fix: reduce copy ambiguity in chat sessions

The session view exposed copy affordances in a way that implied the
whole conversation would be copied, while user prompts had no copy
path at all. This changes copy to a per-message footer action so the
scope is explicit for both prompts and assistant replies, and reuses
the shared clipboard helper instead of maintaining a second copy path.

Constraint: The worktree contains unrelated in-progress changes that must stay uncommitted
Rejected: Keep a floating action above each message | it interrupted reading order and still looked session-scoped
Confidence: high
Scope-risk: narrow
Reversibility: clean
Directive: Keep copy affordances attached to individual message blocks so the copied scope stays obvious
Tested: bun run test src/components/chat/MessageList.test.tsx; bun run lint
Not-tested: Manual desktop visual QA across dense multi-message sessions

desktop/src/components/chat/AssistantMessage.tsx

@@ -1,5 +1,5 @@
 import { MarkdownRenderer } from '../markdown/MarkdownRenderer'
-import { useState } from 'react'
+import { MessageActionBar } from './MessageActionBar'
 
 type Props = {
   content: string
@@ -7,35 +7,19 @@ type Props = {
 }
 
 export function AssistantMessage({ content, isStreaming }: Props) {
-  const [copied, setCopied] = useState(false)
-
-  const handleCopy = async () => {
-    try {
-      await navigator.clipboard.writeText(content)
-      setCopied(true)
-      window.setTimeout(() => setCopied(false), 1500)
-    } catch {
-      setCopied(false)
-    }
-  }
-
   return (
-    <div className="group relative mb-3 ml-10">
-      {/* Copy button — absolute positioned, no reserved space */}
-      {!isStreaming && content.trim() && (
-        <button
-          onClick={handleCopy}
-          className="absolute -right-1 -top-1 rounded-md border border-[var(--color-border)]/60 bg-[var(--color-surface)] px-2 py-0.5 text-[10px] text-[var(--color-text-tertiary)] opacity-0 shadow-sm transition-opacity hover:text-[var(--color-text-primary)] group-hover:opacity-100"
-        >
-          {copied ? 'Copied' : 'Copy'}
-        </button>
-      )}
-      <div className="text-sm text-[var(--color-text-primary)]">
+    <div className="mb-5 ml-10 max-w-[calc(100%-2.5rem)]">
+      <div className="rounded-[20px] rounded-tl-[8px] border border-[var(--color-border)]/60 bg-[var(--color-surface)] px-4 py-3 text-sm text-[var(--color-text-primary)] shadow-sm">
         <MarkdownRenderer content={content} />
         {isStreaming && (
-          <span className="inline-block w-0.5 h-4 bg-[var(--color-brand)] animate-shimmer ml-0.5 align-text-bottom" />
+          <span className="ml-0.5 inline-block h-4 w-0.5 animate-shimmer bg-[var(--color-brand)] align-text-bottom" />
         )}
       </div>
+
+      <MessageActionBar
+        copyText={isStreaming ? undefined : content}
+        copyLabel="Copy reply"
+      />
     </div>
   )
 }

desktop/src/components/chat/MessageActionBar.tsx

@@ -0,0 +1,31 @@
+import { CopyButton } from '../shared/CopyButton'
+
+type Props = {
+  copyText?: string
+  copyLabel: string
+  align?: 'start' | 'end'
+}
+
+export function MessageActionBar({
+  copyText,
+  copyLabel,
+  align = 'start',
+}: Props) {
+  const hasCopy = Boolean(copyText?.trim())
+
+  if (!hasCopy) return null
+
+  return (
+    <div
+      className={`mt-1.5 flex ${align === 'end' ? 'justify-end' : 'justify-start'}`}
+    >
+      <CopyButton
+        text={copyText!}
+        label={copyLabel}
+        displayLabel="Copy"
+        displayCopiedLabel="Copied"
+        className="inline-flex min-h-7 items-center rounded-full border border-[var(--color-border)]/70 bg-[var(--color-surface)] px-2.5 text-[11px] font-medium text-[var(--color-text-tertiary)] transition-colors hover:border-[var(--color-brand)]/35 hover:text-[var(--color-text-primary)] focus-visible:outline-none focus-visible:ring-2 focus-visible:ring-[var(--color-brand)]/35"
+      />
+    </div>
+  )
+}

