diff --git a/build/check-perf-thresholds.cjs b/build/check-perf-thresholds.cjs
new file mode 100644
index 00000000..52f509f5
--- /dev/null
+++ b/build/check-perf-thresholds.cjs
@@ -0,0 +1,99 @@
+// PERF-17 regression gate for the PERF-11 full-App benchmark report.
+// Usage: node build/check-perf-thresholds.cjs <result.json | report directory> [thresholds.json]
+// Exit codes: 0 all limits met, 1 a limit, scenario or status failed,
+// 2 the report cannot be judged (throttled run, unreadable input).
+const { existsSync, readFileSync, statSync } = require('node:fs')
+const { join, resolve } = require('node:path')
+
+const defaultThresholdsPath = join(__dirname, 'perf-thresholds.json')
+
+function metricValue(scenario, path) {
+  let value = scenario
+  for (const key of path.split('.')) {
+    if (value === null || value === undefined) return undefined
+    value = value[key]
+  }
+  return value
+}
+
+// Pure: judges a parsed report against parsed thresholds.
+// Returns { ok, refused, reason?, rows: [{ scenario, metric, value, limit, pass, note? }] }.
+function evaluate(report, thresholds) {
+  const rows = []
+  const throttle = Number(report?.environment?.rendererCpuThrottle ?? 1)
+  const maxThrottle = Number(thresholds.maxRendererCpuThrottle ?? 1)
+  if (!(throttle <= maxThrottle)) {
+    return { ok: false, refused: true, reason: `renderer CPU throttle ${throttle}x exceeds ${maxThrottle}x; thresholds apply to non-throttled runs only`, rows }
+  }
+  const requiredStatus = thresholds.requiredStatus ?? 'passed'
+  rows.push({ scenario: '(report)', metric: 'status', value: report?.status ?? null, limit: `= ${requiredStatus}`, pass: report?.status === requiredStatus })
+  const scenarios = Array.isArray(report?.scenarios) ? report.scenarios : []
+  for (const [id, limits] of Object.entries(thresholds.scenarios ?? {})) {
+    const scenario = scenarios.find(entry => entry?.id === id)
+    if (!scenario) {
+      const optional = limits.optional === true
+      rows.push({ scenario: id, metric: '(present)', value: 'missing', limit: optional ? 'optional' : 'required', pass: optional })
+      continue
+    }
+    for (const [metric, bound] of Object.entries(limits)) {
+      if (metric === 'optional') continue
+      const raw = metricValue(scenario, metric)
+      const value = typeof raw === 'number' && Number.isFinite(raw) ? raw : null
+      let pass = value !== null
+      const limitParts = []
+      if (typeof bound.max === 'number') { limitParts.push(`<= ${bound.max}`); pass = pass && value <= bound.max }
+      if (typeof bound.min === 'number') { limitParts.push(`>= ${bound.min}`); pass = pass && value >= bound.min }
+      rows.push({ scenario: id, metric, value, limit: limitParts.join(', '), pass, ...(value === null ? { note: 'metric missing' } : {}) })
+    }
+  }
+  return { ok: rows.every(row => row.pass), refused: false, rows }
+}
+
+function resolveReportPath(input) {
+  const path = resolve(input)
+  return existsSync(path) && statSync(path).isDirectory() ? join(path, 'result.json') : path
+}
+
+function formatValue(value) {
+  if (typeof value !== 'number') return String(value)
