diff --git a/.changeset/serene-koalas-hum.md b/.changeset/serene-koalas-hum.md
new file mode 100644
index 000000000..df6fe75eb
--- /dev/null
+++ b/.changeset/serene-koalas-hum.md
@@ -0,0 +1,5 @@
+---
+type: Fixed
+pr: 2324
+---
+**The context-monitor hook no longer fails Codex's Stop hook** — GSD wires `gsd-context-monitor` to Codex lifecycle events including `Stop`, but the hook emitted a `hookSpecificOutput.additionalContext` envelope that Codex's Stop schema rejects ("hook returned invalid stop hook JSON output") exactly when context was low. The hook now emits that envelope only for context-injection events (PostToolUse / AfterTool) and exits silently for Stop and every other lifecycle event, while its debounce and critical-session bookkeeping still run. (#2289)
diff --git a/hooks/gsd-context-monitor.js b/hooks/gsd-context-monitor.js
index 991535f40..656540cde 100644
--- a/hooks/gsd-context-monitor.js
+++ b/hooks/gsd-context-monitor.js
@@ -180,15 +180,33 @@ process.stdin.on('end', () => {
           'starting new complex work.';
     }
 
-    const output = {
-      hookSpecificOutput: {
-        hookEventName: (data.hook_event_name && data.hook_event_name.trim())
-          || (process.env.GEMINI_API_KEY ? "AfterTool" : "PostToolUse"),
-        additionalContext: message
-      }
-    };
+    // #2289: the hookSpecificOutput.additionalContext envelope is only a valid
+    // output shape for the context-injection events (PostToolUse, and AfterTool
+    // for the Gemini dialect). This hook is also wired to other lifecycle events
+    // on some hosts — Codex registers it under Stop / SubagentStart /
+    // SubagentStop / PreCompact (#772) — and those reject the envelope
+    // ("hook returned invalid stop hook JSON output"). Use a POSITIVE allowlist:
+    // emit only for injection-capable events; every other event, and a
+    // missing/unrecognized name, exits 0 with no stdout. A Stop-only blacklist is
+    // not enough — a missing name would still fall through to the injection path.
+    // All side effects above (debounce counter, one-time critical-session
+    // recording) have already run regardless of whether output is emitted.
+    const eventName = (data.hook_event_name && data.hook_event_name.trim()) || "";
+    // Preserve the pre-#2289 Gemini fallback: a missing event name under a
+    // Gemini-dialect runtime (GEMINI_API_KEY set) still means AfterTool, so its
+    // advisory output is unchanged. A missing name on any other host is silent.
+    const geminiFallback = eventName === "" && !!process.env.GEMINI_API_KEY;
+    const injectionSupported = eventName === "PostToolUse" || eventName === "AfterTool" || geminiFallback;
 
