diff --git a/plans/versioned-localstorage-sessions.md b/plans/versioned-localstorage-sessions.md
index 961afc45a1..b1069aae5b 100644
--- a/plans/versioned-localstorage-sessions.md
+++ b/plans/versioned-localstorage-sessions.md
@@ -287,6 +287,8 @@ All credential-changing commits use the same per-account lock:
 
 Because all persisted values share one localStorage blob, every write also takes a root persisted-storage lock. The root lock prevents an unrelated preference write from racing the session read-modify-write; the per-account lock expresses credential ownership and gives account operations a consistent order.
 
+Feature-detect the Web Locks API. If `navigator.locks.request` is unavailable, run the operation without a lock rather than failing startup or session operations. Generation-specific conditional commits still reject stale work in this fallback mode, but localStorage read-modify-write is not fully serialized across tabs.
+
 If Tab A holds the locks, Tab B waits. Once Tab A writes and releases them, Tab B acquires them and rereads Tab A's new localStorage state before deciding what to commit.
 
 The complete refresh flow is:
diff --git a/src/state/persisted/__tests__/session-lock.web-test.ts b/src/state/persisted/__tests__/session-lock.web-test.ts
new file mode 100644
index 0000000000..6d9aafdc1d
--- /dev/null
+++ b/src/state/persisted/__tests__/session-lock.web-test.ts
@@ -0,0 +1,42 @@
+import {afterEach, describe, expect, it, jest} from '@jest/globals'
+
+import {runWithSessionCredentialLock} from '../session-lock.web'
+
+const originalNavigatorDescriptor = Object.getOwnPropertyDescriptor(
+  globalThis,
+  'navigator',
+)
+
+function setNavigator(value: unknown) {
+  Object.defineProperty(globalThis, 'navigator', {
+    configurable: true,
+    value,
+  })
+}
+
+afterEach(() => {
+  if (originalNavigatorDescriptor) {
+    Object.defineProperty(globalThis, 'navigator', originalNavigatorDescriptor)
+  } else {
+    Reflect.deleteProperty(globalThis, 'navigator')
+  }
+})
+
+describe('session credential locks on unsupported browsers', () => {
+  it.each([
+    ['navigator is unavailable', undefined],
+    ['navigator.locks is unavailable', {}],
+    ['navigator.locks.request is unavailable', {locks: {}}],
+  ])('runs without a lock when %s', async (_, navigatorValue) => {
+    setNavigator(navigatorValue)
+    const operation = jest.fn(() => 'result')
+
+    await expect(
+      runWithSessionCredentialLock({
+        accountDids: ['did:plc:example'],
+        operation,
+      }),
+    ).resolves.toBe('result')
+    expect(operation).toHaveBeenCalledTimes(1)
+  })
+})
diff --git a/src/state/persisted/session-lock.web.ts b/src/state/persisted/session-lock.web.ts
index 26ce495cf7..153a95321b 100644
--- a/src/state/persisted/session-lock.web.ts
+++ b/src/state/persisted/session-lock.web.ts
@@ -8,7 +8,7 @@ export function runWithSessionCredentialLock<T>({
   accountDids: string[]
   operation: () => T | Promise<T>
 }): Promise<T> {
-  const lockManager = navigator.locks
+  const lockManager = getLockManager()
   if (!lockManager) {
     try {
       return Promise.resolve(operation())
@@ -35,3 +35,11 @@ export function runWithSessionCredentialLock<T>({
 
   return run(0)
 }
+
+function getLockManager(): LockManager | undefined {
+  if (typeof navigator === 'undefined' || !('locks' in navigator)) {
+    return undefined
+  }
+  const lockManager = navigator.locks
+  return typeof lockManager?.request === 'function' ? lockManager : undefined
+}