+  return Number.isInteger(value) ? String(value) : value.toFixed(value < 1 ? 3 : 1)
+}
+
+function run(argv) {
+  const [input, thresholdsArg] = argv
+  if (!input) {
+    console.error('usage: node build/check-perf-thresholds.cjs <result.json | report directory> [thresholds.json]')
+    return 2
+  }
+  const reportPath = resolveReportPath(input)
+  const thresholdsPath = resolve(thresholdsArg || defaultThresholdsPath)
+  let report
+  let thresholds
+  try {
+    report = JSON.parse(readFileSync(reportPath, 'utf8'))
+    thresholds = JSON.parse(readFileSync(thresholdsPath, 'utf8'))
+  } catch (error) {
+    console.error(`cannot read report or thresholds: ${error instanceof Error ? error.message : error}`)
+    return 2
+  }
+  const result = evaluate(report, thresholds)
+  console.log(`\nperf thresholds: ${reportPath}`)
+  if (result.refused) {
+    console.error(`refused: ${result.reason}`)
+    return 2
+  }
+  console.table(result.rows.map(row => ({
+    scenario: row.scenario,
+    metric: row.metric,
+    value: formatValue(row.value),
+    limit: row.limit,
+    pass: row.pass ? 'ok' : `FAIL${row.note ? ` (${row.note})` : ''}`
+  })))
+  const failed = result.rows.filter(row => !row.pass).length
+  console.log(failed ? `${failed} of ${result.rows.length} checks failed` : `all ${result.rows.length} checks passed`)
+  return result.ok ? 0 : 1
+}
+
+module.exports = { evaluate, run, defaultThresholdsPath }
+
+if (require.main === module) process.exitCode = run(process.argv.slice(2))
diff --git a/build/perf-thresholds.json b/build/perf-thresholds.json
new file mode 100644
index 00000000..04027517
--- /dev/null
+++ b/build/perf-thresholds.json
@@ -0,0 +1,89 @@
+{
+  "note": "PERF-17 regression ceilings for the PERF-11 full-App benchmark (npm run perf:app). They apply ONLY to non-throttled runs (environment.rendererCpuThrottle = 1, the default) with the default GB_PERF_* scale, on the reference machine of the 2026-10-02 baseline (Intel Core Ultra X7 358H x16, 32 GB, DPR 1, 120 Hz). Limits are about 2-3x the 2026-10-02 values so that run-to-run noise does not fail; they catch regressions of the size seen before PERF-12 (e.g. ~130 long tasks / 9 s while streaming). Tighten them as PERF-12..PERF-14 land; do not loosen them to make a regression pass. Every listed scenario must be present unless marked optional, and report.status must equal requiredStatus.",
+  "requiredStatus": "passed",
+  "maxRendererCpuThrottle": 1,
+  "scenarios": {
+    "idle": {
+      "renderer.longTasks.count": { "max": 2 },
+      "renderer.longTasks.totalMs": { "max": 200 },
+      "renderer.frames.p95": { "max": 20 },
+      "renderer.frames.jankRatio": { "max": 0.05 },
+      "main.eventLoopDelay.p99": { "max": 100 }
+    },
+    "typing-small": {
+      "renderer.longTasks.count": { "max": 3 },
+      "renderer.longTasks.totalMs": { "max": 300 },
+      "renderer.frames.p95": { "max": 20 },
+      "renderer.frames.jankRatio": { "max": 0.05 },
+      "latency.p95": { "max": 25 },
+      "latency.max": { "max": 100 },
+      "main.eventLoopDelay.p99": { "max": 100 }
+    },
+    "idle-seeded": {
+      "renderer.longTasks.count": { "max": 2 },
