diff --git a/src/main/browser/browser-session-registry.persistence.test.ts b/src/main/browser/browser-session-registry.persistence.test.ts
index f95469b58c8..9c20fdc0d2c 100644
--- a/src/main/browser/browser-session-registry.persistence.test.ts
+++ b/src/main/browser/browser-session-registry.persistence.test.ts
@@ -164,10 +164,9 @@ function installModuleMocks(
         fsState.files.set(targetKey, value)
       }
     }),
+    // Nothing on this path calls writeFileAtomically; it is here only to keep the module shape
+    // complete. The identity write goes through node:fs above, which is where failure is injected.
     writeFileAtomically: vi.fn((pathValue: string, data: string) => {
-      if (failIdentityWrite && pathValue.endsWith('browser-identity-mode.json')) {
-        throw new Error('read-only userData')
-      }
       const key = fsKey(pathValue)
       fsState.files.set(key, data)
       fsState.present.add(key)
diff --git a/src/main/runtime/orca-runtime-tests/browser-capabilities.spec.ts b/src/main/runtime/orca-runtime-tests/browser-capabilities.spec.ts
index 6b09d1263b1..ffe98483585 100644
--- a/src/main/runtime/orca-runtime-tests/browser-capabilities.spec.ts
+++ b/src/main/runtime/orca-runtime-tests/browser-capabilities.spec.ts
@@ -23,8 +23,29 @@ import {
   attachClientBrowserHost,
   publishClientHostedPage
 } from '../orca-runtime-test-scenario-builders.spec'
+import { mkdtempSync } from 'node:fs'
+import { tmpdir } from 'node:os'
+import { join } from 'node:path'
+import {
+  initializeBrowserIdentityModeStore,
+  resetBrowserIdentityModeStoreForTests
+} from '../../browser/browser-identity-mode-store'
 
 describe('OrcaRuntimeService', () => {
+  // The mixed-version guarantee: a host that never initialized the identity store must not
+  // advertise a method that can only throw there.
+  it('advertises the browser identity capability only where an identity store exists', () => {
+    resetBrowserIdentityModeStoreForTests()
+    expect(createRuntime().getStatus().capabilities).not.toContain('browser.identity.v1')
+
+    initializeBrowserIdentityModeStore(mkdtempSync(join(tmpdir(), 'orca-identity-capability-')))
+    try {
+      expect(createRuntime().getStatus().capabilities).toContain('browser.identity.v1')
+    } finally {
+      resetBrowserIdentityModeStoreForTests()
+    }
+  })
+
   it('advertises headless browser capability when an offscreen backend backs a windowless host', () => {
     const runtime = createRuntime()
     runtime.setOffscreenBrowserBackend({ createTab: vi.fn(), closeTab: vi.fn() })
diff --git a/src/main/runtime/rpc/methods/browser.test.ts b/src/main/runtime/rpc/methods/browser.test.ts
index e6b9d8dd891..b5df3a6616b 100644
--- a/src/main/runtime/rpc/methods/browser.test.ts
+++ b/src/main/runtime/rpc/methods/browser.test.ts
@@ -77,8 +77,12 @@ describe('browser RPC methods', () => {
   // The schema check above proves the shape; this proves an older client actually gets the
   // rejection over the wire instead of a success with the field quietly dropped.
   it('rejects the retired profile user-agent field through the dispatcher', async () => {
-    // oxlint-disable-next-line typescript/consistent-type-assertions -- SAFETY: params parsing fails before any runtime member is read.
-    const runtime = { getRuntimeId: () => 'test-runtime' } as unknown as OrcaRuntimeService
+    const browserProfileCreate = vi.fn().mockResolvedValue({ id: 'profile-1' })
+    // oxlint-disable-next-line typescript/consistent-type-assertions -- SAFETY: the dispatcher reads only getRuntimeId and the single browser method stubbed here.
+    const runtime = {
+      getRuntimeId: () => 'test-runtime',
+      browserProfileCreate
+    } as unknown as OrcaRuntimeService
     const dispatcher = new RpcDispatcher({ runtime, methods: BROWSER_CORE_METHODS })
 
     const response = await dispatcher.dispatch(
@@ -89,8 +93,11 @@ describe('browser RPC methods', () => {
       })
     )
 
+    // Why a working runtime stub: if the field were accepted and stripped again the call would
+    // succeed, so every assertion below is load-bearing rather than passing on a missing method.
     expect(response).toMatchObject({ ok: false })
     expect(JSON.stringify(response)).toContain('browser_profile_user_agent_mode_is_now_app_wide')
+    expect(browserProfileCreate).not.toHaveBeenCalled()
   })
 
   it('routes core browser automation commands to the runtime server', async () => {