-    process.stdout.write(JSON.stringify(output));
+    if (injectionSupported) {
+      const output = {
+        hookSpecificOutput: {
+          hookEventName: eventName || "AfterTool",
+          additionalContext: message
+        }
+      };
+      process.stdout.write(JSON.stringify(output));
+    }
   } catch (e) {
     // Silent fail -- never block tool execution
     process.exit(0);
diff --git a/tests/claude-orchestration.test.cjs b/tests/claude-orchestration.test.cjs
index abc7f4453..2eaa89e6f 100644
--- a/tests/claude-orchestration.test.cjs
+++ b/tests/claude-orchestration.test.cjs
@@ -546,11 +546,20 @@ describe('emitWorkflowScript — per-plan use_worktree (#2772 / #2285 finding 1)
         { minLength: 1, maxLength: 4 },
       ).filter((plans) => new Set(plans.map((p) => p.id)).size === plans.length), // unique ids
       (planSpecs) => {
+        // Only plan IDs are unique — briefs may legitimately collide (fast-check
+        // shrinks toward short/empty strings, so duplicate briefs are common).
+        // emitWorkflowScript emits one agent() per plan keyed by the brief label
+        // and is correct for duplicate briefs, but the per-plan line lookup below
+        // (indexOf) would find the FIRST occurrence and misattribute its isolation
+        // when two plans share a brief. Make each agent() label unique by suffixing
+        // the unique id, so the lookup is unambiguous — this disambiguates the TEST
+        // probe, it does not change what the code under test does.
+        const labelFor = (p) => p.brief + ' [' + p.id + ']';
         const waves = [{
           id: 'w1',
           plans: planSpecs.map((p, i) => ({
             id: p.id,
-            brief: p.brief,
+            brief: labelFor(p),
             files_modified: ['src/file' + i + '.cts'], // disjoint -> no overlap-driven staging noise
             use_worktree: p.useWorktree,
           })),
@@ -558,7 +567,7 @@ describe('emitWorkflowScript — per-plan use_worktree (#2772 / #2285 finding 1)
         const r = emitWorkflowScript({ phaseDir: '.p', runId: 'r', waves });
         assert.strictEqual(r.ok, true);
         for (const p of planSpecs) {
-          const briefEsc = JSON.stringify(p.brief);
+          const briefEsc = JSON.stringify(labelFor(p));
           const idx = r.script.indexOf('agent(' + briefEsc + ',');
           assert.ok(idx !== -1, 'agent() call for plan must exist');
           const lineEnd = r.script.indexOf('\n', idx);
diff --git a/tests/fix-2289-context-monitor-event-allowlist.test.cjs b/tests/fix-2289-context-monitor-event-allowlist.test.cjs
new file mode 100644
index 000000000..d25ad3f36
--- /dev/null
+++ b/tests/fix-2289-context-monitor-event-allowlist.test.cjs
@@ -0,0 +1,204 @@
+'use strict';
+
+/**
+ * #2289 — gsd-context-monitor lifecycle-event output allowlist.
+ *
+ * The context monitor emits a `hookSpecificOutput.additionalContext` envelope
+ * to inject context warnings. That shape is only valid for the context-injection
+ * events (PostToolUse, and AfterTool for the Gemini dialect). Codex also wires
+ * this hook to Stop / SubagentStart / SubagentStop / PreCompact (#772), and
+ * Codex's Stop schema REJECTS the envelope ("hook returned invalid stop hook
+ * JSON output"). The fix uses a positive allowlist: emit only for
+ * injection-capable events; every other event — and a missing/unknown name —
+ * exits 0 with NO stdout, while side effects (debounce, critical-session
+ * recording) still run.
+ *
+ * These tests drive the real hook script end-to-end (spawn + stdin + a fresh
+ * metrics bridge file), asserting behavior, not source text.
+ */
+
+const { test, describe } = require('node:test');
+const assert = require('node:assert/strict');
+const fs = require('node:fs');
+const os = require('node:os');
+const path = require('node:path');
+const { execFileSync } = require('node:child_process');
+
+const HOOK_PATH = path.join(__dirname, '..', 'hooks', 'gsd-context-monitor.js');
+
+// Run the monitor with a synthetic, fresh metrics bridge file.
+// Returns { stdout, warnData } and cleans up the bridge + sentinel files.
+// opts: { event, remaining, used = 80, gemini = false, gsdActive = false }
+function runMonitor(opts) {
+  const {
+    event,
+    remaining,
+    used = 80,
+    gemini = false,
+    gsdActive = false,
+  } = opts;
+
+  const sessionId = `fix-2289-${Date.now()}-${Math.random().toString(36).slice(2)}`;
+  const tmpDir = os.tmpdir();
+  const metricsPath = path.join(tmpDir, `claude-ctx-${sessionId}.json`);
+  const warnPath = path.join(tmpDir, `claude-ctx-${sessionId}-warned.json`);
+
+  // Fresh (non-stale) metrics: timestamp is "now" in seconds.
+  fs.writeFileSync(metricsPath, JSON.stringify({
+    timestamp: Math.floor(Date.now() / 1000),
+    remaining_percentage: remaining,
+    used_pct: used,
+  }));
+
+  // Optional GSD-active project dir (STATE.md present) so the critical-session
+  // recording side effect is reachable.
+  let cwd = tmpDir;
+  let projDir = null;
+  if (gsdActive) {
+    projDir = fs.mkdtempSync(path.join(tmpDir, 'fix-2289-proj-'));
+    fs.mkdirSync(path.join(projDir, '.planning'), { recursive: true });
