diff --git a/apps/desktop/src/app/session/hooks/use-message-stream.ts b/apps/desktop/src/app/session/hooks/use-message-stream.ts
index c07222c689..3ee52ec8eb 100644
--- a/apps/desktop/src/app/session/hooks/use-message-stream.ts
+++ b/apps/desktop/src/app/session/hooks/use-message-stream.ts
@@ -1102,8 +1102,13 @@ export function useMessageStream({
 
         if (looksLikeProviderSetup) {
           requestDesktopOnboarding(errorMessage)
-        } else if (isActiveEvent) {
+        } else {
+          // Toast globally, not just when the failing thread is focused: a
+          // turn-ending error (e.g. out of funds) blocks every thread, so the
+          // inline error alone is too easy to miss. The stable id collapses the
+          // same error from multiple blocked threads into one toast.
           notify({
+            id: `gateway-error:${errorMessage}`,
             kind: 'error',
             title: 'Hermes error',
             message: errorMessage
diff --git a/apps/desktop/src/app/session/hooks/use-session-state-cache.test.tsx b/apps/desktop/src/app/session/hooks/use-session-state-cache.test.tsx
index e2a9735827..681334aa2d 100644
--- a/apps/desktop/src/app/session/hooks/use-session-state-cache.test.tsx
+++ b/apps/desktop/src/app/session/hooks/use-session-state-cache.test.tsx
@@ -2,12 +2,14 @@ import { act, cleanup, render } from '@testing-library/react'
 import type { MutableRefObject } from 'react'
 import { afterEach, beforeEach, describe, expect, it, vi } from 'vitest'
 
