diff --git a/mcp-server/observer.ts b/mcp-server/observer.ts
index 9984fb5..4df134f 100644
--- a/mcp-server/observer.ts
+++ b/mcp-server/observer.ts
@@ -17,6 +17,8 @@
  * agents do and helps them not repeat mistakes.
  */
 
+import { filterChunks } from "./relevance";
+
 const GATEWAY = process.env.GATEWAY_URL || "http://localhost:3700";
 const LAKEHOUSE = process.env.LAKEHOUSE_URL || "http://localhost:3100";
 const CYCLE_SECS = parseInt(process.env.OBSERVER_CYCLE || "30");
@@ -578,6 +580,24 @@ function startHttpListener() {
         }).catch((e: Error) =>
           new Response(JSON.stringify({ error: e.message }), { status: 400 }));
       }
+      // ─── Relevance filter (2026-04-25) ───
+      // Drops "adjacency pollution" from matrix-retrieved chunks before
+      // they reach the reviewer LLM. Caller (scrum/agent) posts the
+      // focus file + candidate chunks; observer scores via heuristic
+      // (path/symbol/token signals) and returns kept + dropped lists.
+      // Pure function — no I/O, safe to call hot.
+      if (req.method === "POST" && url.pathname === "/relevance") {
+        return req.json().then((body: any) => {
+          const focus = body.focus_file ?? body.focus ?? {};
+          const chunks = body.chunks ?? [];
+          const threshold = typeof body.threshold === "number" ? body.threshold : 0.3;
+          const result = filterChunks(focus, chunks, threshold);
+          return new Response(JSON.stringify(result), {
+            headers: { "content-type": "application/json" },
+          });
+        }).catch((e: Error) =>
+          new Response(JSON.stringify({ error: e.message }), { status: 400 }));
+      }
       return new Response("not found", { status: 404 });
     },
   });
diff --git a/mcp-server/relevance.test.ts b/mcp-server/relevance.test.ts
new file mode 100644
index 0000000..93b91fa
--- /dev/null
+++ b/mcp-server/relevance.test.ts
@@ -0,0 +1,129 @@
+import { test, expect } from "bun:test";
+import {
+  scoreRelevance,
+  filterChunks,
+  extractDefinedSymbols,
+  extractImportedSymbols,
+  jaccard,
+  tokenize,
+} from "./relevance";
+
+const RUST_FOCUS = `
+use queryd::context::build_context;
+use catalogd::Registry;
+use shared::types::{Tombstone, ModelProfile};
+
+pub struct GatewayState {
+    catalog: Registry,
+}
+
+pub async fn handle_query(state: &GatewayState, sql: &str) -> Result<QueryResponse, Error> {
+    let ctx = build_context(&state.catalog).await?;
+    ctx.sql(sql).await.map(QueryResponse::from)
+}
+
+pub fn shutdown(state: GatewayState) {
+    drop(state);
+}
+`;
+
+test("extractDefinedSymbols pulls pub fn / struct names", () => {
+  const syms = extractDefinedSymbols(RUST_FOCUS);
+  expect(syms).toContain("handle_query");
+  expect(syms).toContain("shutdown");
+  expect(syms).toContain("GatewayState");
+});
+
+test("extractImportedSymbols pulls names from use statements", () => {
+  const syms = extractImportedSymbols(RUST_FOCUS);
+  expect(syms).toContain("build_context");
+  expect(syms).toContain("Registry");
+  expect(syms).toContain("Tombstone");
+  expect(syms).toContain("ModelProfile");
+  // Should not include keywords
+  expect(syms).not.toContain("use");
+  expect(syms).not.toContain("crate");
+});
+
+test("path_match dominates when chunk encodes focus path", () => {
+  const focus = { path: "crates/gateway/src/main.rs", content: RUST_FOCUS };
+  const chunk = {
+    source: "distilled_factual_v20260423095819",
+    doc_id: "crates/gateway/src/main.rs:42",
+    text: "Some chunk content unrelated to anything",
+    score: 0.5,
+  };
+  const { score, reasons } = scoreRelevance(focus, chunk);
+  expect(score).toBeGreaterThanOrEqual(1.0);
+  expect(reasons).toContain("path_match");
+});
+
+test("import_only adjacency pollution gets penalized", () => {
+  // Chunk talks about queryd::context::build_context (imported by focus)
+  // but never mentions any focus-defined symbol — classic pollution.
