diff --git a/src/github/app.ts b/src/github/app.ts
index 5c859c9366..5a9850c9b0 100644
--- a/src/github/app.ts
+++ b/src/github/app.ts
@@ -361,9 +361,11 @@ async function mintInstallationToken(
     throw new Error(
       "GitHub installation token response did not include a token.",
     );
-  const expiresAtMs = payload.expires_at
-    ? Date.parse(payload.expires_at)
-    : Date.now() + 50 * 60_000;
+  // A present-but-UNPARSEABLE expires_at must fall back like an absent one (#10026): Date.parse returns NaN for
+  // a malformed string, and a NaN expiry makes `expiresAtMs - MARGIN > Date.now()` forever false — silently
+  // disabling the cache so every job re-mints and re-triggers the thundering-herd this cache exists to prevent.
+  const parsedExpiry = payload.expires_at ? Date.parse(payload.expires_at) : Number.NaN;
+  const expiresAtMs = Number.isFinite(parsedExpiry) ? parsedExpiry : Date.now() + 50 * 60_000;
   await writeCachedToken(installationId, { token: payload.token, expiresAtMs });
   return payload.token;
 }
diff --git a/test/unit/github-app.test.ts b/test/unit/github-app.test.ts
index 13e2a8d791..b9063793ac 100644
--- a/test/unit/github-app.test.ts
+++ b/test/unit/github-app.test.ts
@@ -200,6 +200,47 @@ describe("GitHub check runs", () => {
     expect(mints).toBe(1);
   });
 
+  it("REGRESSION: an unparseable expires_at must not disable the installation-token cache (#10026)", async () => {
+    const privateKey = await generatePrivateKeyPem();
+    let mints = 0;
+    vi.stubGlobal("fetch", async (input: RequestInfo | URL) => {
+      const url = input.toString();
+      if (url.includes("/access_tokens")) {
+        mints += 1;
+        return Response.json({ token: `installation-token-${mints}`, expires_at: "not-a-date" }); // present but unparseable
+      }
+      return new Response("not found", { status: 404 });
+    });
+
+    const env = createTestEnv({ GITHUB_APP_PRIVATE_KEY: privateKey });
+    const first = await createInstallationToken(env, 777);
+    const second = await createInstallationToken(env, 777);
+    // The NaN expiry used to make every cache comparison false, re-minting on every call. With the finite
+    // fallback the entry is honored: exactly one mint, the same token reused.
+    expect(first).toBe("installation-token-1");
+    expect(second).toBe("installation-token-1");
+    expect(mints).toBe(1); // NOT re-minted
+  });
+
+  it("a well-formed expires_at is unchanged: token reused on the second call (#10026)", async () => {
+    const privateKey = await generatePrivateKeyPem();
+    let mints = 0;
+    const expiresAt = "2030-01-01T00:00:00Z";
+    vi.stubGlobal("fetch", async (input: RequestInfo | URL) => {
+      const url = input.toString();
+      if (url.includes("/access_tokens")) {
+        mints += 1;
+        return Response.json({ token: `installation-token-${mints}`, expires_at: expiresAt });
+      }
+      return new Response("not found", { status: 404 });
+    });
+
+    const env = createTestEnv({ GITHUB_APP_PRIVATE_KEY: privateKey });
+    expect(await createInstallationToken(env, 888)).toBe("installation-token-1");
+    expect(await createInstallationToken(env, 888)).toBe("installation-token-1");
+    expect(mints).toBe(1); // cached against the far-future, well-formed expiry
+  });
+
   it("(#3811) samples clock skew from the installation-token mint response's Date header", async () => {
     const privateKey = await generatePrivateKeyPem();
     vi.useFakeTimers();