+    fs.writeFileSync(path.join(projDir, '.planning', 'STATE.md'), '# State\n');
+    cwd = projDir;
+  }
+
+  const payload = { session_id: sessionId, cwd };
+  if (event !== undefined) payload.hook_event_name = event;
+
+  const env = { ...process.env };
+  if (gemini) env.GEMINI_API_KEY = 'test-key';
+  else delete env.GEMINI_API_KEY;
+
+  let stdout = '';
+  try {
+    stdout = execFileSync(process.execPath, [HOOK_PATH], {
+      input: JSON.stringify(payload),
+      env,
+      encoding: 'utf8',
+      timeout: 8000,
+    });
+  } catch (e) {
+    stdout = e.stdout || '';
+  }
+
+  let warnData = null;
+  try {
+    warnData = JSON.parse(fs.readFileSync(warnPath, 'utf8'));
+  } catch { /* sentinel may not exist */ }
+
+  // Cleanup
+  for (const p of [metricsPath, warnPath]) {
+    try { fs.unlinkSync(p); } catch { /* ignore */ }
+  }
+  if (projDir) {
+    // Retry-tolerant teardown: the critical path fires a detached, unref()'d
+    // `state record-session` grandchild against projDir, and execFileSync does
+    // not wait for it. maxRetries/retryDelay absorbs the transient
+    // EBUSY/ENOTEMPTY window while that process exits, so cleanup can neither
+    // flake nor leak the temp dir (mirrors tests/helpers.cjs cleanup(); see the
+    // #2289 review and the prior fix in perf-317-context-monitor-fs.test.cjs).
+    // eslint-disable-next-line local/no-raw-rmsync-in-tests -- test fixture teardown of a unique mkdtemp dir
+    try { fs.rmSync(projDir, { recursive: true, force: true, maxRetries: 20, retryDelay: 100 }); } catch { /* ignore */ }
+  }
+
+  return { stdout, warnData };
+}
+
+describe('#2289 context-monitor: non-injection events exit silently', () => {
+  // Boundary coverage around WARNING (35) and CRITICAL (25) — Stop must stay
+  // silent at limit-1 / limit / limit+1 for BOTH thresholds.
+  for (const remaining of [40, 36, 35, 34, 26, 25, 24, 20]) {
+    test(`Stop event at remaining=${remaining}% → exit 0, empty stdout`, () => {
+      const { stdout } = runMonitor({ event: 'Stop', remaining });
+      assert.strictEqual(stdout, '', `Stop must emit nothing at remaining=${remaining}% (Codex rejects the envelope)`);
+    });
+  }
+
+  test('missing hook_event_name (no Gemini) at 30% → empty stdout', () => {
+    const { stdout } = runMonitor({ event: undefined, remaining: 30 });
+    assert.strictEqual(stdout, '', 'a missing event name must not fall through to the injection envelope');
+  });
+
+  test('empty-string hook_event_name (no Gemini) at 30% → empty stdout', () => {
+    const { stdout } = runMonitor({ event: '   ', remaining: 30 });
+    assert.strictEqual(stdout, '', 'a blank event name must be treated as missing → silent');
+  });
+
+  for (const event of ['SubagentStart', 'SubagentStop', 'PreCompact', 'SessionStart', 'BeforeTool']) {
+    test(`unknown/non-injection event "${event}" at 30% → empty stdout`, () => {
+      const { stdout } = runMonitor({ event, remaining: 30 });
+      assert.strictEqual(stdout, '', `${event} is not injection-capable and must emit nothing`);
+    });
+  }
+});
+
+describe('#2289 context-monitor: injection events still warn (unchanged)', () => {
+  test('PostToolUse at 30% → WARNING envelope with hookEventName PostToolUse', () => {
+    const { stdout } = runMonitor({ event: 'PostToolUse', remaining: 30, used: 70 });
+    assert.notStrictEqual(stdout, '', 'PostToolUse must still emit a warning envelope');
+    const parsed = JSON.parse(stdout);
+    assert.strictEqual(parsed.hookSpecificOutput.hookEventName, 'PostToolUse');
+    assert.match(parsed.hookSpecificOutput.additionalContext, /CONTEXT WARNING/);
+  });
+
+  test('PostToolUse at 20% → CRITICAL envelope', () => {
+    const { stdout } = runMonitor({ event: 'PostToolUse', remaining: 20, used: 80 });
+    const parsed = JSON.parse(stdout);
+    assert.strictEqual(parsed.hookSpecificOutput.hookEventName, 'PostToolUse');
+    assert.match(parsed.hookSpecificOutput.additionalContext, /CONTEXT CRITICAL/);
+  });
+
+  test('AfterTool at 30% → WARNING envelope with hookEventName AfterTool', () => {
+    const { stdout } = runMonitor({ event: 'AfterTool', remaining: 30 });
+    const parsed = JSON.parse(stdout);
+    assert.strictEqual(parsed.hookSpecificOutput.hookEventName, 'AfterTool');
+    assert.match(parsed.hookSpecificOutput.additionalContext, /CONTEXT WARNING/);
+  });
+
+  test('missing event name WITH Gemini env at 30% → AfterTool envelope (fallback preserved)', () => {
+    const { stdout } = runMonitor({ event: undefined, remaining: 30, gemini: true });
+    assert.notStrictEqual(stdout, '', 'Gemini AfterTool fallback must still emit when the event name is absent');
+    const parsed = JSON.parse(stdout);
+    assert.strictEqual(parsed.hookSpecificOutput.hookEventName, 'AfterTool');
+  });
+
+  test('explicit PostToolUse WITH Gemini env → explicit name wins over the AfterTool fallback', () => {
+    // Precedence guard: the Gemini fallback only applies to a MISSING name; an
+    // explicit PostToolUse must still report as PostToolUse even under GEMINI_API_KEY.
+    const { stdout } = runMonitor({ event: 'PostToolUse', remaining: 30, gemini: true });
+    const parsed = JSON.parse(stdout);