+      "renderer.longTasks.totalMs": { "max": 200 },
+      "renderer.frames.p95": { "max": 20 },
+      "renderer.frames.jankRatio": { "max": 0.05 },
+      "main.eventLoopDelay.p99": { "max": 100 }
+    },
+    "typing-seeded": {
+      "renderer.longTasks.count": { "max": 3 },
+      "renderer.longTasks.totalMs": { "max": 300 },
+      "renderer.frames.p95": { "max": 20 },
+      "renderer.frames.jankRatio": { "max": 0.05 },
+      "latency.p95": { "max": 25 },
+      "latency.max": { "max": 100 },
+      "main.eventLoopDelay.p99": { "max": 100 }
+    },
+    "switch-conversations": {
+      "renderer.longTasks.count": { "max": 5 },
+      "renderer.longTasks.totalMs": { "max": 500 },
+      "renderer.frames.jankRatio": { "max": 0.15 },
+      "latency.p95": { "max": 350 },
+      "main.eventLoopDelay.p99": { "max": 100 }
+    },
+    "switch-conversations-warm": {
+      "renderer.longTasks.count": { "max": 3 },
+      "renderer.longTasks.totalMs": { "max": 300 },
+      "renderer.frames.jankRatio": { "max": 0.15 },
+      "latency.p95": { "max": 200 },
+      "main.eventLoopDelay.p99": { "max": 100 }
+    },
+    "switch-conversations-hot": {
+      "optional": true,
+      "renderer.longTasks.count": { "max": 3 },
+      "renderer.longTasks.totalMs": { "max": 200 },
+      "renderer.frames.jankRatio": { "max": 0.1 },
+      "latency.p95": { "max": 100 },
+      "main.eventLoopDelay.p99": { "max": 100 }
+    },
+    "open-long-conversation": {
+      "renderer.longTasks.count": { "max": 4 },
+      "renderer.longTasks.totalMs": { "max": 400 },
+      "latency.p95": { "max": 400 },
+      "main.eventLoopDelay.p99": { "max": 100 }
+    },
+    "scroll-long-conversation": {
+      "renderer.longTasks.count": { "max": 2 },
+      "renderer.longTasks.totalMs": { "max": 200 },
+      "renderer.frames.p95": { "max": 20 },
+      "renderer.frames.jankRatio": { "max": 0.05 },
+      "main.eventLoopDelay.p99": { "max": 100 }
+    },
+    "stream-seeded": {
+      "renderer.longTasks.count": { "max": 10 },
+      "renderer.longTasks.totalMs": { "max": 1500 },
+      "renderer.frames.p95": { "max": 34 },
+      "renderer.frames.jankRatio": { "max": 0.15 },
+      "main.eventLoopDelay.p99": { "max": 100 }
+    },
+    "stream-and-type": {
+      "renderer.longTasks.count": { "max": 10 },
+      "renderer.longTasks.totalMs": { "max": 1500 },
+      "renderer.frames.p95": { "max": 34 },
+      "renderer.frames.jankRatio": { "max": 0.2 },
+      "latency.p95": { "max": 50 },
+      "main.eventLoopDelay.p99": { "max": 100 }
+    }
+  }
+}
diff --git a/build/run-app-perf.cjs b/build/run-app-perf.cjs
index 5a884e66..65c59749 100644
--- a/build/run-app-perf.cjs
+++ b/build/run-app-perf.cjs
@@ -208,6 +208,9 @@ async function main() {
   console.log(`\nreport: ${resultPath}`)
   if (!keepProfile) rmSync(join(runDirectory, 'profile'), { recursive: true, force: true, maxRetries: 5, retryDelay: 200 })
   process.exitCode = String(report.status).startsWith('passed') ? 0 : 1
+  // PERF-17: GB_PERF_CHECK=1 gates the run on build/perf-thresholds.json
+  // (exit 1 on a regression, 2 when the report cannot be judged, e.g. throttled).
