diff --git a/src/resources/extensions/gsd/auto.ts b/src/resources/extensions/gsd/auto.ts
index bcf9c6582..4fd9f2dad 100644
--- a/src/resources/extensions/gsd/auto.ts
+++ b/src/resources/extensions/gsd/auto.ts
@@ -16,7 +16,7 @@ import type {
   ExtensionCommandContext,
 } from "@gsd/pi-coding-agent";
 
-import { deriveState } from "./state.js";
+import { deriveState, invalidateStateCache } from "./state.js";
 import type { GSDState } from "./types.js";
 import { loadFile, parseContinue, parsePlan, parseRoadmap, parseSummary, extractUatType, inlinePriorMilestoneSummary, getManifestStatus } from "./files.js";
 export { inlinePriorMilestoneSummary };
@@ -574,6 +574,7 @@ export async function startAuto(
     } catch { /* non-fatal */ }
     // Self-heal: clear stale runtime records where artifacts already exist
     await selfHealRuntimeRecords(base, ctx);
+    invalidateStateCache();
     await dispatchNextUnit(ctx, pi);
     return;
   }
@@ -763,6 +764,10 @@ export async function handleAgentEnd(
   // Unit completed — clear its timeout
   clearUnitTimeout();
 
+  // Invalidate deriveState() cache — the unit just completed and may have
+  // written planning files (task summaries, roadmap checkboxes, etc.)
+  invalidateStateCache();
+
   // Small delay to let files settle (git commits, file writes)
   await new Promise(r => setTimeout(r, 500));
 
@@ -1332,6 +1337,7 @@ async function dispatchNextUnit(
           }
         }
         // Re-derive state from the now-merged working tree
+        invalidateStateCache();
         state = await deriveState(basePath);
         mid = state.activeMilestone?.id;
         midTitle = state.activeMilestone?.title;
@@ -1396,6 +1402,7 @@ async function dispatchNextUnit(
               "info",
             );
             // Re-derive state from main so downstream logic sees merged state
+            invalidateStateCache();
             state = await deriveState(basePath);
             mid = state.activeMilestone?.id;
             midTitle = state.activeMilestone?.title;
diff --git a/src/resources/extensions/gsd/state.ts b/src/resources/extensions/gsd/state.ts
index 662545093..9aa14e85c 100644
--- a/src/resources/extensions/gsd/state.ts
+++ b/src/resources/extensions/gsd/state.ts
@@ -53,6 +53,27 @@ export function isMilestoneComplete(roadmap: Roadmap): boolean {
 
 // ─── State Derivation ──────────────────────────────────────────────────────
 
+// ── deriveState memoization ─────────────────────────────────────────────────
+// Cache the most recent deriveState() result keyed by basePath. Within a single
+// dispatch cycle (~100ms window), repeated calls return the cached value instead
+// of re-reading the entire .gsd/ tree from disk.
+
+interface StateCache {
+  basePath: string;
+  result: GSDState;
+  timestamp: number;
+}
+
+const CACHE_TTL_MS = 100;
+let _stateCache: StateCache | null = null;
+
+/**
+ * Invalidate the deriveState() cache. Call this whenever planning files on disk
+ * may have changed (unit completion, merges, file writes).
+ */
+export function invalidateStateCache(): void {
+  _stateCache = null;
+}
 
 /**
  * Returns the ID of the first incomplete milestone, or null if all are complete.
@@ -86,6 +107,21 @@ export async function getActiveMilestoneId(basePath: string): Promise<string | n
  * Falls back to sequential JS file reads when the native module is absent.
  */
 export async function deriveState(basePath: string): Promise<GSDState> {
+  // Return cached result if within the TTL window for the same basePath
+  if (
+    _stateCache &&
+    _stateCache.basePath === basePath &&
+    Date.now() - _stateCache.timestamp < CACHE_TTL_MS
+  ) {
+    return _stateCache.result;
+  }
+
+  const result = await _deriveStateImpl(basePath);
+  _stateCache = { basePath, result, timestamp: Date.now() };
+  return result;
+}
+
+async function _deriveStateImpl(basePath: string): Promise<GSDState> {
   const milestoneIds = findMilestoneIds(basePath);
 
   // ── Batch-parse file cache ──────────────────────────────────────────────
diff --git a/src/resources/extensions/gsd/tests/complete-milestone.test.ts b/src/resources/extensions/gsd/tests/complete-milestone.test.ts
index 0055cb862..af7389701 100644
--- a/src/resources/extensions/gsd/tests/complete-milestone.test.ts
+++ b/src/resources/extensions/gsd/tests/complete-milestone.test.ts
@@ -4,6 +4,7 @@ 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';
 
 // loadPrompt reads from ~/.gsd/agent/extensions/gsd/prompts/ (main checkout).
 // In a worktree the file may not exist there yet, so we resolve prompts
@@ -147,7 +148,7 @@ async function main(): Promise<void> {
   // ─── deriveState integration: completing-milestone dispatches correctly ─
   console.log("\n=== deriveState completing-milestone integration ===");
   {
-    const { deriveState, isMilestoneComplete } = await import("../state.ts");
+    const { deriveState, isMilestoneComplete, invalidateStateCache } = await import("../state.ts");
     const { parseRoadmap } = await import("../files.ts");
 
     const base = createFixtureBase();
@@ -181,6 +182,7 @@ async function main(): Promise<void> {
       // Now add the summary and verify it transitions to complete
       writeMilestoneSummary(base, "M001", "# M001 Summary\n\nDone.");
       clearPathCache();
+      invalidateStateCache();
       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 1037d9809..4ea6f976c 100644
--- a/src/resources/extensions/gsd/tests/draft-promotion.test.ts
+++ b/src/resources/extensions/gsd/tests/draft-promotion.test.ts
@@ -2,7 +2,7 @@ import { mkdtempSync, mkdirSync, rmSync, writeFileSync, existsSync } from "node:
 import { join } from "node:path";
 import { tmpdir } from "node:os";
 
-import { deriveState } from "../state.js";
+import { deriveState, invalidateStateCache } from "../state.js";
 import { resolveMilestoneFile, clearPathCache } from "../paths.js";
 
 let passed = 0;
@@ -41,6 +41,7 @@ const contextPath = join(gsd, "milestones", "M001", "M001-CONTEXT.md");
 writeFileSync(contextPath, "# M001: Full Context\n\nDeep discussion output.\n");
 
 clearPathCache();
+invalidateStateCache();
 const state2 = await deriveState(tmpBase);
 assert(
   state2.phase === "pre-planning",
@@ -67,6 +68,7 @@ assert(
 
 // Step 4: After cleanup, state is still pre-planning (CONTEXT.md exists)
 clearPathCache();
+invalidateStateCache();
 const state3 = await deriveState(tmpBase);
 assert(
   state3.phase === "pre-planning",