diff --git a/mcp/src/auth/grant-age.ts b/mcp/src/auth/grant-age.ts
new file mode 100644
index 0000000000..dfc1b0875a
--- /dev/null
+++ b/mcp/src/auth/grant-age.ts
@@ -0,0 +1,42 @@
+import { OAuthError } from "@cloudflare/workers-oauth-provider";
+import { z } from "zod";
+
+/**
+ * Absolute grant lifetime. The library bounds tokens (1h access, 30d idle refresh) but a grant
+ * that keeps being refreshed would live forever, holding an upstream session that cannot be
+ * refreshed — only held or deleted. So every token issuance re-checks the grant's age from
+ * `grantIssuedAt` in its (encrypted) props and fails closed with `invalid_grant`, which is what
+ * Claude expects when a refresh is no longer valid.
+ *
+ * Props without a timestamp are treated as expired: fail closed, never open.
+ */
+
+/**
+ * Absolute lifetime of a grant regardless of refresh activity: 90 days. Also the runway for
+ * swapping the upstream credential later (mcp/CLAUDE.md → Lifetimes).
+ */
+export const GRANT_MAX_AGE_SECONDS = 90 * 24 * 60 * 60;
+
+/** Props key holding the grant's issue time (epoch ms); set by consent, checked on every exchange. */
+export const GRANT_ISSUED_AT_PROP = "grantIssuedAt";
+
+const IssuedAtSchema = z.object({ [GRANT_ISSUED_AT_PROP]: z.number().int().positive() });
+
+export function assertGrantWithinMaxAge(
+  props: unknown,
+  nowMs: number,
+  maxAgeSeconds: number = GRANT_MAX_AGE_SECONDS,
+): void {
+  const parsed = IssuedAtSchema.safeParse(props);
+  if (!parsed.success) {
+    throw new OAuthError("invalid_grant", {
+      description: "grant has no issue time and is treated as expired",
+    });
+  }
+  const ageMs = nowMs - parsed.data[GRANT_ISSUED_AT_PROP];
+  if (ageMs > maxAgeSeconds * 1000) {
+    throw new OAuthError("invalid_grant", {
+      description: "grant is older than the maximum lifetime; the user must connect again",
+    });
+  }
+}
diff --git a/mcp/src/auth/oauth.ts b/mcp/src/auth/oauth.ts
index a179654aff..2a5ba7f3c8 100644
--- a/mcp/src/auth/oauth.ts
+++ b/mcp/src/auth/oauth.ts
@@ -1,5 +1,7 @@
 import type { OAuthProviderOptions } from "@cloudflare/workers-oauth-provider";
 
+import { assertGrantWithinMaxAge } from "./grant-age";
+
 /**
  * Configuration of the OAuth 2.1 authorization server that fronts the MCP endpoint. The
  * library (`@cloudflare/workers-oauth-provider`) owns discovery metadata, client registration
@@ -27,13 +29,6 @@ export type Scope = (typeof SCOPES)[number];
 export const ACCESS_TOKEN_TTL_SECONDS = 60 * 60;
 export const REFRESH_TOKEN_TTL_SECONDS = 30 * 24 * 60 * 60;
 
-/**
- * Absolute lifetime of a grant regardless of refresh activity. The upstream session cannot
- * be refreshed, only held or deleted, so grants must re-consent eventually; this is also the
- * runway for swapping the upstream credential later (mcp/CLAUDE.md → Lifetimes).
- */
-export const GRANT_MAX_AGE_SECONDS = 90 * 24 * 60 * 60;
-
 export const RESOURCE_NAME = "Artcraft";
 
 /** An ExportedHandler that definitely has `fetch` — what the provider accepts for handlers. */
@@ -75,6 +70,12 @@ export function createOAuthProviderOptions(handlers: OAuthHandlers): OAuthProvid
     accessTokenTTL: ACCESS_TOKEN_TTL_SECONDS,
     refreshTokenTTL: REFRESH_TOKEN_TTL_SECONDS,
 