+  if (process.env.GB_PERF_CHECK === '1') process.exitCode = require('./check-perf-thresholds.cjs').run([resultPath])
 }
 
 main().catch(error => {
diff --git a/docs/roadmap/product-performance-experience-improvement-plan.md b/docs/roadmap/product-performance-experience-improvement-plan.md
index a16ed90b..74129168 100644
--- a/docs/roadmap/product-performance-experience-improvement-plan.md
+++ b/docs/roadmap/product-performance-experience-improvement-plan.md
@@ -701,6 +701,29 @@ Renderer 主线程降速 4 倍（Main 不降速，每次 reload 后重新应用
 - **React 规则：** 缓存会话保持滚动位置（`bf6c4a9`）在渲染期间写 ref，即当前 lint 的 3 条
   `react-hooks/refs` 错误；启用 React Compiler 前需改为由状态派生顺序。
 
+#### 2026-10-02 并行修复：补全分帧、远程批量写库、架构守护
+
+- **补全分帧（`ChatHistoryPane.tsx`）：** 新面板补全改为每帧 +20 条（20→40→60→80），每步后
+  贴底或按高度差保持阅读位置；“加载更早”、批次变化或笔记跳转到达时立即渲染完整批次。
+- **渲染期 ref（`App.tsx`、`pane-order.ts`）：** 会话面板顺序改为 state，用纯函数
+  `reconcilePaneOrder` 在渲染期按需调整，顺序不变时返回原数组；消除 3 条 `react-hooks/refs`。
+- **远程批量写库（`remote-event-batcher.ts`、`assistant-database.ts`、`ipc.ts`）：** 远程语义事件
+  按 checkpoint 一个事务提交（Agent 在 checkpoint 之后才可能 ACK），可恢复会话每批只读写一次
+  消息；非远程事件、`question`、`done`/`error`、显式 flush、256 条或 50 ms 计时器也会先提交。
+  只转发新写入的事件，顺序不变；提交失败整批回滚且之后拒绝继续提交。交互、后台/定时/通道
+  和重启恢复三条路径都已改用。当前 checkpoint 粒度是每条 transcript 记录而非每页，按页批量
+  需要改 `acp-remote-runtime.ts`。
+- **架构守护（提前实施 `PERF-17` 的一部分）：** `eslint.config.js` 新增只减不增的白名单规则：
+  `DatabaseSync`（19 个文件）、`src/main` 同步 fs（8 个文件）、组件直接订阅
+  `window.goodbuddy.*.on*`（6 个文件）、`App.tsx` `max-lines` 11700；文档演示和 vendor 脚本
+  不再参与 lint，`npx eslint .` 现为 0 错误。`build/perf-thresholds.json` +
+  `npm run perf:check`（或 `GB_PERF_CHECK=1 npm run perf:app`）检查不降速报告的阈值。
+- **`npm run perf:app` A/B（改动前为 `9d8477b`）：** 不降速 2 轮两版均通过 60 项阈值；打开长会话
+  从 1 个 Long Task / 56–61 ms、最大帧 50–58 ms 降到 0 个、最大帧 25–33 ms，其余持平。4× 降速
+  3 轮：打开长会话最大帧从 475–567 ms 降到 225–283 ms（帧 p95 因多出几帧小提交升到
+  175–258 ms）；切换会话持平；流式场景两版方差都很大，未见一致变化。远程批量写库不在该基准
+  覆盖范围内，由单元测试和 `ipc.test.ts` 验证事务次数、顺序与回放去重。
+
 ### PERF-14 长列表虚拟化
 
 - **优先级 / 状态：** P2 / 待开始
diff --git a/eslint.config.js b/eslint.config.js
index 5dd03b78..501e59ff 100644
--- a/eslint.config.js
+++ b/eslint.config.js
@@ -3,6 +3,86 @@ import reactHooks from 'eslint-plugin-react-hooks'
 import globals from 'globals'
 import tseslint from 'typescript-eslint'
 
+// ---------------------------------------------------------------------------
+// Architecture ratchets (docs/roadmap/product-performance-experience-improvement-plan.md,
+// batch 3C, PERF-17). Existing code still violates these rules, so each rule
+// carries an allowlist of the files that violated it when the rule was added.
+// The allowlists must ONLY SHRINK: remove a file once it is fixed, never add
+// one. New code that needs the forbidden API belongs in the place the plan
+// assigns it (data process, async fs, domain store, layout shell).
+// ---------------------------------------------------------------------------
+
+// PERF-16: SQLite belongs to a dedicated data process (utilityProcess); Main
+// only routes messages. Files that imported DatabaseSync from 'node:sqlite'
+// (including `import type`) on 2026-10-02. Shrink only.
