diff --git a/src/resources/extensions/gsd/auto.ts b/src/resources/extensions/gsd/auto.ts
index ca9543edf..de76458dc 100644
--- a/src/resources/extensions/gsd/auto.ts
+++ b/src/resources/extensions/gsd/auto.ts
@@ -18,7 +18,7 @@ import type {
 
 import { deriveState, invalidateStateCache } from "./state.js";
 import type { BudgetEnforcementMode, GSDState } from "./types.js";
-import { loadFile, parseRoadmap, getManifestStatus, clearParseCache } from "./files.js";
+import { loadFile, parseRoadmap, getManifestStatus } from "./files.js";
 export { inlinePriorMilestoneSummary } from "./files.js";
 import { collectSecretsFromManifest } from "../get-secrets-from-user.js";
 import {
@@ -27,8 +27,8 @@ import {
   relMilestoneFile, relSliceFile, relSlicePath, relMilestonePath,
   milestonesDir,
   buildMilestoneFileName, buildSliceFileName, buildTaskFileName,
-  clearPathCache,
 } from "./paths.js";
+import { invalidateAllCaches } from "./cache.js";
 import { saveActivityLog } from "./activity-log.js";
 import { synthesizeCrashRecovery, getDeepDiagnostic } from "./session-forensics.js";
 import { writeLock, clearLock, readCrashLock, formatCrashInfo, isLockProcessAlive } from "./crash-recovery.js";
@@ -492,9 +492,7 @@ export async function startAuto(
     } catch { /* non-fatal */ }
     // Self-heal: clear stale runtime records where artifacts already exist
     await selfHealRuntimeRecords(base, ctx, completedKeySet);
-    invalidateStateCache();
-    clearParseCache();
-    clearPathCache();
+    invalidateAllCaches();
     await dispatchNextUnit(ctx, pi);
     return;
   }
@@ -763,11 +761,9 @@ export async function handleAgentEnd(
   // Unit completed — clear its timeout
   clearUnitTimeout();
 
-  // Invalidate deriveState() cache — the unit just completed and may have
+  // Invalidate all caches — the unit just completed and may have
   // written planning files (task summaries, roadmap checkboxes, etc.)
-  invalidateStateCache();
-  clearParseCache();
-  clearPathCache();
+  invalidateAllCaches();
 
   // Small delay to let files settle (git commits, file writes)
   await new Promise(r => setTimeout(r, 500));
@@ -1120,11 +1116,10 @@ async function dispatchNextUnit(
     await new Promise(r => setTimeout(r, 200));
   }
 
-  // Clear stale directory listing cache so deriveState sees fresh disk state (#431)
-  clearPathCache();
-  // Clear parsed roadmap/plan cache — doctor may have re-populated it with
+  // Clear all caches so deriveState sees fresh disk state (#431).
+  // Parse cache is also cleared — doctor may have re-populated it with
   // stale data between handleAgentEnd and this dispatch call (Path B fix).
-  clearParseCache();
+  invalidateAllCaches();
 
   let state = await deriveState(basePath);
   let mid = state.activeMilestone?.id;
@@ -1171,9 +1166,7 @@ async function dispatchNextUnit(
 