+    // Runs on every code exchange and refresh: enforces the absolute grant lifetime
+    // (grant-age.ts) by failing closed with invalid_grant.
+    tokenExchangeCallback(options) {
+      assertGrantWithinMaxAge(options.props, Date.now());
+    },
+
     onError(error) {
       // Codes and categories only: descriptions can echo client-supplied input.
       console.warn(
diff --git a/mcp/test/auth/grant-age.test.ts b/mcp/test/auth/grant-age.test.ts
new file mode 100644
index 0000000000..8b2d6a05d1
--- /dev/null
+++ b/mcp/test/auth/grant-age.test.ts
@@ -0,0 +1,68 @@
+import { OAuthError } from "@cloudflare/workers-oauth-provider";
+import { describe, expect, it } from "vitest";
+
+import {
+  assertGrantWithinMaxAge,
+  GRANT_ISSUED_AT_PROP,
+  GRANT_MAX_AGE_SECONDS,
+} from "../../src/auth/grant-age";
+
+const NOW = 1_800_000_000_000;
+const DAY_MS = 24 * 60 * 60 * 1000;
+const MAX_AGE_MS = GRANT_MAX_AGE_SECONDS * 1000;
+
+function check(props: unknown, maxAgeSeconds?: number): () => void {
+  return () => {
+    assertGrantWithinMaxAge(props, NOW, maxAgeSeconds);
+  };
+}
+
+function issuedAt(ageMs: number): { [GRANT_ISSUED_AT_PROP]: number } {
+  return { [GRANT_ISSUED_AT_PROP]: NOW - ageMs };
+}
+
+describe("assertGrantWithinMaxAge", () => {
+  it("passes a fresh grant", () => {
+    expect(check(issuedAt(0))).not.toThrow();
+  });
+
+  it("passes a grant one second inside the limit", () => {
+    expect(check(issuedAt(MAX_AGE_MS - 1000))).not.toThrow();
+  });
+
+  it("fires one second past the limit with invalid_grant", () => {
+    expect(check(issuedAt(MAX_AGE_MS + 1000))).toThrow(OAuthError);
+    expect(check(issuedAt(MAX_AGE_MS + 1000))).toThrow(/older than the maximum lifetime/);
+    try {
+      check(issuedAt(MAX_AGE_MS + 1000))();
+    } catch (error) {
+      expect(error).toBeInstanceOf(OAuthError);
+      expect((error as OAuthError).code).toBe("invalid_grant");
+    }
+  });
+
+  it("uses 90 days as the default maximum", () => {
+    expect(GRANT_MAX_AGE_SECONDS).toBe(90 * 24 * 60 * 60);
+    expect(check(issuedAt(89 * DAY_MS))).not.toThrow();
+    expect(check(issuedAt(91 * DAY_MS))).toThrow(OAuthError);
+  });
+
+  it("fails closed when the timestamp is missing or malformed", () => {
+    const malformed: unknown[] = [
+      {},
+      null,
+      undefined,
+      { [GRANT_ISSUED_AT_PROP]: "yesterday" },
+      { [GRANT_ISSUED_AT_PROP]: -1 },
+      { [GRANT_ISSUED_AT_PROP]: 1.5 },
+    ];
+    for (const props of malformed) {
+      expect(check(props)).toThrow(/no issue time/);
+    }
+  });
+
+  it("honours an explicit maximum", () => {
+    expect(check(issuedAt(2 * DAY_MS), DAY_MS / 1000)).toThrow(OAuthError);
+    expect(check(issuedAt(2 * DAY_MS), (3 * DAY_MS) / 1000)).not.toThrow();
+  });
+});
diff --git a/mcp/test/auth/token-flow.test.ts b/mcp/test/auth/token-flow.test.ts
index e97309a176..b5a941a1e4 100644
--- a/mcp/test/auth/token-flow.test.ts
+++ b/mcp/test/auth/token-flow.test.ts
@@ -74,6 +74,8 @@ async function consent(
   clientId: string,
   challenge: string,
   scope: readonly string[] = SCOPES,
+  /** `null` omits the timestamp entirely (a legacy or hand-made grant). */
+  grantIssuedAt: number | null = Date.now(),
 ): Promise<URL> {
   const params = new URLSearchParams({
     response_type: "code",
@@ -96,7 +98,7 @@ async function consent(
     scope: [...scope],
     props: {
       credential: createSessionCredential(SIGNED_SESSION).toProps(),
-      grantIssuedAt: Date.now(),
+      ...(grantIssuedAt === null ? {} : { grantIssuedAt }),
     },
   });
   return new URL(redirectTo);
@@ -261,3 +263,33 @@ describe("storage", () => {
     }
   });
 });
+
+describe("absolute grant lifetime", () => {
+  const NINETY_ONE_DAYS_MS = 91 * 24 * 60 * 60 * 1000;
+
+  it("refuses to exchange a code for a grant older than 90 days", async () => {
+    const clientId = await registerClient();
+    const { verifier, challenge } = await pkce();
+    const redirect = await consent(clientId, challenge, SCOPES, Date.now() - NINETY_ONE_DAYS_MS);
+    const response = await exchangeCode(
+      clientId,
+      redirect.searchParams.get("code") ?? "",
+      verifier,
+    );
+    expect(response.status).toBe(400);
+    expect((await response.json<TokenError>()).error).toBe("invalid_grant");
+  });
+
+  it("refuses a grant that carries no issue time (fail closed)", async () => {
+    const clientId = await registerClient();
+    const { verifier, challenge } = await pkce();
+    const redirect = await consent(clientId, challenge, SCOPES, null);
+    const response = await exchangeCode(
+      clientId,
+      redirect.searchParams.get("code") ?? "",
+      verifier,
+    );
+    expect(response.status).toBe(400);
+    expect((await response.json<TokenError>()).error).toBe("invalid_grant");
+  });
+});