+const databaseSyncAllowlist = [
+  'src/agent-daemon/agent-model-gateway.ts',
+  'src/agent-daemon/event-journal.ts',
+  'src/agent-daemon/installation-cleanup.ts',
+  'src/agent-daemon/runtime-owner-registry.ts',
+  'src/agent-daemon/semantic-prompt-store.ts',
+  'src/main/agent/runtime-session-binding-store.ts',
+  'src/main/assistant/assistant-database.ts',
+  'src/main/assistant/assistant-storage-upgrade.ts',
+  'src/main/assistant/story-graph-reader.ts',
+  'src/main/assistant/subagent-progress-storage.ts',
+  'src/main/assistant/supervision-experiences.ts',
+  'src/main/assistant/supervision-review-store.ts',
+  'src/main/assistant/supervision-stories.ts',
+  'src/main/assistant/supervision-suggestions.ts',
+  'src/main/assistant/supervision-timeline.ts',
+  'src/main/conversation-attachment-storage.ts',
+  'src/main/knowledge/external/external-knowledge-store.ts',
+  'src/main/knowledge/knowledge-database.ts',
+  'src/shared/node/private-sqlite-database.ts'
+]
+
+// PERF-15/PERF-16 rule 1: Main must not block its event loop on file IO.
+// Synchronous fs functions; also caught as `fs.xxxSync` member access.
+const syncFsFunctions = [
+  'accessSync', 'appendFileSync', 'chmodSync', 'chownSync', 'closeSync', 'copyFileSync', 'cpSync',
+  'existsSync', 'fchmodSync', 'fchownSync', 'fdatasyncSync', 'fstatSync', 'fsyncSync', 'ftruncateSync',
+  'futimesSync', 'globSync', 'lchmodSync', 'lchownSync', 'linkSync', 'lstatSync', 'lutimesSync',
+  'mkdirSync', 'mkdtempSync', 'opendirSync', 'openSync', 'readdirSync', 'readFileSync', 'readlinkSync',
+  'readSync', 'readvSync', 'realpathSync', 'renameSync', 'rmdirSync', 'rmSync', 'statfsSync', 'statSync',
+  'symlinkSync', 'truncateSync', 'unlinkSync', 'utimesSync', 'writeFileSync', 'writeSync', 'writevSync'
+]
+const syncFsMessage = 'Synchronous fs APIs block the Main event loop (PERF-17). Use node:fs/promises, or move the work to the data process (PERF-16).'
+const databaseSyncRestriction = [{
+  name: 'node:sqlite',
+  importNames: ['DatabaseSync'],
+  message: 'DatabaseSync may only be used by the data process (PERF-16); other code must go through its async interface (PERF-15). The allowlist in eslint.config.js must only shrink.'
+}]
+const syncFsRestriction = ['node:fs', 'fs', 'original-fs'].map(name => ({ name, importNames: syncFsFunctions, message: syncFsMessage }))
+// Files under src/main (non-test) that used synchronous fs APIs on 2026-10-02. Shrink only.
+const mainSyncFsAllowlist = [
+  'src/main/agent/paged-output-store.ts',
+  'src/main/assistant/assistant-database.ts',
+  'src/main/assistant/assistant-storage-upgrade.ts',
+  'src/main/conversation-attachment-storage.ts',
+  'src/main/magic-notes/magic-note-storage.ts',
+  'src/main/portable-user-data.ts',
+  'src/main/remote-agent/control-plane-package-installer.ts',
+  'src/main/windows-notification-identity.ts'
+]
+
+// PERF-13 rule 2: IPC subscriptions live in domain stores (*.ts), components
+// subscribe to stores through selectors. React component files (*.tsx) that
+// subscribed directly to window.goodbuddy.*.on* on 2026-10-02. Shrink only.