+    assert.strictEqual(parsed.hookSpecificOutput.hookEventName, 'PostToolUse');
+    assert.match(parsed.hookSpecificOutput.additionalContext, /CONTEXT WARNING/);
+  });
+
+  // Threshold boundaries on the emit path: 36 = no warn, 35 = warn, 25 = critical, 26 = warn.
+  test('PostToolUse at 36% (above WARNING) → empty stdout', () => {
+    const { stdout } = runMonitor({ event: 'PostToolUse', remaining: 36 });
+    assert.strictEqual(stdout, '', 'no warning above the 35% threshold');
+  });
+
+  test('PostToolUse at 35% (WARNING boundary) → WARNING envelope', () => {
+    const { stdout } = runMonitor({ event: 'PostToolUse', remaining: 35 });
+    assert.match(JSON.parse(stdout).hookSpecificOutput.additionalContext, /CONTEXT WARNING/);
+  });
+
+  test('PostToolUse at 25% (CRITICAL boundary) → CRITICAL envelope', () => {
+    const { stdout } = runMonitor({ event: 'PostToolUse', remaining: 25 });
+    assert.match(JSON.parse(stdout).hookSpecificOutput.additionalContext, /CONTEXT CRITICAL/);
+  });
+});
+
+describe('#2289 context-monitor: side effects still fire on silent events (no output ≠ no side effect)', () => {
+  test('Stop at 30% still writes the debounce sentinel (bookkeeping runs)', () => {
+    const { stdout, warnData } = runMonitor({ event: 'Stop', remaining: 30 });
+    assert.strictEqual(stdout, '', 'Stop emits nothing');
+    assert.ok(warnData, 'the debounce sentinel must still be written on a silenced Stop event');
+    assert.strictEqual(warnData.lastLevel, 'warning', 'debounce level bookkeeping runs regardless of output');
+  });
+
+  test('Stop at 20% in a GSD project still records the critical-session sentinel', () => {
+    const { stdout, warnData } = runMonitor({ event: 'Stop', remaining: 20, used: 80, gsdActive: true });
+    assert.strictEqual(stdout, '', 'Stop emits nothing even at critical context');
+    assert.ok(warnData, 'sentinel must be written');
+    assert.strictEqual(warnData.criticalRecorded, true, 'critical-session recording side effect fires on the silent Stop event');
+  });
+});
diff --git a/tests/fixtures/golden-install-parity/antigravity.json b/tests/fixtures/golden-install-parity/antigravity.json
index a2d5dc2a0..5b1534d47 100644
--- a/tests/fixtures/golden-install-parity/antigravity.json
+++ b/tests/fixtures/golden-install-parity/antigravity.json
@@ -314,7 +314,7 @@
   "hooks/gsd-check-update-worker.js": "fa301e6366270d5f",
   "hooks/gsd-check-update.js": "4617a98bf529e4c3",
   "hooks/gsd-config-reload.js": "96546e0e8bb47904",
-  "hooks/gsd-context-monitor.js": "6d81d7326e5b2710",
+  "hooks/gsd-context-monitor.js": "31a4d99fd3b260ad",
   "hooks/gsd-cursor-post-tool.js": "9168e0a09de1972a",
   "hooks/gsd-cursor-pre-tool.js": "873998b25e308c29",
   "hooks/gsd-cursor-session-start.js": "9b2e6f4f0c405375",
diff --git a/tests/fixtures/golden-install-parity/augment.json b/tests/fixtures/golden-install-parity/augment.json
index a67da0c77..4742483ab 100644
--- a/tests/fixtures/golden-install-parity/augment.json
+++ b/tests/fixtures/golden-install-parity/augment.json
@@ -385,7 +385,7 @@
   "hooks/gsd-check-update-worker.js": "cc1ef5f840f9dfc9",
   "hooks/gsd-check-update.js": "7b3a7983d5f1f5d3",
   "hooks/gsd-config-reload.js": "96546e0e8bb47904",
-  "hooks/gsd-context-monitor.js": "44ff1bbf292747af",
+  "hooks/gsd-context-monitor.js": "493e24c139c61129",
   "hooks/gsd-cursor-post-tool.js": "9168e0a09de1972a",
   "hooks/gsd-cursor-pre-tool.js": "873998b25e308c29",
   "hooks/gsd-cursor-session-start.js": "9b2e6f4f0c405375",
diff --git a/tests/fixtures/golden-install-parity/claude-local.json b/tests/fixtures/golden-install-parity/claude-local.json
index 13a384d94..ca1d0e981 100644
--- a/tests/fixtures/golden-install-parity/claude-local.json
+++ b/tests/fixtures/golden-install-parity/claude-local.json
@@ -384,7 +384,7 @@
   "hooks/gsd-check-update-worker.js": "a530efdb5fdc0da3",
   "hooks/gsd-check-update.js": "25cde66a12d6b886",
   "hooks/gsd-config-reload.js": "96546e0e8bb47904",
-  "hooks/gsd-context-monitor.js": "ecbe9747e4a442e0",
+  "hooks/gsd-context-monitor.js": "f2ac6a1033d140de",
   "hooks/gsd-cursor-post-tool.js": "8a8a249c0642cc71",
   "hooks/gsd-cursor-pre-tool.js": "8cb8e8f895edaec9",
   "hooks/gsd-cursor-session-start.js": "05a14e903c5edafa",
diff --git a/tests/fixtures/golden-install-parity/claude.json b/tests/fixtures/golden-install-parity/claude.json
index ed0118837..1f7126836 100644
--- a/tests/fixtures/golden-install-parity/claude.json
+++ b/tests/fixtures/golden-install-parity/claude.json
@@ -313,7 +313,7 @@
   "hooks/gsd-check-update-worker.js": "a530efdb5fdc0da3",
   "hooks/gsd-check-update.js": "25cde66a12d6b886",
   "hooks/gsd-config-reload.js": "96546e0e8bb47904",
-  "hooks/gsd-context-monitor.js": "ecbe9747e4a442e0",
+  "hooks/gsd-context-monitor.js": "f2ac6a1033d140de",
   "hooks/gsd-cursor-post-tool.js": "8a8a249c0642cc71",
   "hooks/gsd-cursor-pre-tool.js": "8cb8e8f895edaec9",
   "hooks/gsd-cursor-session-start.js": "05a14e903c5edafa",
diff --git a/tests/fixtures/golden-install-parity/codebuddy.json b/tests/fixtures/golden-install-parity/codebuddy.json
index d792532aa..5ea483dd8 100644
--- a/tests/fixtures/golden-install-parity/codebuddy.json