desktop/src/components/chat/MessageList.test.tsx

@@ -0,0 +1,179 @@
+import { beforeEach, describe, expect, it, vi } from 'vitest'
+import { fireEvent, render, screen, waitFor } from '@testing-library/react'
+import { MessageList, buildRenderItems } from './MessageList'
+import { useChatStore } from '../../stores/chatStore'
+import type { UIMessage } from '../../types/chat'
+
+describe('MessageList nested tool calls', () => {
+  beforeEach(() => {
+    useChatStore.setState({
+      messages: [],
+      chatState: 'idle',
+      streamingText: '',
+      streamingToolInput: '',
+      activeToolUseId: null,
+      activeToolName: null,
+      activeThinkingId: null,
+      pendingPermission: null,
+      elapsedSeconds: 0,
+    })
+  })
+
+  it('renders sub-agent tool calls inline beneath the parent agent tool call', () => {
+    useChatStore.setState({
+      messages: [
+        {
+          id: 'tool-agent',
+          type: 'tool_use',
+          toolName: 'Agent',
+          toolUseId: 'agent-1',
+          input: { description: 'Inspect src/components' },
+          timestamp: 1,
+        },
+        {
+          id: 'tool-read',
+          type: 'tool_use',
+          toolName: 'Read',
+          toolUseId: 'read-1',
+          input: { file_path: '/tmp/example.ts' },
+          timestamp: 2,
+          parentToolUseId: 'agent-1',
+        },
+        {
+          id: 'result-read',
+          type: 'tool_result',
+          toolUseId: 'read-1',
+          content: 'const answer = 42',
+          isError: false,
+          timestamp: 3,
+          parentToolUseId: 'agent-1',
+        },
+      ],
+    })
+
+    const { container } = render(<MessageList />)
+
+    expect(screen.getByText('Running')).toBeTruthy()
+    expect(screen.getByText(/Read .*example\.ts.*done/i)).toBeTruthy()
+    expect(container.textContent).toContain('Agent')
+  })
+
+  it('keeps root tool runs split when nested child tool calls appear between them', () => {
+    const messages: UIMessage[] = [
+      {
+        id: 'tool-agent',
+        type: 'tool_use',
+        toolName: 'Agent',
+        toolUseId: 'agent-1',
+        input: { description: 'Inspect src/components' },
+        timestamp: 1,
+      },
+      {
+        id: 'tool-read',
+        type: 'tool_use',
+        toolName: 'Read',
+        toolUseId: 'read-1',
+        input: { file_path: '/tmp/example.ts' },
+        timestamp: 2,
+        parentToolUseId: 'agent-1',
+      },
+      {
+        id: 'result-read',
+        type: 'tool_result',
+        toolUseId: 'read-1',
+        content: 'const answer = 42',
+        isError: false,
+        timestamp: 3,
+        parentToolUseId: 'agent-1',
+      },
+      {
+        id: 'tool-write',
+        type: 'tool_use',
+        toolName: 'Write',
+        toolUseId: 'write-1',
+        input: { file_path: '/tmp/out.ts', content: 'export const value = 1' },
+        timestamp: 4,
+      },
+    ]
+
+    const toolUseIds = new Set(messages.filter((message) => message.type === 'tool_use').map((message) => message.toolUseId))
+    const renderItems = buildRenderItems(messages, toolUseIds)
+    const toolGroups = renderItems.filter((item) => item.kind === 'tool_group')
+
+    expect(toolGroups).toHaveLength(2)
+    expect(toolGroups.map((item) => item.toolCalls[0]?.toolUseId)).toEqual(['agent-1', 'write-1'])
+  })
+
+  it('shows failed agent status and compact unavailable summary for Explore launch errors', () => {
+    useChatStore.setState({
+      messages: [
+        {
+          id: 'tool-agent',
+          type: 'tool_use',
+          toolName: 'Agent',
+          toolUseId: 'agent-1',
+          input: { description: '探索整体架构', subagent_type: 'Explore' },
+          timestamp: 1,
+        },
+        {
+          id: 'result-agent',
+          type: 'tool_result',
+          toolUseId: 'agent-1',
+          content: `Agent type 'Explore' not found. Available agents: general-purpose`,
+          isError: true,
+          timestamp: 2,
+        },
+      ],
+    })
+
+    render(<MessageList />)
+
+    expect(screen.getByText('Failed')).toBeTruthy()
+    expect(screen.getByText('Explore agent unavailable in this session')).toBeTruthy()
+  })
+
+  it('renders copy controls for user messages and scopes assistant copy to a single reply', async () => {
+    const writeText = vi.fn().mockResolvedValue(undefined)
+    Object.assign(navigator, {
+      clipboard: {
+        writeText,
+      },
+    })
+
+    useChatStore.setState({
+      messages: [
+        {
+          id: 'user-1',
+          type: 'user_text',
+          content: '请帮我探索整体架构',
+          timestamp: 1,
+        },
+        {
+          id: 'assistant-1',
+          type: 'assistant_text',
+          content: '先看 CLI 和服务端入口。',
+          timestamp: 2,
+        },
+        {
+          id: 'assistant-2',
+          type: 'assistant_text',
+          content: '再看 desktop 前后端边界。',
+          timestamp: 3,
+        },
+      ],
+    })
+
+    render(<MessageList />)
+
+    expect(screen.getByRole('button', { name: 'Copy prompt' })).toBeTruthy()
+
+    fireEvent.click(screen.getAllByRole('button', { name: 'Copy reply' })[1]!)
+
+    await waitFor(() => {
+      expect(writeText).toHaveBeenCalledWith('再看 desktop 前后端边界。')
+    })
+    expect(writeText).not.toHaveBeenCalledWith(
+      '先看 CLI 和服务端入口。\n再看 desktop 前后端边界。'
+    )
+  })
+})