+const rendererIpcSubscriptionAllowlist = [
+  'src/renderer/src/App.tsx',
+  'src/renderer/src/DocumentResultPreview.tsx',
+  'src/renderer/src/MagicNotesPanel.tsx',
+  'src/renderer/src/MagicNotesWorkspace.tsx',
+  'src/renderer/src/RightAssistantSidebar.tsx',
+  'src/renderer/src/StaticHtmlPreview.tsx'
+]
+const rendererIpcSubscriptionMessage = 'Components must not subscribe to window.goodbuddy.*.on* directly (PERF-17). Move the subscription into a domain store and read it with a selector (PERF-13).'
+
+// PERF-13 goal: App.tsx shrinks to a layout shell. 11,635 lines on
+// 2026-10-02, ceiling rounded up to the next 100. Only ever lower this number.
+const appTsxMaxLines = 11700
+
 export default tseslint.config(
   {
     ignores: [
@@ -12,6 +92,8 @@ export default tseslint.config(
       'coverage/**',
       'dist/**',
       'docs/**/*-demo.*',
+      'docs/**/*-demo-*.js',
+      'docs/**/vendor/**',
       'node_modules/**',
       'out/**',
       'shareserver/**'
@@ -84,5 +166,57 @@ export default tseslint.config(
         ...globals.vitest
       }
     }
+  },
+  // --- Architecture ratchets (PERF-17), see allowlists at the top. ---------
+  // Rules a (DatabaseSync) and b (sync fs) share `no-restricted-imports`, and a
+  // later flat-config block replaces an earlier block's options for the same
+  // rule. The blocks below therefore partition the files so that each file
+  // gets exactly the restrictions it is subject to.
+  // DatabaseSync outside the data process (all of src, both value and type imports).
+  {
+    files: ['src/**/*.{ts,tsx}'],
+    ignores: ['**/*.test.{ts,tsx}', ...databaseSyncAllowlist],
+    rules: { 'no-restricted-imports': ['error', { paths: databaseSyncRestriction }] }
+  },
+  // Synchronous fs in Main: files not allowlisted for either rule get both restrictions.
+  {
+    files: ['src/main/**/*.ts'],
+    ignores: ['**/*.test.ts', ...mainSyncFsAllowlist, ...databaseSyncAllowlist],
+    rules: { 'no-restricted-imports': ['error', { paths: [...databaseSyncRestriction, ...syncFsRestriction] }] }
+  },
+  // Main files allowlisted for DatabaseSync but not for sync fs.
+  {
+    files: databaseSyncAllowlist.filter(file => file.startsWith('src/main/')),
+    ignores: mainSyncFsAllowlist,
+    rules: { 'no-restricted-imports': ['error', { paths: syncFsRestriction }] }
+  },
+  // `fs.readFileSync(...)` style access through a namespace/default import.
+  {
+    files: ['src/main/**/*.ts'],
+    ignores: ['**/*.test.ts', ...mainSyncFsAllowlist],
+    rules: {
+      'no-restricted-syntax': ['error', {
+        selector: `MemberExpression[object.name=/^(fs|nodeFs|fsSync|originalFs)$/][property.name=/^(${syncFsFunctions.join('|')})$/]`,
+        message: syncFsMessage
+      }]
+    }
+  },
+  // Direct IPC subscriptions in renderer components.
+  {
+    files: ['src/renderer/src/**/*.tsx'],
+    ignores: ['**/*.test.tsx', ...rendererIpcSubscriptionAllowlist],
+    rules: {
+      'no-restricted-syntax': ['error', {
+        selector: "CallExpression[callee.property.name=/^on[A-Z]/] > MemberExpression.callee MemberExpression[object.name='window'][property.name='goodbuddy']",
+        message: rendererIpcSubscriptionMessage
+      }]
+    }
+  },
+  // App.tsx size ceiling.