+  const focus = { path: "crates/gateway/src/main.rs", content: RUST_FOCUS };
+  const chunk = {
+    source: "distilled_procedural_v20260423102847",
+    doc_id: "proc_8421",
+    text: "When build_context fails the Registry must be invalidated. The Tombstone fields drive the merge-on-read filter — caller should not retry on stale fingerprints.",
+    score: 0.65,
+  };
+  const { score, reasons } = scoreRelevance(focus, chunk);
+  expect(reasons.some(r => r.startsWith("import_only("))).toBe(true);
+  expect(score).toBeLessThan(0.3);  // below default threshold → dropped
+});
+
+test("defined_match keeps a chunk that's actually about the focus", () => {
+  const focus = { path: "crates/gateway/src/main.rs", content: RUST_FOCUS };
+  const chunk = {
+    source: "distilled_factual_v20260423095819",
+    doc_id: "fact_12",
+    text: "handle_query in GatewayState must return QueryResponse, not anyhow::Error. The shutdown path drops state synchronously.",
+    score: 0.4,
+  };
+  const { score, reasons } = scoreRelevance(focus, chunk);
+  expect(reasons.some(r => r.startsWith("defined_match"))).toBe(true);
+  expect(score).toBeGreaterThan(0.3);  // above threshold → kept
+});
+
+test("filterChunks bucket-sorts kept vs dropped", () => {
+  const focus = { path: "crates/gateway/src/main.rs", content: RUST_FOCUS };
+  const chunks = [
+    { source: "x", doc_id: "crates/gateway/src/main.rs:1", text: "anything", score: 0.5 },                              // path_match — kept
+    { source: "x", doc_id: "y", text: "build_context Tombstone Registry adjacent only", score: 0.7 },                   // import_only — dropped
+    { source: "x", doc_id: "z", text: "handle_query and GatewayState are at fault here", score: 0.4 },                  // defined_match — kept
+    { source: "x", doc_id: "w", text: "completely unrelated content about chicago permits", score: 0.6 },               // nothing — dropped
+  ];
+  const result = filterChunks(focus, chunks);
+  expect(result.kept.length).toBe(2);
+  expect(result.dropped.length).toBe(2);
+  expect(result.kept.map(c => c.doc_id)).toContain("crates/gateway/src/main.rs:1");
+  expect(result.kept.map(c => c.doc_id)).toContain("z");
+});
+
+test("threshold override changes filter behavior", () => {
+  const focus = { path: "crates/queryd/src/x.rs", content: "pub fn foo() {}" };
+  const weak = { source: "x", doc_id: "y", text: "foo is referenced here briefly", score: 0.2 };
+  const result_strict = filterChunks(focus, [weak], 0.95);
+  const result_loose = filterChunks(focus, [weak], 0.1);
+  expect(result_strict.kept.length).toBe(0);
+  expect(result_loose.kept.length).toBe(1);
+});
+
+test("empty defined/imported gracefully scores by tokens only", () => {
+  const focus = { path: "doc.md", content: "This is plain prose about welders in Chicago." };
+  const chunk = { source: "x", doc_id: "y", text: "Welders working in Chicago need OSHA certs.", score: 0.5 };
+  const { score, reasons } = scoreRelevance(focus, chunk);
+  expect(score).toBeGreaterThan(0);
+  expect(reasons.some(r => r.startsWith("token_overlap"))).toBe(true);
+});
+
+test("jaccard / tokenize basic sanity", () => {
+  const a = tokenize("the quick brown fox jumps over the lazy dog");
+  const b = tokenize("a fast brown wolf runs over a tired dog");
+  expect(a.has("the")).toBe(false);  // stopword
+  expect(a.has("brown")).toBe(true);
+  const j = jaccard(a, b);
+  expect(j).toBeGreaterThan(0);
+  expect(j).toBeLessThan(1);
+});
diff --git a/mcp-server/relevance.ts b/mcp-server/relevance.ts
new file mode 100644
index 0000000..f99d985
--- /dev/null
+++ b/mcp-server/relevance.ts
@@ -0,0 +1,246 @@
+/**
+ * Heuristic relevance filter for matrix-retrieved chunks.
+ *
+ * Drops "adjacency pollution" — chunks that scored well on cosine but
+ * are actually about code the focus file IMPORTS, not the focus file
+ * itself. Without this, the reviewer LLM hallucinates imported-crate
+ * internals as belonging to the focus file ("I see main.rs does X"
+ * when X is in queryd::context that main.rs only calls through).
+ *
+ * Pure functions here; HTTP wiring lives in observer.ts.