+++ b/tests/fixtures/golden-install-parity/codebuddy.json
@@ -385,7 +385,7 @@
   "hooks/gsd-check-update-worker.js": "bdc9324a2f080ddd",
   "hooks/gsd-check-update.js": "b7669f605631e506",
   "hooks/gsd-config-reload.js": "96546e0e8bb47904",
-  "hooks/gsd-context-monitor.js": "f372804867cabe40",
+  "hooks/gsd-context-monitor.js": "8d512ba0b08fc4e3",
   "hooks/gsd-cursor-post-tool.js": "9168e0a09de1972a",
   "hooks/gsd-cursor-pre-tool.js": "873998b25e308c29",
   "hooks/gsd-cursor-session-start.js": "9b2e6f4f0c405375",
diff --git a/tests/fixtures/golden-install-parity/codex.json b/tests/fixtures/golden-install-parity/codex.json
index 2dcb4d563..3c92bb509 100644
--- a/tests/fixtures/golden-install-parity/codex.json
+++ b/tests/fixtures/golden-install-parity/codex.json
@@ -418,7 +418,7 @@
   "gsd-core/workflows/verify-phase.md": "2e12c3cb97a9122a",
   "gsd-core/workflows/verify-work.md": "5b348e73fc0d0829",
   "hooks/gsd-check-update.js": "ef48957eb6ac6a10",
-  "hooks/gsd-context-monitor.js": "76fecaaa2babd6c1",
+  "hooks/gsd-context-monitor.js": "ec76b23f45d29b02",
   "scripts/changeset/README.md": "86ff89331dfd94b2",
   "scripts/changeset/cli.cjs": "68f92a344b199271",
   "scripts/changeset/github-release-notes.cjs": "795677f0c009b132",
diff --git a/tests/fixtures/golden-install-parity/hermes.json b/tests/fixtures/golden-install-parity/hermes.json
index 5b169e580..90875b0fe 100644
--- a/tests/fixtures/golden-install-parity/hermes.json
+++ b/tests/fixtures/golden-install-parity/hermes.json
@@ -314,7 +314,7 @@
   "hooks/gsd-check-update-worker.js": "7989cc2bedd1138d",
   "hooks/gsd-check-update.js": "25f5ad726f76fc11",
   "hooks/gsd-config-reload.js": "880b696458e85e9b",
-  "hooks/gsd-context-monitor.js": "41d28e0db7b20968",
+  "hooks/gsd-context-monitor.js": "b829f618e82406de",
   "hooks/gsd-cursor-post-tool.js": "8a8a249c0642cc71",
   "hooks/gsd-cursor-pre-tool.js": "8cb8e8f895edaec9",
   "hooks/gsd-cursor-session-start.js": "05a14e903c5edafa",
diff --git a/tests/fixtures/golden-install-parity/kimi.json b/tests/fixtures/golden-install-parity/kimi.json
index 757d1ab89..85eb40534 100644
--- a/tests/fixtures/golden-install-parity/kimi.json
+++ b/tests/fixtures/golden-install-parity/kimi.json
@@ -4,7 +4,7 @@
   ".kimi/hooks/gsd-check-update-worker.js": "6593f80914f4edee",
   ".kimi/hooks/gsd-check-update.js": "aac7612ee04ff5fe",
   ".kimi/hooks/gsd-config-reload.js": "96546e0e8bb47904",
-  ".kimi/hooks/gsd-context-monitor.js": "783c93351e82151f",
+  ".kimi/hooks/gsd-context-monitor.js": "4c0535bc269e130e",
   ".kimi/hooks/gsd-cursor-post-tool.js": "9168e0a09de1972a",
   ".kimi/hooks/gsd-cursor-pre-tool.js": "873998b25e308c29",
   ".kimi/hooks/gsd-cursor-session-start.js": "9b2e6f4f0c405375",
diff --git a/tests/fixtures/golden-install-parity/opencode.json b/tests/fixtures/golden-install-parity/opencode.json
index 0389a4fab..12b64cfd1 100644
--- a/tests/fixtures/golden-install-parity/opencode.json
+++ b/tests/fixtures/golden-install-parity/opencode.json
@@ -385,7 +385,7 @@
   "hooks/gsd-check-update-worker.js": "385fb7c67810baf6",
   "hooks/gsd-check-update.js": "4549451414ffa7d7",
   "hooks/gsd-config-reload.js": "96546e0e8bb47904",
-  "hooks/gsd-context-monitor.js": "7a9787868a39b76d",
+  "hooks/gsd-context-monitor.js": "91ce47cd2dc3bbe8",
   "hooks/gsd-cursor-post-tool.js": "9168e0a09de1972a",
   "hooks/gsd-cursor-pre-tool.js": "873998b25e308c29",
   "hooks/gsd-cursor-session-start.js": "9b2e6f4f0c405375",
diff --git a/tests/fixtures/golden-install-parity/pi.json b/tests/fixtures/golden-install-parity/pi.json
index cfe486f79..ef2dbc2a1 100644
--- a/tests/fixtures/golden-install-parity/pi.json
+++ b/tests/fixtures/golden-install-parity/pi.json
@@ -281,7 +281,7 @@
   "hooks/gsd-check-update-worker.js": "55376b5b9335a580",
   "hooks/gsd-check-update.js": "a89562537a41f83d",
   "hooks/gsd-config-reload.js": "96546e0e8bb47904",
-  "hooks/gsd-context-monitor.js": "1c48eb0f38a24318",
+  "hooks/gsd-context-monitor.js": "e72cba19eeab9036",
   "hooks/gsd-cursor-post-tool.js": "9168e0a09de1972a",
   "hooks/gsd-cursor-pre-tool.js": "873998b25e308c29",
   "hooks/gsd-cursor-session-start.js": "9b2e6f4f0c405375",
diff --git a/tests/fixtures/golden-install-parity/qwen.json b/tests/fixtures/golden-install-parity/qwen.json
index d04b00d06..2149e8638 100644
--- a/tests/fixtures/golden-install-parity/qwen.json
+++ b/tests/fixtures/golden-install-parity/qwen.json
@@ -314,7 +314,7 @@
   "hooks/gsd-check-update-worker.js": "4bb354044e0dff91",
   "hooks/gsd-check-update.js": "d2065cb3e725a42a",
   "hooks/gsd-config-reload.js": "4f52b8a0120bb1b8",
-  "hooks/gsd-context-monitor.js": "437a33e6e3058640",
+  "hooks/gsd-context-monitor.js": "b71785540277e9d7",
   "hooks/gsd-cursor-post-tool.js": "8a8a249c0642cc71",
   "hooks/gsd-cursor-pre-tool.js": "8cb8e8f895edaec9",
   "hooks/gsd-cursor-session-start.js": "05a14e903c5edafa",
diff --git a/tests/perf-317-context-monitor-fs.test.cjs b/tests/perf-317-context-monitor-fs.test.cjs
index 31da5bae7..3c580566e 100644
--- a/tests/perf-317-context-monitor-fs.test.cjs
+++ b/tests/perf-317-context-monitor-fs.test.cjs
@@ -64,7 +64,10 @@ function runMonitorRaw(opts) {
     fs.writeFileSync(warnPath, JSON.stringify(wd));
   }
 