+  {
+    files: ['src/renderer/src/App.tsx'],
+    rules: {
+      'max-lines': ['error', { max: appTsxMaxLines, skipBlankLines: false, skipComments: false }]
+    }
   }
 )
diff --git a/package.json b/package.json
index 08c590dd..df6221ee 100644
--- a/package.json
+++ b/package.json
@@ -26,6 +26,7 @@
     "test": "vitest run",
     "test:watch": "vitest",
     "perf:app": "node build/run-app-perf.cjs",
+    "perf:check": "node build/check-perf-thresholds.cjs",
     "perf:knowledge": "node build/run-knowledge-perf.cjs",
     "eval:retrieval": "vitest run --config tests/support/knowledge-retrieval-evaluation.ts tests/knowledge-retrieval-metrics.test.ts tests/knowledge-retrieval-evaluation.test.ts",
     "build": "npm run typecheck && npm run build:bundle",
diff --git a/tests/perf-thresholds.test.ts b/tests/perf-thresholds.test.ts
new file mode 100644
index 00000000..67925484
--- /dev/null
+++ b/tests/perf-thresholds.test.ts
@@ -0,0 +1,136 @@
+import { readFileSync } from 'node:fs'
+import { createRequire } from 'node:module'
+import { join } from 'node:path'
+import { describe, expect, it } from 'vitest'
+
+interface ThresholdRow {
+  scenario: string
+  metric: string
+  value: number | string | null
+  limit: string
+  pass: boolean
+  note?: string
+}
+
+interface EvaluateResult {
+  ok: boolean
+  refused: boolean
+  reason?: string
+  rows: ThresholdRow[]
+}
+
+interface PerfThresholds {
+  requiredStatus: string
+  scenarios: Record<string, Record<string, { max?: number } | boolean>>
+}
+
+interface PerfCheckerModule {
+  evaluate(report: unknown, thresholds: PerfThresholds): EvaluateResult
+  defaultThresholdsPath: string
+}
+
+const require = createRequire(import.meta.url)
+const checker = require('../build/check-perf-thresholds.cjs') as PerfCheckerModule
+const thresholds = JSON.parse(readFileSync(checker.defaultThresholdsPath, 'utf8')) as PerfThresholds
+
+// A report shaped like tests/support/app-perf-driver.mjs output, with values
+// close to the 2026-10-02 non-throttled baseline.
+function scenario(id: string, overrides: { longTasks?: number, longTaskMs?: number, frameP95?: number, latencyP95?: number | null } = {}) {
+  const latencyP95 = overrides.latencyP95 === undefined ? 10 : overrides.latencyP95
+  return {
+    id,
+    durationMs: 3000,
+    renderer: {
+      longTasks: { count: overrides.longTasks ?? 0, totalMs: overrides.longTaskMs ?? 0, maxMs: overrides.longTaskMs ?? 0 },
+      frames: { p50: 8.3, p95: overrides.frameP95 ?? 8.5, max: 17, jankRatio: 0.01 }
+    },
+    latency: latencyP95 === null ? null : { p50: latencyP95 / 2, p95: latencyP95, max: latencyP95 + 2, count: 10 },
+    main: { eventLoopDelay: { p99: 20, max: 30 } }
+  }
+}
+
+function passingReport() {
+  return {
+    status: 'passed',
+    environment: { rendererCpuThrottle: 1 },
+    scenarios: [
+      scenario('idle', { latencyP95: null }),
+      scenario('typing-small', { latencyP95: 8.5 }),
+      scenario('idle-seeded', { latencyP95: null }),
+      scenario('typing-seeded', { latencyP95: 8.7 }),
+      scenario('switch-conversations', { longTasks: 1, longTaskMs: 120, latencyP95: 160 }),
+      scenario('switch-conversations-warm', { latencyP95: 68 }),
+      scenario('switch-conversations-hot', { latencyP95: 32 }),
+      scenario('open-long-conversation', { longTasks: 1, longTaskMs: 104, latencyP95: 140 }),
+      scenario('scroll-long-conversation', { latencyP95: null }),
