diff --git a/src/headless-query.ts b/src/headless-query.ts
index b3faf9a8c..c9aa6ae2b 100644
--- a/src/headless-query.ts
+++ b/src/headless-query.ts
@@ -71,7 +71,7 @@ export async function handleQuery(basePath: string): Promise<QueryResult> {
 
   // Derive next dispatch action
   let next: QuerySnapshot['next']
-  if (!state.activeMilestone) {
+  if (!state.activeMilestone?.id) {
     next = {
       action: 'stop',
       reason: state.phase === 'complete' ? 'All milestones complete.' : state.nextAction,
diff --git a/src/resources/extensions/gsd/guided-flow.ts b/src/resources/extensions/gsd/guided-flow.ts
index 4c655e745..2cda89d72 100644
--- a/src/resources/extensions/gsd/guided-flow.ts
+++ b/src/resources/extensions/gsd/guided-flow.ts
@@ -517,8 +517,9 @@ export async function showDiscuss(
 
   const state = await deriveState(basePath);
 
-  // No active milestone — check for pending milestones to discuss instead
-  if (!state.activeMilestone) {
+  // No active milestone (or corrupted milestone with undefined id) —
+  // check for pending milestones to discuss instead
+  if (!state.activeMilestone?.id) {
     const pendingMilestones = state.registry.filter(m => m.status === "pending");
     if (pendingMilestones.length === 0) {
       ctx.ui.notify("No active milestone. Run /gsd to create one first.", "warning");
@@ -1043,7 +1044,7 @@ export async function showSmartEntry(
 
   const state = await deriveState(basePath);
 
-  if (!state.activeMilestone) {
+  if (!state.activeMilestone?.id) {
     // Guard: if a discuss session is already in flight, don't re-inject the prompt.
     // Both /gsd and /gsd auto reach this branch when no milestone exists yet.
     // Without this guard, every subsequent /gsd call overwrites pendingAutoStart
diff --git a/src/resources/extensions/gsd/tests/active-milestone-id-guard.test.ts b/src/resources/extensions/gsd/tests/active-milestone-id-guard.test.ts
new file mode 100644
index 000000000..10bf649d5
--- /dev/null
+++ b/src/resources/extensions/gsd/tests/active-milestone-id-guard.test.ts
@@ -0,0 +1,91 @@
+/**
+ * Regression test for #2773 — activeMilestone.id guard
+ *
+ * When activeMilestone is a non-null object with `id: undefined` (corrupted
+ * state), the old `!state.activeMilestone` truthiness check passed through,
+ * causing a downstream crash when code assumed `.id` was a valid string.
+ *
+ * The fix uses optional chaining (`!state.activeMilestone?.id`) so all three
+ * "no usable milestone" shapes are caught:
+ *   1. activeMilestone === null
+ *   2. activeMilestone === undefined
+ *   3. activeMilestone === { id: undefined, title: "..." }
+ */
+
+import { describe, it } from 'node:test'
+import assert from 'node:assert/strict'
+
+import type { GSDState, ActiveRef } from '../types.ts'
+
+// ─── Guard Under Test ────────────────────────────────────────────────────────
+// Extracted guard logic identical to headless-query.ts (line 74) and
+// guided-flow.ts (lines 522, 1047).
+
+function activeMilestoneIsUsable(activeMilestone: ActiveRef | null | undefined): boolean {
+  return !!activeMilestone?.id
+}
+
+// ─── Tests ───────────────────────────────────────────────────────────────────
+
+describe('activeMilestone?.id guard (#2773)', () => {
+  it('rejects null activeMilestone', () => {
+    assert.equal(activeMilestoneIsUsable(null), false)
+  })
+
+  it('rejects undefined activeMilestone', () => {
+    assert.equal(activeMilestoneIsUsable(undefined), false)
+  })
+
+  it('rejects malformed activeMilestone with id: undefined', () => {
+    // This is the crash case from #2773 — object exists but id is undefined
+    const malformed = { id: undefined, title: 'Ghost Milestone' } as unknown as ActiveRef
+    assert.equal(activeMilestoneIsUsable(malformed), false)
+  })
+
+  it('rejects malformed activeMilestone with id: empty string', () => {
+    const malformed = { id: '', title: 'Empty ID Milestone' } as unknown as ActiveRef
+    assert.equal(activeMilestoneIsUsable(malformed), false)
+  })
+
+  it('accepts valid activeMilestone with a real id', () => {
+    const valid: ActiveRef = { id: 'M001', title: 'Real Milestone' }
+    assert.equal(activeMilestoneIsUsable(valid), true)
+  })
+})
+
+describe('headless-query stop behavior with corrupted milestone', () => {
+  // Simulates the decision logic from handleQuery (headless-query.ts:74-78)
+  function deriveNextAction(activeMilestone: ActiveRef | null | undefined, phase: string) {
+    if (!activeMilestone?.id) {
+      return {
+        action: 'stop' as const,
+        reason: phase === 'complete' ? 'All milestones complete.' : 'No active milestone.',
+      }
+    }
+    return { action: 'dispatch' as const, unitId: activeMilestone.id }
+  }
+
+  it('returns stop when activeMilestone is null', () => {
+    const result = deriveNextAction(null, 'pre-planning')
+    assert.equal(result.action, 'stop')
+  })
+
+  it('returns stop when activeMilestone has undefined id', () => {
+    const corrupted = { id: undefined, title: 'Corrupted' } as unknown as ActiveRef
+    const result = deriveNextAction(corrupted, 'executing')
+    assert.equal(result.action, 'stop')
+    assert.equal(result.reason, 'No active milestone.')
+  })
+
+  it('returns dispatch with valid milestone id', () => {
+    const valid: ActiveRef = { id: 'M001', title: 'Valid' }
+    const result = deriveNextAction(valid, 'executing')
+    assert.equal(result.action, 'dispatch')
+  })
+
+  it('returns correct stop reason when phase is complete', () => {
+    const result = deriveNextAction(null, 'complete')
+    assert.equal(result.action, 'stop')
+    assert.equal(result.reason, 'All milestones complete.')
+  })
+})