diff --git a/packages/core/src/__tests__/foreign-session-timeclip.test.ts b/packages/core/src/__tests__/foreign-session-timeclip.test.ts
new file mode 100644
index 0000000000..921bd46a1c
--- /dev/null
+++ b/packages/core/src/__tests__/foreign-session-timeclip.test.ts
@@ -0,0 +1,54 @@
+/*
+ * Licensed to the Apache Software Foundation (ASF) under one
+ * or more contributor license agreements.  See the NOTICE file
+ * distributed with this work for additional information
+ * regarding copyright ownership.  The ASF licenses this file
+ * to you under the Apache License, Version 2.0 (the
+ * "License"); you may not use this file except in compliance
+ * with the License.  You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing,
+ * software distributed under the License is distributed on an
+ * "AS IS" BASIS, WITHOUT WARRANTIES OR CONDITIONS OF ANY
+ * KIND, either express or implied.  See the License for the
+ * specific language governing permissions and limitations
+ * under the License.
+ */
+
+import assert from 'node:assert/strict';
+import { describe, it } from 'node:test';
+import {
+  createDigestAccumulator,
+  finishDigest,
+  normalizeCodexThreadRow,
+  renderForeignSessionDigestForPrompt,
+} from '../foreign-session.js';
+
+describe('foreign session timestamp bounds', () => {
+  it('rejects timestamps outside the ECMAScript TimeClip range', () => {
+    assert.equal(
+      normalizeCodexThreadRow({ id: 'future', rollout_path: '/p', updated_at_ms: 1e16 })
+        ?.updatedAtMs,
+      0,
+    );
+    assert.equal(
+      normalizeCodexThreadRow({ id: 'past', rollout_path: '/p', updated_at_ms: -1e16 })
+        ?.updatedAtMs,
+      0,
+    );
+  });
+
+  it('renders out-of-range finite timestamps as unknown instead of throwing', () => {
+    const digest = finishDigest(createDigestAccumulator(), {
+      source: 'codex',
+      id: 'safeid',
+      title: 'Future session',
+      cwd: '/repo',
+      updatedAtMs: 1e16,
+    });
+
+    assert.match(renderForeignSessionDigestForPrompt(digest), /^updated_at=unknown$/m);
+  });
+});
diff --git a/packages/core/src/foreign-session.ts b/packages/core/src/foreign-session.ts
index 8975847ae7..06d226ff05 100644
--- a/packages/core/src/foreign-session.ts
+++ b/packages/core/src/foreign-session.ts
@@ -400,6 +400,12 @@ export const CODEX_SUPPORTED_THREAD_SOURCES = ['cli', 'vscode', 'atlas', 'chatgp
  * `updated_at_ms` in ms on newer ones; this lets one path normalize both.
  */
 export const FOREIGN_SESSION_MIN_EPOCH_MS = 1_577_836_800_000;
+/** Largest absolute timestamp accepted by ECMAScript Date's TimeClip operation. */
+const FOREIGN_SESSION_MAX_EPOCH_MS = 8_640_000_000_000_000;
+
+function isValidForeignEpochMs(value: number): boolean {
+  return Number.isFinite(value) && Math.abs(value) <= FOREIGN_SESSION_MAX_EPOCH_MS;
+}
 
 function normalizeEpochMs(value: unknown): number | undefined {
   const n =
@@ -409,7 +415,8 @@ function normalizeEpochMs(value: unknown): number | undefined {
         ? Number(value)
         : undefined;
   if (n === undefined) return undefined;
-  return n > 0 && n < FOREIGN_SESSION_MIN_EPOCH_MS ? n * 1000 : n;
+  const normalized = n > 0 && n < FOREIGN_SESSION_MIN_EPOCH_MS ? n * 1000 : n;
+  return isValidForeignEpochMs(normalized) ? normalized : undefined;
 }
 
 /**
@@ -630,9 +637,9 @@ export function renderForeignSessionDigestForPrompt(digest: ForeignSessionDigest
     `title=${safe(digest.title)}`,
     `cwd=${safe(digest.cwd)}`,
     ...(digest.gitBranch ? [`git_branch=${safe(digest.gitBranch)}`] : []),
-    // A non-finite timestamp (corrupt store row) would make new Date().toISOString()
-    // throw RangeError, so guard it rather than let the render crash.
-    `updated_at=${Number.isFinite(digest.updatedAtMs) ? new Date(digest.updatedAtMs).toISOString() : 'unknown'}`,
+    // Invalid timestamps (corrupt store rows) make toISOString() throw RangeError,
+    // including finite values outside Date's TimeClip range.
+    `updated_at=${isValidForeignEpochMs(digest.updatedAtMs) ? new Date(digest.updatedAtMs).toISOString() : 'unknown'}`,
     '',
     '## User messages (chronological)',
     ...digest.userMessages.map((m, i) => `${i + 1}. ${safe(m)}`),