-  const input = JSON.stringify({ session_id: sessionId, cwd });
+  // #2289: explicit hook_event_name is required — the hook now emits its
+  // envelope ONLY for the PostToolUse/AfterTool allowlist; a missing name
+  // (non-Gemini) is silent. These callers model PostToolUse invocations.
+  const input = JSON.stringify({ session_id: sessionId, cwd, hook_event_name: 'PostToolUse' });
   let stdout = '';
   let exitCode = 0;
 
@@ -185,8 +188,11 @@ describe('perf #317: config.json absent (exercises config-missing → defaults p
     let exitCode = 0;
     let stdout = '';
     try {
+      // #2289: send hook_event_name: 'PostToolUse' so the silence asserted below
+      // is attributable ONLY to context_warnings=false, not to the hook's
+      // non-injection-event silence path.
       stdout = execFileSync(process.execPath, [MONITOR_PATH], {
-        input: JSON.stringify({ session_id: sessionId, cwd: testCwd }),
+        input: JSON.stringify({ session_id: sessionId, cwd: testCwd, hook_event_name: 'PostToolUse' }),
         encoding: 'utf-8',
         timeout: 5000,
       });
@@ -351,9 +357,13 @@ function runHook(sessionId, remainingPct, cwd) {
     timestamp: Math.floor(Date.now() / 1000),
   }));
 
