("metric-grid");
   clear(grid);
+  // Kick off pathway fetch in parallel; render when it resolves so the
+  // rest of the metrics grid appears immediately. The cards append to
+  // the grid after the synchronous block below — they'll show up at
+  // the bottom of the grid within a tick of first render.
+  fetch("/data/pathway_stats").then(r => r.ok ? r.json() : null).then(j => {
+    if (!j || !j.stats) return;
+    const s = j.stats;
+    const w = j.scrum_window ?? {};
+    // Activity metric — is the hot-swap firing at all?
+    grid.append(metricBox("pathway reuse rate", `${Math.round((w.pathway_reuse_rate ?? 0) * 100)}%`,
+      (w.pathway_reuse_rate ?? 0) > 0.1 ? "good" : (w.pathway_reuse_rate ?? 0) > 0 ? "warn" : "bad", {
+      explain: "% of recent reviews where a pathway hot-swap fired (narrow fingerprint match + 0.80 success rate + ≥3 replays + audit_consensus pass + 0.90 similarity).",
+      source: `scrum_reviews.jsonl .pathway_hot_swap_hit over last ${w.reviews ?? 0} reviews (${w.hot_swap_hits ?? 0} hits)`,
+      good: "≥10% sustained = index earning its keep. <10% over many iters = fingerprint too narrow or probation too strict. 0% on fresh install is expected (no replays yet).",
+    }));
+    // Value metric — how much compute did hot-swap actually save?
+    const saved = w.avg_rungs_saved_per_commit ?? 0;
+    grid.append(metricBox("avg rungs saved", saved.toFixed(2),
+      saved >= 1 ? "good" : saved > 0 ? "warn" : "bad", {
+      explain: "Average ladder rungs skipped per committed review by hot-swap. Rungs_saved = recommended_rung - 1 when the recommended model succeeded (otherwise 0).",
+      source: "scrum_reviews.jsonl .rungs_saved averaged",
+      good: "Every 1.0 here ≈ one less model call per review. At 21 files/iter, 1.0 saved = 21 cloud calls avoided. Value only counts when the replay actually succeeded.",
+    }));
+    // Stability metric — retired pathways indicate the learning loop is correcting itself.
+    grid.append(metricBox("pathways tracked", String(s.total_pathways),
+      s.total_pathways > 0 ? "good" : "warn", {
+      explain: `Total pathway traces stored. ${s.retired} retired (below 0.80 success after ≥3 replays). ${s.with_audit_pass} audit-passed, eligible for hot-swap probation.`,
+      source: "/vectors/pathway/stats",
+      good: `Grows monotonically with scrum runs. Retired=${s.retired} is HEALTHY — it means the learning loop is pruning pathways that stopped working. replay_success_rate=${(s.replay_success_rate*100).toFixed(0)}% aggregates all historical replays.`,
+    }));
+  }).catch(() => {});
   const byTier = { auto:0, dry_run:0, simulation:0, block:0, unknown:0 };
   state.reviews.forEach(r => { const t = r.gradient_tier ?? "unknown"; if (byTier[t] != null) byTier[t]++; });
   const total = state.reviews.length || 1;