fix: Render structured agent fallbacks readably

Agent fallback output can arrive as structured result arrays when no terminal task report is available. The desktop card now formats that shape into a readable Markdown list instead of exposing raw transport JSON in the preview or result modal.

Constraint: Some Agent executions still only provide structured tool_result content without a terminal Markdown report
Rejected: Keep pretty-printed JSON as the fallback | still exposes implementation shape instead of user-facing results
Confidence: high
Scope-risk: narrow
Directive: Structured Agent result arrays are display data; keep them readable before falling back to raw serialization
Tested: cd desktop && bun run test -- run src/components/chat/MessageList.test.tsx -t "formats structured agent fallback"
Tested: cd desktop && bun run test -- run src/components/chat/MessageList.test.tsx -t agent
Not-tested: Full desktop gate per request to avoid broad gate/test runs

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

@@ -1134,6 +1134,60 @@ describe('MessageList nested tool calls', () => {
     expect(within(dialog).queryByText(/raw structured JSON should not be shown/)).toBeNull()
   })
 
+  it('formats structured agent fallback results as readable markdown', () => {
+    useChatStore.setState({
+      sessions: {
+        [ACTIVE_TAB]: makeSessionState({
+          messages: [
+            {
+              id: 'tool-agent',
+              type: 'tool_use',
+              toolName: 'Agent',
+              toolUseId: 'agent-1',
+              input: { description: '审查安全风险' },
+              timestamp: 1,
+            },
+            {
+              id: 'result-agent',
+              type: 'tool_result',
+              toolUseId: 'agent-1',
+              content: {
+                results: [
+                  {
+                    file: 'git:v0.2.6..v0.2.7',
+                    line: 0,
+                    snippet: 'v0.2.7 tag = a4c92ec7',
+                    context: '版本范围判断：release-notes/v0.2.7.md 明确相比 v0.2.6。',
+                  },
+                  {
+                    file: '/tmp/example/src/lib.rs',
+                    line: 220,
+                    context: '中风险：服务默认监听 0.0.0.0。',
+                  },
+                ],
+              },
+              isError: false,
+              timestamp: 2,
+            },
+          ],
+        }),
+      },
+    })
+
+    render(<MessageList />)
+
+    expect(screen.getByText(/git:v0\.2\.6\.\.v0\.2\.7:0/)).toBeTruthy()
+    expect(screen.queryByText(/\{"results"/)).toBeNull()
+
+    fireEvent.click(screen.getByRole('button', { name: 'View result' }))
+
+    const dialog = screen.getByRole('dialog')
+    expect(within(dialog).getByText('git:v0.2.6..v0.2.7:0')).toBeTruthy()
+    expect(within(dialog).getByText('/tmp/example/src/lib.rs:220')).toBeTruthy()
+    expect(within(dialog).getByText(/服务默认监听 0\.0\.0\.0/)).toBeTruthy()
+    expect(within(dialog).queryByText(/\{"results"/)).toBeNull()
+  })
+
   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, {

desktop/src/components/chat/ToolCallGroup.tsx

@@ -877,7 +877,72 @@ function getAgentOutputSummary(content: string): string {
 }
 
 function extractAgentDisplayText(content: unknown): string {
-  return stripAgentResultMetadata(extractTextContent(content))
+  return stripAgentResultMetadata(formatAgentStructuredResult(content) || extractTextContent(content))
+}
+
+function formatAgentStructuredResult(content: unknown): string {
+  const structured = parseStructuredAgentContent(content)
+  if (!structured || Array.isArray(structured)) return ''
+
+  const results = structured.results
+  if (!Array.isArray(results) || results.length === 0) return ''
+
+  const items = results
+    .map((result, index) => formatAgentStructuredResultItem(result, index))
+    .filter(Boolean)
+
+  return items.join('\n')
+}
+
+function parseStructuredAgentContent(content: unknown): Record<string, unknown> | unknown[] | null {
+  if (typeof content === 'string') {
+    const trimmed = content.trim()
+    if (!trimmed.startsWith('{') && !trimmed.startsWith('[')) return null
+    try {
+      const parsed = JSON.parse(trimmed) as unknown
+      return typeof parsed === 'object' && parsed !== null ? parsed as Record<string, unknown> | unknown[] : null
+    } catch {
+      return null
+    }
+  }
+
+  return typeof content === 'object' && content !== null ? content as Record<string, unknown> | unknown[] : null
+}
+
+function formatAgentStructuredResultItem(result: unknown, index: number): string {
+  if (!result || typeof result !== 'object' || Array.isArray(result)) {
+    const text = extractTextContent(result).trim()
+    return text ? `${index + 1}. ${text}` : ''
+  }
+
+  const record = result as Record<string, unknown>
+  const location = formatAgentResultLocation(record)
+  const context = getStringField(record, 'context')
+  const snippet = getStringField(record, 'snippet')
+  const message = getStringField(record, 'message') || getStringField(record, 'text') || getStringField(record, 'summary')
+  const lines = [`${index + 1}. ${location ? formatInlineCode(location) : 'Result'}`]
+
+  if (message) lines.push(`   - ${message}`)
+  if (context) lines.push(`   - ${context}`)
+  if (snippet) lines.push(`   - ${snippet}`)
+
+  return lines.join('\n')
+}
+
+function formatAgentResultLocation(record: Record<string, unknown>): string {
+  const file = getStringField(record, 'file')
+  if (!file) return ''
+  const line = typeof record.line === 'number' ? record.line : null
+  return line !== null ? `${file}:${line}` : file
+}
+
+function getStringField(record: Record<string, unknown>, key: string): string {
+  const value = record[key]
+  return typeof value === 'string' ? value.trim() : ''
+}
+
+function formatInlineCode(value: string): string {
+  return `\`${value.replace(/`/g, '\\`')}\``
 }
 
 function stripAgentResultMetadata(text: string): string {