+  // #2289: explicit hook_event_name: 'PostToolUse' so the hook takes the
+  // emitting/allowlisted path — the tests in this block assert on stdout
+  // content and record-session side effects, not event-name plumbing.
   const input = JSON.stringify({
     session_id: sessionId,
     cwd,
+    hook_event_name: 'PostToolUse',
   });
 
   const result = spawnSync(process.execPath, [HOOK_PATH], {
@@ -640,7 +650,10 @@ function runMonitorHook(remainingPct, usedPct) {
     timestamp: Math.floor(Date.now() / 1000),
   }));
 
-  const input = JSON.stringify({ session_id: sessionId, cwd: os.tmpdir() });
+  // #2289: explicit hook_event_name: 'PostToolUse' — this helper's callers
+  // assert on emitted message content (used_pct wording), which requires
+  // the allowlisted emitting path.
+  const input = JSON.stringify({ session_id: sessionId, cwd: os.tmpdir(), hook_event_name: 'PostToolUse' });
   let stdout = '';
   try {
     stdout = execFileSync(process.execPath, [MONITOR_PATH], {
@@ -826,36 +839,26 @@ function makeSessionId(suffix) {
 
 // ─── hookEventName echoing ────────────────────────────────────────────────────
 
-describe('bug #925: context monitor echoes the invoking hook event name', () => {
-  test('hookEventName is "Stop" when payload contains hook_event_name: "Stop"', () => {
+describe('bug #925: context monitor echoes the invoking hook event name (superseded for non-injection events by #2289)', () => {
+  test('Stop is a non-injection event → silent (#2289)', () => {
+    // #2289: Codex's Stop schema rejects the hookSpecificOutput envelope
+    // entirely ("invalid stop hook JSON output"), so the hook must emit
+    // NOTHING for Stop rather than echo it. This supersedes bug #925's
+    // "echo the triggering event name" behavior for Stop specifically.
     const out = runMonitor({ hookEventName: 'Stop', sessionId: makeSessionId('stop') });
-    assert.ok(out, 'hook must emit output when context is below WARNING threshold (remaining=30)');
-    assert.strictEqual(
-      out.hookSpecificOutput?.hookEventName,
-      'Stop',
-      `Expected hookEventName "Stop" but got "${out.hookSpecificOutput?.hookEventName}". ` +
-      'The hook must echo the hook_event_name from stdin, not hardcode "PostToolUse".'
-    );
+    assert.strictEqual(out, null, 'Stop is a non-injection event → silent (#2289)');
   });
 
-  test('hookEventName is "SubagentStop" when payload contains hook_event_name: "SubagentStop"', () => {
+  test('SubagentStop is a non-injection event → silent (#2289)', () => {
+    // #2289: same rationale as Stop above — non-injection events get no envelope.
     const out = runMonitor({ hookEventName: 'SubagentStop', sessionId: makeSessionId('subagent-stop') });
-    assert.ok(out, 'hook must emit output when context is below WARNING threshold');
-    assert.strictEqual(
-      out.hookSpecificOutput?.hookEventName,
-      'SubagentStop',
-      `Expected hookEventName "SubagentStop" but got "${out.hookSpecificOutput?.hookEventName}".`
-    );
+    assert.strictEqual(out, null, 'SubagentStop is a non-injection event → silent (#2289)');
   });
 
-  test('hookEventName is "PreCompact" when payload contains hook_event_name: "PreCompact"', () => {
+  test('PreCompact is a non-injection event → silent (#2289)', () => {
+    // #2289: same rationale as Stop above — non-injection events get no envelope.
     const out = runMonitor({ hookEventName: 'PreCompact', sessionId: makeSessionId('precompact') });
-    assert.ok(out, 'hook must emit output when context is below WARNING threshold');
-    assert.strictEqual(
-      out.hookSpecificOutput?.hookEventName,
-      'PreCompact',
-      `Expected hookEventName "PreCompact" but got "${out.hookSpecificOutput?.hookEventName}".`
-    );
+    assert.strictEqual(out, null, 'PreCompact is a non-injection event → silent (#2289)');
   });
 
   test('hookEventName is "PostToolUse" when payload contains hook_event_name: "PostToolUse"', () => {
@@ -871,8 +874,12 @@ describe('bug #925: context monitor echoes the invoking hook event name', () =>
 