+import type { ChatMessage } from '@/lib/chat-messages'
 import {
   $currentFastMode,
   $currentModel,
   $currentProvider,
   $currentReasoningEffort,
   $currentServiceTier,
+  $messages,
   $turnStartedAt,
   setCurrentFastMode,
   setCurrentModel,
@@ -213,3 +215,113 @@ describe('useSessionStateCache — per-session turn timer', () => {
     expect($currentFastMode.get()).toBe(false)
   })
 })
+
+function userMessage(id: string, text: string): ChatMessage {
+  return { id, role: 'user', parts: [{ type: 'text', text }] }
+}
+
+function assistantText(id: string, text: string): ChatMessage {
+  return { id, role: 'assistant', parts: [{ type: 'text', text }] }
+}
+
+function assistantError(id: string, error: string): ChatMessage {
+  return { id, role: 'assistant', parts: [], error, pending: false }
+}
+
+interface ViewHarnessProps {
+  activeSessionId: string | null
+  onReady: (cache: Cache) => void
+}
+
+function ViewHarness({ activeSessionId, onReady }: ViewHarnessProps) {
+  const busyRef: MutableRefObject<boolean> = { current: false }
+  const cache = useSessionStateCache({
+    activeSessionId,
+    busyRef,
+    selectedStoredSessionId: null,
+    setAwaitingResponse: () => undefined,
+    setBusy: () => undefined,
+    // Wire the published view back into the real $messages atom the flush
+    // reads from, so the round-trip matches production.
+    setMessages: messages => $messages.set(messages)
+  })
+
+  onReady(cache)
+
+  return null
+}
+
+describe('useSessionStateCache — cross-thread error isolation', () => {
+  afterEach(() => {
+    cleanup()
+    $messages.set([])
+  })
+
+  it('does not leak a failed turn into another thread on switch', () => {
+    $messages.set([])
+    let cache!: Cache
+    const { rerender } = render(<ViewHarness activeSessionId="thread-A" onReady={c => (cache = c)} />)
+
+    // Thread A ends its turn with an out-of-funds error and is on screen.
+    act(() => {
+      cache.updateSessionState(
+        'thread-A',
+        state => ({
+          ...state,
+          busy: false,
+          messages: [userMessage('user-a', 'do the thing'), assistantError('assistant-a-error', 'Out of funds')]
+        }),
+        'stored-A'
+      )
+    })
+
+    expect($messages.get().some(message => message.error === 'Out of funds')).toBe(true)
+
+    // Switch to thread B (which completed cleanly). Its cached state syncs to
+    // the view while $messages still holds thread A's transcript.
+    rerender(<ViewHarness activeSessionId="thread-B" onReady={c => (cache = c)} />)
+    act(() => {
+      cache.updateSessionState(
+        'thread-B',
+        state => ({
+          ...state,
+          busy: false,
+          messages: [userMessage('user-b', 'hello'), assistantText('assistant-b', 'hi there')]
+        }),
+        'stored-B'
+      )
+    })
+
+    expect($messages.get().map(message => message.id)).toEqual(['user-b', 'assistant-b'])
+    expect($messages.get().some(message => message.error === 'Out of funds')).toBe(false)
+  })
+
+  it('still preserves a same-session local error a heartbeat dropped', () => {
+    $messages.set([])
+    let cache!: Cache
+    render(<ViewHarness activeSessionId="thread-A" onReady={c => (cache = c)} />)
+
+    // First paint establishes thread A as the on-screen session.
+    act(() => {
+      cache.updateSessionState(
+        'thread-A',
+        state => ({ ...state, busy: false, messages: [userMessage('user-a', 'do the thing')] }),
+        'stored-A'
+      )
+    })
+
+    // A local error lands in the view (e.g. failAssistantMessage wrote it).
+    $messages.set([userMessage('user-a', 'do the thing'), assistantError('assistant-a-error', 'OpenRouter 403')])
+
+    // A later same-session heartbeat carries cached state that lost the error.
+    act(() => {
+      cache.updateSessionState('thread-A', state => ({
+        ...state,
+        busy: false,
+        messages: [userMessage('user-a', 'do the thing')]
+      }))
+    })
+
+    expect($messages.get().some(message => message.error === 'OpenRouter 403')).toBe(true)
+  })
+})
diff --git a/apps/desktop/src/app/session/hooks/use-session-state-cache.ts b/apps/desktop/src/app/session/hooks/use-session-state-cache.ts
index a08eb1f16c..1445dd17a7 100644
--- a/apps/desktop/src/app/session/hooks/use-session-state-cache.ts
+++ b/apps/desktop/src/app/session/hooks/use-session-state-cache.ts
@@ -79,6 +79,9 @@ export function useSessionStateCache({
   const runtimeIdByStoredSessionIdRef = useRef(new Map<string, string>())
   const pendingViewStateRef = useRef<{ sessionId: string; state: ClientSessionState } | null>(null)
   const viewSyncRafRef = useRef<number | null>(null)
+  // Runtime id whose transcript currently occupies `$messages` — lets the
+  // flush below tell a same-session refresh from a thread switch.
+  const viewSessionIdRef = useRef<string | null>(null)
 
   useEffect(() => {
     activeSessionIdRef.current = activeSessionId
@@ -142,12 +145,22 @@ export function useSessionStateCache({
     // jerks the scroll position while the user is reading. Skip the publish when
     // the merged result is content-identical to what's already on screen.
     const currentMessages = $messages.get()
-    const nextMessages = preserveLocalAssistantErrors(pending.state.messages, currentMessages)
+    // On a thread switch `$messages` still holds the *previous* thread, so
+    // preserving its local errors would graft that thread's failed turn (e.g.
+    // an out-of-funds error) onto this one — then cascade it everywhere as the
+    // polluted view becomes the next switch's baseline. Only carry errors
+    // across a same-session refresh; our cached state already keeps its own.
+    const nextMessages =
+      viewSessionIdRef.current === pending.sessionId
+        ? preserveLocalAssistantErrors(pending.state.messages, currentMessages)
+        : pending.state.messages
 
     if (!sameMessageList(nextMessages, currentMessages)) {
       setMessages(nextMessages)
     }
 
+    viewSessionIdRef.current = pending.sessionId
+
     syncRuntimeMetadataToView(pending.state)
     setBusy(pending.state.busy)
     setMutableRef(busyRef, pending.state.busy)