+ *
+ * Scoring signals (all 0..1, additive then clamped):
+ *   path_match     +1.0  chunk.source/doc_id encodes focus.path
+ *   defined_match  +0.6  chunk text mentions focus.defined_symbols
+ *   token_overlap  +0.4  jaccard of non-stopword tokens
+ *   prefix_match   +0.3  chunk source shares first-2-segment prefix
+ *   import_penalty -0.5  mentions ONLY imported symbols, no defined ones
+ *
+ * Threshold default 0.3 — empirically tuned to keep direct hits and drop
+ * the obvious adjacency cases. Caller can override per-request.
+ */
+
+const STOPWORDS = new Set([
+  "the","a","an","and","or","but","if","then","else","is","are","was","were",
+  "be","been","being","of","in","on","at","to","for","with","by","from","as",
+  "that","this","these","those","it","its","they","them","their","we","our",
+  "you","your","i","me","my","not","no","so","do","does","did","done",
+  "will","would","could","should","can","may","might","must","shall",
+  "fn","let","mut","pub","use","mod","struct","enum","trait","impl","self",
+  "type","const","static","async","await","return","match","ok","err","some",
+  "none","into","from","ref","box","arc","rc","vec","string","str",
+]);
+
+export interface FocusFile {
+  path: string;
+  content?: string;
+  defined_symbols?: string[];
+  imported_symbols?: string[];
+}
+
+export interface CandidateChunk {
+  source: string;       // corpus name or producer file
+  doc_id: string;       // chunk identifier
+  text: string;
+  score: number;        // upstream cosine score
+}
+
+export interface ScoredChunk extends CandidateChunk {
+  relevance: number;
+  reasons: string[];
+}
+
+export interface FilterResult {
+  kept: ScoredChunk[];
+  dropped: ScoredChunk[];
+  threshold: number;
+  focus_path: string;
+  total_in: number;
+}
+
+export function tokenize(text: string): Set<string> {
+  const out = new Set<string>();
+  if (!text) return out;
+  const words = text.toLowerCase().match(/[a-z_][a-z0-9_]{2,}/g) ?? [];
+  for (const w of words) {
+    if (!STOPWORDS.has(w)) out.add(w);
+  }
+  return out;
+}
+
+export function jaccard(a: Set<string>, b: Set<string>): number {
+  if (a.size === 0 || b.size === 0) return 0;
+  let inter = 0;
+  for (const x of a) if (b.has(x)) inter++;
+  const union = a.size + b.size - inter;
+  return union === 0 ? 0 : inter / union;
+}
+
+function collectMatches(content: string, re: RegExp, group: number): string[] {
+  const out: string[] = [];
+  for (const m of content.matchAll(re)) {
+    if (m[group]) out.push(m[group]);
+  }
+  return out;
+}
+
+/**
+ * Extract pub-symbol names from Rust/TS source. Conservative — we'd
+ * rather miss a symbol than over-match on something unrelated.
+ */
+export function extractDefinedSymbols(content: string): string[] {
+  if (!content) return [];
+  const out = new Set<string>();
+  const patterns: Array<[RegExp, number]> = [
+    [/\bpub\s+(?:async\s+)?fn\s+([a-z_][a-z0-9_]*)/gi, 1],
+    [/\bpub\s+struct\s+([A-Z][A-Za-z0-9_]*)/g, 1],
+    [/\bpub\s+enum\s+([A-Z][A-Za-z0-9_]*)/g, 1],
+    [/\bpub\s+trait\s+([A-Z][A-Za-z0-9_]*)/g, 1],
+    [/\bpub\s+const\s+([A-Z_][A-Z0-9_]*)/g, 1],
+    [/\bpub\s+type\s+([A-Z][A-Za-z0-9_]*)/g, 1],
+    [/\bexport\s+(?:async\s+)?function\s+([a-z_][a-zA-Z0-9_]*)/g, 1],
+    [/\bexport\s+class\s+([A-Z][A-Za-z0-9_]*)/g, 1],
+    [/\bexport\s+interface\s+([A-Z][A-Za-z0-9_]*)/g, 1],
+    [/\bexport\s+(?:const|let|var)\s+([a-zA-Z_][a-zA-Z0-9_]*)/g, 1],
+  ];
+  for (const [re, g] of patterns) {
+    for (const sym of collectMatches(content, re, g)) out.add(sym);
+  }
+  return [...out];
+}
+
+/**
+ * Extract imported symbol names from Rust/TS source. Used as the
+ * negative signal — chunks about THESE belong to other files.