+      scenario('stream-seeded', { longTasks: 1, longTaskMs: 250, latencyP95: null }),
+      scenario('stream-and-type', { longTasks: 1, longTaskMs: 230, frameP95: 16.6, latencyP95: 17 })
+    ]
+  }
+}
+
+describe('perf threshold checker', () => {
+  it('accepts a report at baseline levels', () => {
+    const result = checker.evaluate(passingReport(), thresholds)
+    expect(result.refused).toBe(false)
+    expect(result.rows.filter(row => !row.pass)).toEqual([])
+    expect(result.ok).toBe(true)
+  })
+
+  it('every configured scenario has a check for each metric it lists', () => {
+    const result = checker.evaluate(passingReport(), thresholds)
+    for (const [id, limits] of Object.entries(thresholds.scenarios)) {
+      const metrics = Object.keys(limits).filter(key => key !== 'optional')
+      expect(result.rows.filter(row => row.scenario === id).map(row => row.metric).sort()).toEqual(metrics.sort())
+    }
+  })
+
+  it('fails a streaming regression of the pre-PERF-12 size and a slow switch', () => {
+    const report = passingReport()
+    report.scenarios[9] = scenario('stream-seeded', { longTasks: 130, longTaskMs: 9000, frameP95: 83, latencyP95: null })
+    report.scenarios[4] = scenario('switch-conversations', { longTasks: 1, longTaskMs: 120, latencyP95: 900 })
+    const result = checker.evaluate(report, thresholds)
+    expect(result.ok).toBe(false)
+    const failures = result.rows.filter(row => !row.pass).map(row => `${row.scenario} ${row.metric}`)
+    expect(failures).toEqual(expect.arrayContaining([
+      'stream-seeded renderer.longTasks.count',
+      'stream-seeded renderer.longTasks.totalMs',
+      'stream-seeded renderer.frames.p95',
+      'switch-conversations latency.p95'
+    ]))
+  })
+
+  it('fails when the report status is not passed', () => {
+    const report = { ...passingReport(), status: 'passed-with-renderer-errors' }
+    const result = checker.evaluate(report, thresholds)
+    expect(result.ok).toBe(false)
+    expect(result.rows.find(row => row.metric === 'status')?.pass).toBe(false)
+  })
+
+  it('fails when a required scenario or metric is missing, but not an optional scenario', () => {
+    const report = passingReport()
+    report.scenarios = report.scenarios.filter(entry => entry.id !== 'stream-and-type' && entry.id !== 'switch-conversations-hot')
+    report.scenarios[1] = { ...scenario('typing-small'), latency: null }
+    const result = checker.evaluate(report, thresholds)
+    expect(result.ok).toBe(false)
+    const failures = result.rows.filter(row => !row.pass)
+    expect(failures).toEqual(expect.arrayContaining([
+      expect.objectContaining({ scenario: 'stream-and-type', metric: '(present)', value: 'missing' }),
+      expect.objectContaining({ scenario: 'typing-small', metric: 'latency.p95', value: null, note: 'metric missing' })
+    ]))
+    expect(failures.some(row => row.scenario === 'switch-conversations-hot')).toBe(false)
+  })
+
+  it('refuses to judge a throttled run', () => {
+    const report = { ...passingReport(), environment: { rendererCpuThrottle: 4 } }
+    const result = checker.evaluate(report, thresholds)
+    expect(result.refused).toBe(true)
+    expect(result.ok).toBe(false)
+    expect(result.reason).toMatch(/throttle 4x/)
+  })
+
+  it('ships thresholds that require a clean, non-throttled report', () => {
+    expect(thresholds.requiredStatus).toBe('passed')
+    expect(join(checker.defaultThresholdsPath)).toMatch(/perf-thresholds\.json$/)
+  })
+})