diff --git a/.changeset/steady-wasps-roam.md b/.changeset/steady-wasps-roam.md
new file mode 100644
index 000000000..e1fdc2083
--- /dev/null
+++ b/.changeset/steady-wasps-roam.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 3139
+---
+**Unusable `last_activity` now emits a diagnostic** — a present-but-unparseable `last_activity` in STATE.md silently suppressed the idle-stranded recommendation. The fallback (`stale_activity: false`) stays for continuity, but a `last_activity_unparseable` warning is now emitted so the degradation is visible. (#3099)
diff --git a/src/smart-entry.cts b/src/smart-entry.cts
index 50e2c358e..20a81c711 100644
--- a/src/smart-entry.cts
+++ b/src/smart-entry.cts
@@ -51,6 +51,9 @@ const { stateExtractField } = stateDocument;
 // eslint-disable-next-line @typescript-eslint/no-require-imports
 import phaseId = require('./phase-id.cjs');
 const { comparePhaseNum, extractPhaseToken, normalizePhaseName, phaseTokenMatches } = phaseId;
+// eslint-disable-next-line @typescript-eslint/no-require-imports
+import unusableInput = require('./unusable-input.cjs');
+const { warnUnusableInput, UNUSABLE_REASON } = unusableInput;
 
 // ─── Types ────────────────────────────────────────────────────────────────────
 
@@ -355,7 +358,16 @@ export function detectSignals(cwd: string, now: () => number = Date.now): SmartE
   // Stale = no recorded activity for IDLE_STALE_MS. Used only by idle-stranded.
   // Computed here (with the clock seam) so the pure classify() stays a function
   // of (signals, staleActivity) and detectSignals owns all disk reads.
+  // #3099 (ADR-1411 amendment): if last_activity is present but unparseable,
+  // emit a diagnostic so the silent fallback (stale_activity: false) is visible.
+  // The fallback itself stays — continuity is correct, the silence was the defect.
   const lastActivityMs = parseActivityTimestamp(lastActivityRaw);
+  if (lastActivityRaw && lastActivityMs === null) {
+    warnUnusableInput({
+      reason: UNUSABLE_REASON.LAST_ACTIVITY_UNPARSEABLE,
+      source: paths.state,
+    });
+  }
   const staleActivity = lastActivityMs !== null && now() - lastActivityMs > IDLE_STALE_MS;
 
   // Verify-failed may be signalled either by STATE.md status or by a failed
diff --git a/src/unusable-input.cts b/src/unusable-input.cts
index 96c7f5af6..83206d385 100644
--- a/src/unusable-input.cts
+++ b/src/unusable-input.cts
@@ -49,6 +49,14 @@ const UNUSABLE_REASON = Object.freeze({
    * simply has no ROADMAP yet: absence returns the same sentinel, silently. (#1881)
    */
   ROADMAP_UNREADABLE: 'roadmap_unreadable',
+  /**
+   * A `last_activity` value in STATE.md frontmatter is present but unparseable as a date.
+   * Distinct from absent: absence means the field was never set, while an unparseable value
+   * means it was set to something Date.parse rejects. Per ADR-1411 amendment: corrupt is not
+   * absent — the fallback (stale_activity: false) stays, but a diagnostic is emitted so the
+   * silent degradation is visible. (#3099, sixth #1879 site)
+   */
+  LAST_ACTIVITY_UNPARSEABLE: 'last_activity_unparseable',
 } as const);
 
 type UnusableReason = (typeof UNUSABLE_REASON)[keyof typeof UNUSABLE_REASON];
@@ -59,6 +67,8 @@ const REASON_PROSE: Readonly<Record<UnusableReason, string>> = Object.freeze({
     'frontmatter opens with "---" but never closes; metadata was NOT applied',
   [UNUSABLE_REASON.ROADMAP_UNREADABLE]:
     'ROADMAP.md exists but could not be read; phase and milestone lookups fell back to defaults',
+  [UNUSABLE_REASON.LAST_ACTIVITY_UNPARSEABLE]:
+    'last_activity in STATE.md is present but unparseable as a date; stale_activity fell back to false (idle-stranded suppressed)',
 });
 