+ */
+export function extractImportedSymbols(content: string): string[] {
+  if (!content) return [];
+  const out = new Set<string>();
+  const ignore = new Set(["use","as","crate","super","self","mod"]);
+  // Rust: use foo::bar::Baz, use foo::{Bar, Baz}, use foo::bar as alias.
+  // Character class must include uppercase or paths like
+  // `use catalogd::Registry;` get skipped because the regex backs off
+  // when it can't extend the captured block past the uppercase letter.
+  const useRe = /\buse\s+([A-Za-z_][A-Za-z0-9_:{}, \n]*?);/g;
+  for (const block of collectMatches(content, useRe, 1)) {
+    for (const ident of block.matchAll(/[A-Za-z_][A-Za-z0-9_]*/g)) {
+      const tok = ident[0];
+      if (tok.length > 2 && !ignore.has(tok)) out.add(tok);
+    }
+  }
+  // TS: import { X, Y } from "foo"; import X from "foo";
+  const tsRe = /\bimport\s+(?:\{([^}]+)\}|([A-Za-z_][A-Za-z0-9_]*))\s+from/g;
+  for (const m of content.matchAll(tsRe)) {
+    const block = m[1] || m[2] || "";
+    for (const ident of block.matchAll(/[A-Za-z_][A-Za-z0-9_]*/g)) {
+      const tok = ident[0];
+      if (tok.length > 2 && tok !== "as") out.add(tok);
+    }
+  }
+  return [...out];
+}
+
+/**
+ * First-2-segment prefix used to compare paths cheaply. Mirrors the
+ * pathway_memory file_prefix() so the same "same crate" notion applies.
+ */
+export function filePrefix(path: string): string {
+  return path.split("/").slice(0, 2).join("/");
+}
+
+export function scoreRelevance(
+  focus: FocusFile,
+  chunk: CandidateChunk,
+): { score: number; reasons: string[] } {
+  const reasons: string[] = [];
+  let score = 0;
+
+  const focusPath = focus.path ?? "";
+  const focusBase = focusPath.split("/").pop() ?? "";
+  const chunkText = chunk.text ?? "";
+  const chunkSource = chunk.source ?? "";
+  const chunkDocId = chunk.doc_id ?? "";
+
+  // path_match: chunk's provenance encodes the focus path or filename.
+  if (focusPath && (chunkSource.includes(focusPath) || chunkDocId.includes(focusPath) || chunkText.includes(focusPath))) {
+    score += 1.0;
+    reasons.push("path_match");
+  } else if (focusBase && focusBase.length > 4 && (chunkText.includes(focusBase) || chunkDocId.includes(focusBase))) {
+    score += 0.6;
+    reasons.push("filename_match");
+  }
+
+  // defined_match: chunk text mentions symbols this file actually defines
+  const defined = focus.defined_symbols ?? (focus.content ? extractDefinedSymbols(focus.content) : []);
+  if (defined.length > 0) {
+    let hits = 0;
+    for (const s of defined) {
+      if (s.length > 2 && chunkText.includes(s)) hits++;
+    }
+    if (hits > 0) {
+      const ratio = Math.min(1, hits / Math.max(1, defined.length));
+      const contrib = 0.6 * ratio;
+      score += contrib;
+      reasons.push(`defined_match(${hits}/${defined.length})`);
+    }
+  }
+
+  // token_overlap: jaccard of non-stopword tokens
+  if (focus.content) {
+    const overlap = jaccard(tokenize(focus.content), tokenize(chunkText));
+    if (overlap > 0.05) {
+      const contrib = 0.4 * overlap;
+      score += contrib;
+      reasons.push(`token_overlap(${overlap.toFixed(2)})`);
+    }
+  }
+
+  // prefix_match: same first-2-segments (e.g. crates/queryd)
+  if (focusPath) {
+    const fp = filePrefix(focusPath);
+    if (fp && (chunkSource.includes(fp) || chunkDocId.includes(fp) || chunkText.includes(fp))) {
+      score += 0.3;
+      reasons.push("prefix_match");
+    }
+  }
+
+  // import_penalty: chunk mentions only symbols this file imports, never
+  // any it defines. Strong signal of adjacency pollution.
