fix: resolve AI chat empty body race — read contextPrompt from closure, not stale ref

contextRef (useRef) was initialized at mount time and synced via useEffect,
which runs after paint. If contextPrompt was empty at mount (tab content
not yet loaded), sendMessage could read stale/empty context during the
window between paint and effect. Removing the ref and reading contextPrompt
directly in the useCallback closure eliminates the race entirely.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/hooks/useAiAgent.test.ts

@@ -1,6 +1,15 @@
-import { describe, it, expect, vi } from 'vitest'
-import { detectFileOperation, parseBashFileCreation } from './useAiAgent'
+import { describe, it, expect, vi, beforeEach } from 'vitest'
+import { renderHook, act } from '@testing-library/react'
+import { detectFileOperation, parseBashFileCreation, useAiAgent } from './useAiAgent'
 import type { AgentFileCallbacks } from './useAiAgent'
+import { streamClaudeAgent } from '../utils/ai-agent'
+
+vi.mock('../utils/ai-agent', () => ({
+  streamClaudeAgent: vi.fn(),
+  buildAgentSystemPrompt: vi.fn(() => 'default-system-prompt'),
+}))
+
+const mockStreamClaudeAgent = vi.mocked(streamClaudeAgent)
 
 const VAULT = '/Users/luca/Laputa'
 
@@ -176,3 +185,56 @@ describe('parseBashFileCreation', () => {
     expect(parseBashFileCreation(input, VAULT)).toBeNull()
   })
 })
+
+describe('useAiAgent', () => {
+  beforeEach(() => {
+    mockStreamClaudeAgent.mockReset()
+    mockStreamClaudeAgent.mockImplementation(async (_msg: string, _sys: string, _vault: string, callbacks: { onDone: () => void }) => {
+      callbacks.onDone()
+    })
+  })
+
+  it('updates sendMessage when contextPrompt changes (no stale closure)', () => {
+    // Mount with empty contextPrompt (simulates race: panel mounts before tab content loads)
+    const { result, rerender } = renderHook(
+      ({ vault, context }) => useAiAgent(vault, context),
+      { initialProps: { vault: VAULT, context: undefined as string | undefined } },
+    )
+
+    const firstSendMessage = result.current.sendMessage
+
+    // Simulate tab content arriving — re-render with real contextPrompt
+    rerender({ vault: VAULT, context: 'You are viewing note with body: Hello world' })
+
+    // sendMessage must be a NEW function that closes over the updated contextPrompt.
+    // If it's the same reference, it may read stale context (the race condition).
+    expect(result.current.sendMessage).not.toBe(firstSendMessage)
+  })
+
+  it('uses the current contextPrompt at send time, not the mount-time value', async () => {
+    const { result, rerender } = renderHook(
+      ({ vault, context }) => useAiAgent(vault, context),
+      { initialProps: { vault: VAULT, context: undefined as string | undefined } },
+    )
+
+    rerender({ vault: VAULT, context: 'You are viewing note with body: Hello world' })
+
+    await act(async () => {
+      await result.current.sendMessage('What does this note contain?')
+    })
+
+    expect(mockStreamClaudeAgent).toHaveBeenCalledTimes(1)
+    expect(mockStreamClaudeAgent.mock.calls[0][1]).toBe('You are viewing note with body: Hello world')
+  })
+
+  it('falls back to buildAgentSystemPrompt when contextPrompt is undefined', async () => {
+    const { result } = renderHook(() => useAiAgent(VAULT, undefined))
+
+    await act(async () => {
+      await result.current.sendMessage('Hello')
+    })
+
+    expect(mockStreamClaudeAgent).toHaveBeenCalledTimes(1)
+    expect(mockStreamClaudeAgent.mock.calls[0][1]).toBe('default-system-prompt')
+  })
+})

src/hooks/useAiAgent.ts

@@ -44,15 +44,11 @@ export function useAiAgent(
   const [messages, setMessages] = useState<AiAgentMessage[]>([])
   const [status, setStatus] = useState<AgentStatus>('idle')
   const abortRef = useRef({ aborted: false })
-  const contextRef = useRef(contextPrompt)
   const responseAccRef = useRef('')
   const fileCallbacksRef = useRef(fileCallbacks)
   // Track tool inputs for file-operation detection on ToolDone
   const toolInputMapRef = useRef<Map<string, { tool: string; input?: string }>>(new Map())
 
-  useEffect(() => {
-    contextRef.current = contextPrompt
-  }, [contextPrompt])
   useEffect(() => {
     fileCallbacksRef.current = fileCallbacks
   }, [fileCallbacks])
@@ -89,7 +85,7 @@ export function useAiAgent(
       update(m => m.reasoningDone ? m : { ...m, reasoningDone: true })
     }
 
-    const systemPrompt = contextRef.current ?? buildAgentSystemPrompt()
+    const systemPrompt = contextPrompt ?? buildAgentSystemPrompt()
 
     await streamClaudeAgent(text.trim(), systemPrompt, vaultPath, {
       onThinking: (chunk) => {
@@ -178,7 +174,7 @@ export function useAiAgent(
         fileCallbacksRef.current?.onVaultChanged?.()
       },
     })
-  }, [status, vaultPath])
+  }, [status, vaultPath, contextPrompt])
 
   const clearConversation = useCallback(() => {
     abortRef.current.aborted = true