   // ── Mid-merge safety check: detect leftover merge state from a prior session ──
   if (reconcileMergeState(basePath, ctx)) {
-    invalidateStateCache();
-    clearParseCache();
-    clearPathCache();
+    invalidateAllCaches();
     state = await deriveState(basePath);
     mid = state.activeMilestone?.id;
     midTitle = state.activeMilestone?.title;
diff --git a/src/resources/extensions/gsd/cache.ts b/src/resources/extensions/gsd/cache.ts
new file mode 100644
index 000000000..0dcef5b4f
--- /dev/null
+++ b/src/resources/extensions/gsd/cache.ts
@@ -0,0 +1,27 @@
+// GSD Extension — Unified Cache Invalidation
+//
+// Three module-scoped caches exist across the GSD extension:
+//   1. State cache (state.ts)  — memoized deriveState() result
+//   2. Path cache  (paths.ts)  — directory listing results (readdirSync)
+//   3. Parse cache (files.ts)  — parsed markdown file results
+//
+// After any file write that changes .gsd/ contents, all three must be
+// invalidated together to prevent stale reads. This module provides a
+// single function that clears all three atomically.
+
+import { invalidateStateCache } from './state.js';
+import { clearPathCache } from './paths.js';
+import { clearParseCache } from './files.js';
+
+/**
+ * Invalidate all GSD runtime caches in one call.
+ *
+ * Call this after file writes, milestone transitions, merge reconciliation,
+ * or any operation that changes .gsd/ contents on disk. Forgetting to clear
+ * any single cache causes stale reads (see #431).
+ */
+export function invalidateAllCaches(): void {
+  invalidateStateCache();
+  clearPathCache();
+  clearParseCache();
+}
diff --git a/src/resources/extensions/gsd/tests/complete-milestone.test.ts b/src/resources/extensions/gsd/tests/complete-milestone.test.ts
index af7389701..8037ef317 100644
--- a/src/resources/extensions/gsd/tests/complete-milestone.test.ts
+++ b/src/resources/extensions/gsd/tests/complete-milestone.test.ts
@@ -3,8 +3,7 @@ import { join, dirname } from "node:path";
 import { tmpdir } from "node:os";
 import { fileURLToPath } from "node:url";
 import { createTestContext } from './test-helpers.ts';
-import { clearPathCache } from '../paths.ts';
-import { invalidateStateCache } from '../state.ts';
+import { invalidateAllCaches } from '../cache.ts';
 
 // loadPrompt reads from ~/.gsd/agent/extensions/gsd/prompts/ (main checkout).
 // In a worktree the file may not exist there yet, so we resolve prompts
@@ -148,7 +147,8 @@ async function main(): Promise<void> {
   // ─── deriveState integration: completing-milestone dispatches correctly ─
   console.log("\n=== deriveState completing-milestone integration ===");
   {
-    const { deriveState, isMilestoneComplete, invalidateStateCache } = await import("../state.ts");
+    const { deriveState, isMilestoneComplete } = await import("../state.ts");
+    const { invalidateAllCaches: invalidateAllCachesDynamic } = await import("../cache.ts");
     const { parseRoadmap } = await import("../files.ts");
 
     const base = createFixtureBase();
@@ -181,8 +181,7 @@ async function main(): Promise<void> {
 
       // Now add the summary and verify it transitions to complete
       writeMilestoneSummary(base, "M001", "# M001 Summary\n\nDone.");
-      clearPathCache();
-      invalidateStateCache();
+      invalidateAllCachesDynamic();
       const stateAfter = await deriveState(base);
       assertEq(stateAfter.phase, "complete", "deriveState returns complete after summary exists");
       assertEq(stateAfter.registry[0]?.status, "complete", "registry shows complete status");
diff --git a/src/resources/extensions/gsd/tests/draft-promotion.test.ts b/src/resources/extensions/gsd/tests/draft-promotion.test.ts
index 0ce24ed50..aaad0e2da 100644
--- a/src/resources/extensions/gsd/tests/draft-promotion.test.ts
+++ b/src/resources/extensions/gsd/tests/draft-promotion.test.ts
@@ -2,8 +2,9 @@ import { mkdtempSync, mkdirSync, rmSync, writeFileSync, existsSync } from "node:
 import { join } from "node:path";
 import { tmpdir } from "node:os";
 
-import { deriveState, invalidateStateCache } from "../state.js";
-import { resolveMilestoneFile, clearPathCache } from "../paths.js";
+import { deriveState } from "../state.js";
+import { resolveMilestoneFile } from "../paths.js";
+import { invalidateAllCaches } from "../cache.js";
 
 let passed = 0;
 let failed = 0;
@@ -40,8 +41,7 @@ assert(
 const contextPath = join(gsd, "milestones", "M001", "M001-CONTEXT.md");
 writeFileSync(contextPath, "# M001: Full Context\n\nDeep discussion output.\n");
 
-clearPathCache();
-invalidateStateCache();
+invalidateAllCaches();
 const state2 = await deriveState(tmpBase);
 assert(
   state2.phase === "pre-planning",
@@ -67,8 +67,7 @@ assert(
 );
 
 // Step 4: After cleanup, state is still pre-planning (CONTEXT.md exists)
-clearPathCache();
-invalidateStateCache();
+invalidateAllCaches();
 const state3 = await deriveState(tmpBase);
 assert(
   state3.phase === "pre-planning",