 // ─── Fallback behaviour (no hook_event_name in payload) ──────────────────────
 
-describe('bug #925: context monitor falls back to heuristic when hook_event_name absent', () => {
-  test('falls back to "PostToolUse" when hook_event_name is absent (non-Gemini)', () => {
+describe('bug #925: context monitor falls back to heuristic when hook_event_name absent (non-Gemini fallback now silent per #2289)', () => {
+  test('absent hook_event_name (non-Gemini) is now silent (#2289)', () => {
+    // #2289: a missing hook_event_name without GEMINI_API_KEY set used to fall
+    // back to "PostToolUse" and emit. It is now a non-injection case → silent,
+    // since we cannot positively confirm this is a context-injection-capable
+    // invocation without either an allowlisted event name or the Gemini signal.
     const env = { ...process.env };
     delete env.GEMINI_API_KEY;
     const out = runMonitor({
@@ -880,15 +887,12 @@ describe('bug #925: context monitor falls back to heuristic when hook_event_name
       sessionId: makeSessionId('fallback-non-gemini'),
       env: { GEMINI_API_KEY: '' }, // ensure unset
     });
-    assert.ok(out, 'hook must emit output when context is below WARNING threshold');
-    assert.strictEqual(
-      out.hookSpecificOutput?.hookEventName,
-      'PostToolUse',
-      `Expected fallback "PostToolUse" for non-Gemini but got "${out.hookSpecificOutput?.hookEventName}".`
-    );
+    assert.strictEqual(out, null, 'absent hook_event_name (non-Gemini) is now silent (#2289)');
   });
 
   test('falls back to "AfterTool" when hook_event_name is absent and GEMINI_API_KEY is set', () => {
+    // Unchanged by #2289: this is the Gemini fallback, which remains an
+    // explicit allowlisted emitting path.
     const out = runMonitor({
       hookEventName: undefined,
       sessionId: makeSessionId('fallback-gemini'),
@@ -902,57 +906,44 @@ describe('bug #925: context monitor falls back to heuristic when hook_event_name
     );
   });
 
-  test('falls back to "PostToolUse" when hook_event_name is an empty string (non-Gemini)', () => {
+  test('empty-string hook_event_name (non-Gemini) is now silent (#2289)', () => {
+    // #2289: an empty hook_event_name without GEMINI_API_KEY is treated the
+    // same as absent — non-injection case → silent.
     const out = runMonitor({
       hookEventName: '',
       sessionId: makeSessionId('fallback-empty'),
       env: { GEMINI_API_KEY: '' },
     });
-    assert.ok(out, 'hook must emit output when context is below WARNING threshold');
-    assert.strictEqual(
-      out.hookSpecificOutput?.hookEventName,
-      'PostToolUse',
-      `Expected fallback "PostToolUse" for empty hook_event_name but got "${out.hookSpecificOutput?.hookEventName}".`
-    );
+    assert.strictEqual(out, null, 'empty-string hook_event_name (non-Gemini) is now silent (#2289)');
   });
 
-  test('falls back to "PostToolUse" when hook_event_name is whitespace-only (non-Gemini)', () => {
-    // trim() makes "   " → "" which is falsy, so the || fallback fires
+  test('whitespace-only hook_event_name (non-Gemini) is now silent (#2289)', () => {
+    // trim() makes "   " → "" which is falsy; #2289: this now takes the
+    // non-injection silent path rather than falling back to "PostToolUse".
     const out = runMonitor({
       hookEventName: '   ',
       sessionId: makeSessionId('fallback-whitespace'),
       env: { GEMINI_API_KEY: '' },
     });
-    assert.ok(out, 'hook must emit output when context is below WARNING threshold');
-    assert.strictEqual(
-      out.hookSpecificOutput?.hookEventName,
-      'PostToolUse',
-      `Expected fallback "PostToolUse" for whitespace-only hook_event_name but got "${out.hookSpecificOutput?.hookEventName}".`
-    );
+    assert.strictEqual(out, null, 'whitespace-only hook_event_name (non-Gemini) is now silent (#2289)');
   });
 });
 
 // ─── Critical threshold also echoes the event name ───────────────────────────
 
 describe('bug #925: critical threshold warning also uses correct hookEventName', () => {
-  test('CRITICAL warning emitted under Stop also echoes "Stop"', () => {
+  test('CRITICAL under Stop is silent (Codex rejects the Stop envelope) (#2289)', () => {
+    // #2289: even at CRITICAL severity, Stop is a non-injection event whose
+    // schema (Codex) rejects the hookSpecificOutput envelope outright. The
+    // hook must emit nothing rather than echo "Stop", superseding bug #925's
+    // "echoes Stop" expectation for this event specifically.
     const out = runMonitor({
       hookEventName: 'Stop',
       sessionId: makeSessionId('critical-stop'),
       remainingPct: 20,
       usedPct: 80,
     });
-    assert.ok(out, 'hook must emit output at critical threshold (remaining=20)');
-    assert.strictEqual(
-      out.hookSpecificOutput?.hookEventName,
-      'Stop',
-      `Expected hookEventName "Stop" at critical threshold, got "${out.hookSpecificOutput?.hookEventName}".`
-    );
-    assert.match(
-      out.hookSpecificOutput?.additionalContext || '',
-      /CONTEXT CRITICAL/,
-      'Output should be a CRITICAL warning at remaining=20'
-    );
+    assert.strictEqual(out, null, 'CRITICAL under Stop must be silent — no envelope for a non-injection event (#2289)');
   });
 });
   });