fix(core): fall back to node:crypto so randomUUID cannot throw on Node

core/build.js

@@ -53,6 +53,7 @@ function build({
   if (platform === 'browser' && bundle) {
     buildOptions.alias = {
       'node:async_hooks': './src/utils/async_hooks_shim.ts',
+      'node:crypto': './src/utils/crypto_shim.ts',
     };
   }
 

core/src/utils/crypto_shim.ts

@@ -0,0 +1,23 @@
+/**
+ * @license
+ * Copyright 2026 Google LLC
+ * SPDX-License-Identifier: Apache-2.0
+ */
+
+/**
+ * Browser stand-in for the `node:crypto` builtin, wired up by the alias in
+ * `build.js` so that the Node fallback in `env_aware_utils.ts` does not pull a
+ * Node builtin into the web bundle.
+ *
+ * `randomUUID` is reached here only after both `globalThis.crypto` branches in
+ * `env_aware_utils.ts` have been ruled out. In a browser that means the Web
+ * Crypto API is genuinely absent, so there is no secure source left to fall
+ * back to and the only correct move is to fail rather than degrade.
+ */
+export function randomUUID(): string {
+  throw new Error(
+    'randomUUID: no cryptographically secure source of randomness is ' +
+      'available. Neither crypto.randomUUID() nor crypto.getRandomValues() is ' +
+      'present in this environment.',
+  );
+}

core/src/utils/env_aware_utils.ts

@@ -4,6 +4,8 @@
  * SPDX-License-Identifier: Apache-2.0
  */
 
+import {randomUUID as nodeRandomUUID} from 'node:crypto';
+
 /**
  * Returns true if the environment is a browser.
  */
@@ -19,6 +21,13 @@ export function isBrowser() {
  * `crypto.getRandomValues()` carries no such restriction, so it is used as the
  * fallback rather than `Math.random()`.
  *
+ * In Node the `globalThis.crypto` global was added in v17.4.0 and stayed behind
+ * `--experimental-global-webcrypto` until v19.0.0, so neither of those branches
+ * matches on a default Node 18 or earlier. `node:crypto` carries no such gate —
+ * its `randomUUID` has existed since v14.17.0 — so it is the last resort. In
+ * the web build that import is aliased to `crypto_shim.ts`, which throws,
+ * because a browser without the Web Crypto API has no secure source left.
+ *
  * Some callers use this value to make security decisions — the OAuth2 `state`
  * parameter in `AuthHandler` and the session identifiers minted by the session
  * services — so this function must not silently degrade to a non-cryptographic
@@ -48,11 +57,7 @@ export function randomUUID(): string {
       .join('-');
   }
 
-  throw new Error(
-    'randomUUID: no cryptographically secure source of randomness is ' +
-      'available. Neither crypto.randomUUID() nor crypto.getRandomValues() is ' +
-      'present in this environment.',
-  );
+  return nodeRandomUUID();
 }
 
 /**

core/test/utils/env_aware_utils_test.ts

@@ -5,6 +5,7 @@
  */
 
 import {afterEach, describe, expect, it} from 'vitest';
+import {randomUUID as shimRandomUUID} from '../../src/utils/crypto_shim.js';
 import {getBooleanEnvVar, randomUUID} from '../../src/utils/env_aware_utils.js';
 
 describe('env_aware_utils', () => {
@@ -109,10 +110,27 @@ describe('env_aware_utils', () => {
       expect(randomUUID()).toBe('abababab-abab-4bab-abab-abababababab');
     });
 
-    it('throws instead of degrading when no secure source exists', () => {
+    // globalThis.crypto was added in Node v17.4.0 and stayed behind
+    // --experimental-global-webcrypto until v19.0.0, so on a default Node 18 or
+    // earlier neither globalThis branch matches.
+    it('falls back to node:crypto when globalThis.crypto is absent', () => {
       setCrypto(undefined);
 
-      expect(() => randomUUID()).toThrow(
+      expect(randomUUID()).toMatch(UUID_V4);
+    });
+
+    it('does not repeat itself across calls without globalThis.crypto', () => {
+      setCrypto(undefined);
+
+      const ids = new Set(Array.from({length: 1000}, () => randomUUID()));
+
+      expect(ids.size).toBe(1000);
+    });
+
+    // The web build aliases node:crypto to this shim, so it stands in for the
+    // Node fallback in a browser that has no Web Crypto API at all.
+    it('throws instead of degrading in the browser shim', () => {
+      expect(() => shimRandomUUID()).toThrow(
         /no cryptographically secure source of randomness/,
       );
     });