desktop/src/components/chat/UserMessage.tsx

@@ -1,5 +1,6 @@
 import type { UIAttachment } from '../../types/chat'
 import { AttachmentGallery } from './AttachmentGallery'
+import { MessageActionBar } from './MessageActionBar'
 
 type Props = {
   content: string
@@ -17,12 +18,19 @@ export function UserMessage({ content, attachments }: Props) {
         )}
 
         {hasText && (
-          <div
-            className="bg-[var(--color-surface-user-msg)] px-4 py-3 text-sm leading-relaxed text-[var(--color-text-primary)] whitespace-pre-wrap break-words"
-            style={{ borderRadius: '18px 4px 18px 18px' }}
-          >
-            {content}
-          </div>
+          <>
+            <div
+              className="bg-[var(--color-surface-user-msg)] px-4 py-3 text-sm leading-relaxed text-[var(--color-text-primary)] whitespace-pre-wrap break-words"
+              style={{ borderRadius: '18px 4px 18px 18px' }}
+            >
+              {content}
+            </div>
+            <MessageActionBar
+              copyText={content}
+              copyLabel="Copy prompt"
+              align="end"
+            />
+          </>
         )}
       </div>
     </div>

desktop/src/components/shared/CopyButton.tsx

@@ -1,12 +1,23 @@
 import { useEffect, useState } from 'react'
+import { copyTextToClipboard } from '../chat/clipboard'
 
 type Props = {
   text: string
   label?: string
+  copiedLabel?: string
+  displayLabel?: string
+  displayCopiedLabel?: string
   className?: string
 }
 
-export function CopyButton({ text, label = 'Copy', className = '' }: Props) {
+export function CopyButton({
+  text,
+  label = 'Copy',
+  copiedLabel = 'Copied',
+  displayLabel,
+  displayCopiedLabel,
+  className = '',
+}: Props) {
   const [copied, setCopied] = useState(false)
 
   useEffect(() => {
@@ -17,17 +28,10 @@ export function CopyButton({ text, label = 'Copy', className = '' }: Props) {
 
   const handleCopy = async () => {
     try {
-      if (navigator.clipboard?.writeText) {
-        await navigator.clipboard.writeText(text)
-      } else {
-        const textarea = document.createElement('textarea')
-        textarea.value = text
-        textarea.style.position = 'fixed'
-        textarea.style.opacity = '0'
-        document.body.appendChild(textarea)
-        textarea.select()
-        document.execCommand('copy')
-        document.body.removeChild(textarea)
+      const ok = await copyTextToClipboard(text)
+      if (!ok) {
+        setCopied(false)
+        return
       }
       setCopied(true)
     } catch {
@@ -35,15 +39,20 @@ export function CopyButton({ text, label = 'Copy', className = '' }: Props) {
     }
   }
 
+  const currentLabel = copied ? copiedLabel : label
+  const buttonText = copied
+    ? (displayCopiedLabel ?? copiedLabel)
+    : (displayLabel ?? label)
+
   return (
     <button
       type="button"
       onClick={handleCopy}
       className={className}
-      aria-label={copied ? 'Copied' : label}
-      title={copied ? 'Copied' : label}
+      aria-label={currentLabel}
+      title={currentLabel}
     >
-      {copied ? 'Copied' : label}
+      {buttonText}
     </button>
   )
 }