fix(server): cache session list summaries

Reduce repeated /api/sessions?limit=400 scans by caching per-file JSONL list summaries keyed by mtime and size. This targets #894 diagnostics where sidebar refreshes reported client_api_request_failed timeouts on session list and inspection paths.

Tested: bun test src/server/__tests__/sessions.test.ts

Tested: bun run check:server

Tested: temporary source server /api/sessions?limit=400 first call 0.80s; after waiting past the 5s list cache TTL, second call 0.03s with identical 399 session IDs.

Not-tested: Windows release build retest for #894 diagnostics timeout.

Confidence: high

Scope-risk: narrow

src/server/__tests__/sessions.test.ts

@@ -607,6 +607,57 @@ describe('SessionService', () => {
     expect(scanCount).toBe(3)
   })
 
+  it('should reuse unchanged file summaries after the list response cache is cleared', async () => {
+    const sessionFiles: Array<{ id: string; filePath: string }> = []
+    for (let i = 0; i < 3; i++) {
+      const id = `2500000${i.toString(16)}-bbbb-cccc-dddd-eeeeeeeeeeee`
+      const filePath = await writeSessionFile('-tmp-file-summary-cache', id, [
+        makeSnapshotEntry(),
+        makeUserEntry(`Cached file summary ${i}`),
+      ])
+      const mtime = new Date(Date.now() - i * 1000)
+      await fs.utimes(filePath, mtime, mtime)
+      sessionFiles.push({ id, filePath })
+    }
+
+    const serviceWithSpy = service as unknown as {
+      scanSessionListSummary: (...args: unknown[]) => Promise<unknown>
+    }
+    const serviceInternals = service as unknown as {
+      sessionListCache: Map<string, unknown>
+    }
+    const originalScanSessionListSummary = serviceWithSpy.scanSessionListSummary.bind(service)
+    let scanCount = 0
+    serviceWithSpy.scanSessionListSummary = async (...args) => {
+      scanCount += 1
+      return originalScanSessionListSummary(...args)
+    }
+
+    await service.listSessions({ limit: 3, offset: 0 })
+    expect(scanCount).toBe(3)
+
+    serviceInternals.sessionListCache.clear()
+    const second = await service.listSessions({ limit: 3, offset: 0 })
+    expect(second.sessions).toHaveLength(3)
+    expect(scanCount).toBe(3)
+
+    await fs.appendFile(
+      sessionFiles[1]!.filePath,
+      `${JSON.stringify({
+        type: 'custom-title',
+        customTitle: 'Changed cached file summary',
+        timestamp: new Date().toISOString(),
+      })}\n`,
+      'utf-8',
+    )
+    serviceInternals.sessionListCache.clear()
+
+    const third = await service.listSessions({ limit: 3, offset: 0 })
+    expect(third.sessions.find((session) => session.id === sessionFiles[1]!.id)?.title)
+      .toBe('Changed cached file summary')
+    expect(scanCount).toBe(4)
+  })
+
   it('should invalidate cached list metadata after writes', async () => {
     const sessionId = '30000000-bbbb-cccc-dddd-eeeeeeeeeeee'
     await writeSessionFile('-tmp-cache-invalidation', sessionId, [

src/server/services/sessionService.ts

@@ -5,7 +5,7 @@
  * 确保 Desktop App 与 CLI 的数据完全互通。
  */
 
-import { createReadStream } from 'node:fs'
+import { createReadStream, type Stats } from 'node:fs'
 import * as fs from 'node:fs/promises'
 import * as path from 'node:path'
 import * as os from 'node:os'
@@ -286,6 +286,12 @@ type SessionListSummary = {
   worktreeSession?: PersistedWorktreeSession | null
 }
 
+type SessionListSummaryCacheEntry = {
+  mtimeMs: number
+  size: number
+  summary: SessionListSummary
+}
+
 const VALID_SESSION_PERMISSION_MODES = new Set([
   'default',
   'acceptEdits',
@@ -318,6 +324,7 @@ export class SessionService {
     expiresAt: number
     result: { sessions: SessionListItem[]; total: number }
   }>()
+  private readonly sessionListSummaryCache = new Map<string, SessionListSummaryCacheEntry>()
 
   private sessionListCacheKey(options?: {
     project?: string
@@ -342,6 +349,35 @@ export class SessionService {
     this.sessionListCache.clear()
   }
 
+  private cloneSessionListSummary(summary: SessionListSummary): SessionListSummary {
+    return {
+      ...summary,
+      repository: summary.repository ? { ...summary.repository } : undefined,
+      worktreeSession: summary.worktreeSession
+        ? { ...summary.worktreeSession }
+        : summary.worktreeSession,
+    }
+  }
+
+  private async getCachedSessionListSummary(
+    filePath: string,
+    projectDir: string,
+    stat: Stats,
+  ): Promise<SessionListSummary> {
+    const cached = this.sessionListSummaryCache.get(filePath)
+    if (cached && cached.mtimeMs === stat.mtimeMs && cached.size === stat.size) {
+      return this.cloneSessionListSummary(cached.summary)
+    }
+
+    const summary = await this.scanSessionListSummary(filePath, projectDir, stat)
+    this.sessionListSummaryCache.set(filePath, {
+      mtimeMs: stat.mtimeMs,
+      size: stat.size,
+      summary: this.cloneSessionListSummary(summary),
+    })
+    return summary
+  }
+
   // --------------------------------------------------------------------------
   // Config helpers
   // --------------------------------------------------------------------------
@@ -1746,7 +1782,7 @@ export class SessionService {
     const items: SessionListItem[] = []
     for (const { filePath, projectDir, sessionId, stat } of paginatedFiles) {
       try {
-        const summary = await this.scanSessionListSummary(filePath, projectDir, stat)
+        const summary = await this.getCachedSessionListSummary(filePath, projectDir, stat)
         const workDir = summary.workDir
         const projectRoot = await this.resolveProjectRootFromSessionMetadata({
           worktreeSession: summary.worktreeSession,