+  const imported = focus.imported_symbols ?? (focus.content ? extractImportedSymbols(focus.content) : []);
+  if (imported.length > 0 && defined.length > 0) {
+    let importHits = 0;
+    let definedHits = 0;
+    for (const s of imported) {
+      if (s.length > 2 && chunkText.includes(s)) importHits++;
+    }
+    for (const s of defined) {
+      if (s.length > 2 && chunkText.includes(s)) definedHits++;
+    }
+    if (importHits > 0 && definedHits === 0) {
+      score -= 0.5;
+      reasons.push(`import_only(${importHits})`);
+    }
+  }
+
+  return { score, reasons };
+}
+
+export function filterChunks(
+  focus: FocusFile,
+  chunks: CandidateChunk[],
+  threshold = 0.3,
+): FilterResult {
+  const scored: ScoredChunk[] = chunks.map((c) => {
+    const { score, reasons } = scoreRelevance(focus, c);
+    return { ...c, relevance: score, reasons };
+  });
+  const kept = scored.filter((c) => c.relevance >= threshold);
+  const dropped = scored.filter((c) => c.relevance < threshold);
+  return {
+    kept,
+    dropped,
+    threshold,
+    focus_path: focus.path,
+    total_in: chunks.length,
+  };
+}
diff --git a/tests/real-world/scrum_master_pipeline.ts b/tests/real-world/scrum_master_pipeline.ts
index d4b16d6..9e956e6 100644
--- a/tests/real-world/scrum_master_pipeline.ts
+++ b/tests/real-world/scrum_master_pipeline.ts
@@ -499,6 +499,7 @@ async function fetchMatrixContext(
   filePath: string,
   topPerCorpus = 3,
   topOverall = 8,
+  focusContent?: string,
 ): Promise<MatrixContext> {
   const t0 = Date.now();
   const corpora = MATRIX_CORPORA_FOR_TASK[taskClass] ?? [];
@@ -548,9 +549,48 @@ async function fetchMatrixContext(
     errors.pathway_memory = e.message;
   }
 
+  // Optional adjacency-pollution filter via observer /relevance.
+  // Drops chunks that scored well on cosine but are about symbols the
+  // focus file only IMPORTS (not defines). Opt-out with
+  // LH_RELEVANCE_FILTER=0 for A/B comparison.
+  const relevanceEnabled = process.env.LH_RELEVANCE_FILTER !== "0";
+  let filteredHits = allHits;
+  let droppedCount = 0;
+  if (relevanceEnabled && allHits.length > 0) {
+    try {
+      const r = await fetch(`${OBSERVER_URL}/relevance`, {
+        method: "POST",
+        headers: { "content-type": "application/json" },
+        body: JSON.stringify({
+          focus_file: { path: filePath, content: focusContent ?? "" },
+          chunks: allHits.map(h => ({
+            source: h.source_corpus,
+            doc_id: h.doc_id,
+            text: h.text,
+            score: h.score,
+          })),
+          threshold: Number(process.env.LH_RELEVANCE_THRESHOLD ?? 0.3),
+        }),
+        signal: AbortSignal.timeout(5000),
+      });
+      if (r.ok) {
+        const fr: any = await r.json();
+        const keptKeys = new Set((fr.kept ?? []).map((c: any) => `${c.source}|${c.doc_id}`));
+        filteredHits = allHits.filter(h => keptKeys.has(`${h.source_corpus}|${h.doc_id}`));
+        droppedCount = (fr.dropped ?? []).length;
+      } else {
+        errors.relevance_filter = `HTTP ${r.status}`;
+      }
+    } catch (e: any) {
+      // Fall-open: filter failure must not block the pipeline.
+      errors.relevance_filter = e.message;
+    }
+  }
+
   // Sort all hits by score desc, take top N
-  allHits.sort((a, b) => b.score - a.score);
-  const topHits = allHits.slice(0, topOverall);
+  filteredHits.sort((a, b) => b.score - a.score);
+  const topHits = filteredHits.slice(0, topOverall);
+  if (droppedCount > 0) byCorpus._relevance_dropped = droppedCount;
 
   return {
     hits: topHits,
@@ -1349,7 +1389,7 @@ Respond with markdown. Be specific, not generic. Cite file-region + PRD-chunk-of
     // source so retrieval anchors against both the metadata and the
     // actual code being reviewed.
     const matrixQuery = `${taskClass} ${rel} ${content.slice(0, 500)}`;
-    const matrixCtx = await fetchMatrixContext(matrixQuery, taskClass, rel, 3, 8);
+    const matrixCtx = await fetchMatrixContext(matrixQuery, taskClass, rel, 3, 8, content);
     provenApproachesPreamble = buildMatrixPreamble(matrixCtx);
     const corporaSummary = Object.entries(matrixCtx.by_corpus)
       .map(([k, v]) => `${k.split("_v")[0]}=${v}`).join(" ");