 // ─── Dedup state ──────────────────────────────────────────────────────────────
diff --git a/tests/smart-entry.unit.test.cjs b/tests/smart-entry.unit.test.cjs
index a76c228ad..2a54e3f2c 100644
--- a/tests/smart-entry.unit.test.cjs
+++ b/tests/smart-entry.unit.test.cjs
@@ -549,3 +549,89 @@ describe('#2427 — roadmap-grounded completion + tightened status regex', () =>
       `legacy fallback must still reject completion when current_phase < total_phases. Got: ${result.situation}`);
   });
 });
+
+// ---------------------------------------------------------------------------
+// #3099: unusable last_activity emits a diagnostic (ADR-1411 amendment:
+// corrupt is not absent — the fallback stays, the silence is the defect)
+// ---------------------------------------------------------------------------
+
+describe('#3099: unusable last_activity emits a diagnostic', () => {
+  const {
+    _resetUnusableInputWarningsForTests,
+    _unusableInputEmissionCountForTests,
+  } = require('../gsd-core/bin/lib/unusable-input.cjs');
+
+  function makeStateWithActivity(activity) {
+    return [
+      '---',
+      'status: executing',
+      `last_activity: ${activity}`,
+      '---',
+      '',
+      '# Project State',
+      '',
+      'Phase: 1',
+      '',
+    ].join('\n');
+  }
+
+  test('unusable last_activity still resolves stale_activity: false (fallback unchanged)', () => {
+    _resetUnusableInputWarningsForTests();
+    const dir = track(makeProject({
+      state: makeStateWithActivity('yesterday - did some work'),
+    }));
+    const signals = detectSignals(dir);
+    assert.equal(signals.stale_activity, false,
+      'unusable last_activity must still resolve stale_activity: false (continuity is correct)');
+  });
+
+  test('unusable last_activity emits LAST_ACTIVITY_UNPARSEABLE diagnostic', () => {
+    _resetUnusableInputWarningsForTests();
+    const dir = track(makeProject({
+      state: makeStateWithActivity('not-a-date-at-all'),
+    }));
+    detectSignals(dir);
+    assert.equal(_unusableInputEmissionCountForTests(), 1,
+      'unusable last_activity must emit exactly one diagnostic');
+  });
+
+  test('absent last_activity emits nothing (distinguishable from unusable)', () => {
+    _resetUnusableInputWarningsForTests();
+    const dir = track(makeProject({
+      state: [
+        '---',
+        'status: executing',
+        '---',
+        '',
+        '# Project State',
+        '',
+        'Phase: 1',
+        '',
+      ].join('\n'),
+    }));
+    detectSignals(dir);
+    assert.equal(_unusableInputEmissionCountForTests(), 0,
+      'absent last_activity must NOT emit a diagnostic (it is genuinely absent, not corrupt)');
+  });
+
+  test('well-formed last_activity emits nothing', () => {
+    _resetUnusableInputWarningsForTests();
+    const dir = track(makeProject({
+      state: makeStateWithActivity('2026-06-13T12:00:00Z'),
+    }));
+    detectSignals(dir);
+    assert.equal(_unusableInputEmissionCountForTests(), 0,
+      'well-formed last_activity must NOT emit a diagnostic');
+  });
+
+  test('unusable last_activity does not re-emit on second call (dedup)', () => {
+    _resetUnusableInputWarningsForTests();
+    const dir = track(makeProject({
+      state: makeStateWithActivity('gibberish'),
+    }));
+    detectSignals(dir);
+    detectSignals(dir);
+    assert.equal(_unusableInputEmissionCountForTests(), 1,
+      'dedup: second call on the same source must not re-emit');
+  });
+});
diff --git a/tests/unusable-input.test.cjs b/tests/unusable-input.test.cjs
index c2da7a9bf..1d9f8549b 100644
--- a/tests/unusable-input.test.cjs
+++ b/tests/unusable-input.test.cjs
@@ -72,7 +72,7 @@ describe('UNUSABLE_REASON', () => {
     // (enum + call site + this assertion) instead of a silent widening.
     assert.deepStrictEqual(
       Object.keys(UNUSABLE_REASON).sort(),
-      ['FRONTMATTER_UNTERMINATED', 'ROADMAP_UNREADABLE'],
+      ['FRONTMATTER_UNTERMINATED', 'LAST_ACTIVITY_UNPARSEABLE', 'ROADMAP_UNREADABLE'],
     );
     assert.strictEqual(UNUSABLE_REASON.FRONTMATTER_UNTERMINATED, 'frontmatter_unterminated');
   });