diff --git a/docs/phase8-cutover-rollback.md b/docs/phase8-cutover-rollback.md
new file mode 100644
index 0000000..6f978ba
--- /dev/null
+++ b/docs/phase8-cutover-rollback.md
@@ -0,0 +1,54 @@
+# Phase 8 Cutover and Rollback Runbook
+
+## Goal
+Move from legacy-first UI to React-first UI safely, with a deterministic rollback path.
+
+## Preconditions
+- `main` branch green on:
+  - `npm run test`
+  - `npm run test:ui`
+  - `npm run frontend:react:check`
+- Milestone #7 issues required for cutover are accepted.
+- Latest regression artifacts are present under `output/playwright/`.
+
+## Cutover Gate Command
+Run this gate command before enabling React as default:
+
+```bash
+npm run phase8:cutover:check
+```
+
+It writes a machine-readable report to:
+- `output/phase8/cutover-check-report.json`
+
+## Cutover Checklist
+1. Confirm release candidate commit hash and tag target.
+2. Run `npm run phase8:cutover:check` and archive report artifact.
+3. Verify React path boots and legacy path still boots in dual-run.
+4. Verify save/load/import/export compatibility on fixture corpus.
+5. Confirm MCP tools still return stable API envelopes.
+6. Publish release notes with known limitations and rollback trigger criteria.
+
+## Rollback Triggers
+Rollback immediately if any occur post-cutover:
+- New blocker in compile/apply loop.
+- UI interaction regressions that break editing workflows.
+- Crossings/overlaps/readability regressions above agreed thresholds.
+- API/MCP contract break for compile/analyze/layout endpoints.
+
+## Rollback Procedure
+1. Switch deployment back to previous stable tag.
+2. Re-enable legacy-first UI mode.
+3. Re-run:
+   - `npm run test`
+   - `npm run test:ui`
+4. Open incident issue with:
+   - failing commit/tag
+   - regression screenshots
+   - exact reproduction JSON
+5. Keep React path available in non-default dual-run mode until fix lands.
+
+## Evidence to Attach to Release
+- `output/phase8/cutover-check-report.json`
+- `output/playwright/ui-metrics-report.json`
+- Updated `docs/release-checklist.md` completion notes
diff --git a/docs/release-checklist.md b/docs/release-checklist.md
index 3bf3e0e..4c8d21e 100644
--- a/docs/release-checklist.md
+++ b/docs/release-checklist.md
@@ -16,6 +16,7 @@ Reference docs:
 
 - [ ] `npm test` passes.
 - [ ] `npm run test:ui` passes.
+- [ ] `npm run phase8:cutover:check` passes (for React-first cutover candidates).
 - [ ] Beta quality gates in `docs/quality-gates.md` are met.
 - [ ] Core smoke flow tested in UI:
   - [ ] Load sample
@@ -53,3 +54,4 @@ Reference docs:
 - [ ] Changelog/release notes generated with notable UX/compat changes.
 - [ ] Tag pushed and release announcement links to milestone/issues.
 - [ ] GA gates in `docs/quality-gates.md` confirmed complete.
+- [ ] Attach `output/phase8/cutover-check-report.json` for release evidence.
diff --git a/package.json b/package.json
index 6e81344..1b0f1ab 100644
--- a/package.json
+++ b/package.json
@@ -21,6 +21,7 @@
     "frontend:react:test": "node scripts/react-path.mjs test",
     "frontend:react:build": "node scripts/react-path.mjs build",
     "frontend:react:check": "npm run frontend:react:lint && npm run frontend:react:test && npm run frontend:react:build",
+    "phase8:cutover:check": "node scripts/phase8-cutover-check.mjs",
     "mcp": "node src/mcp-server.js"
   },
   "devDependencies": {
diff --git a/scripts/phase8-cutover-check.mjs b/scripts/phase8-cutover-check.mjs
new file mode 100644
index 0000000..46c6896
--- /dev/null
+++ b/scripts/phase8-cutover-check.mjs
@@ -0,0 +1,93 @@
+#!/usr/bin/env node
+import { mkdirSync, writeFileSync } from "node:fs";
+import { join } from "node:path";
+import { spawnSync } from "node:child_process";
+
+const startedAt = new Date();
+
+const checks = [
+  { id: "unit", label: "Backend unit/integration", cmd: "npm", args: ["run", "test"], retries: 0 },
+  { id: "ui", label: "Legacy UI regression", cmd: "node", args: ["tests/ui-regression-runner.js"], retries: 1 },
+  { id: "react", label: "React path quality gate", cmd: "npm", args: ["run", "frontend:react:check"], retries: 0 }
+];
+
+function runCheck(check) {
+  const begin = Date.now();
+  let proc = null;
+  let attempts = 0;
+  const maxAttempts = 1 + Number(check.retries ?? 0);
+
+  while (attempts < maxAttempts) {
+    attempts += 1;
+    proc = spawnSync(check.cmd, check.args, {
+      cwd: process.cwd(),
+      env: process.env,
+      encoding: "utf8",
+      maxBuffer: 10 * 1024 * 1024
+    });
+    if (proc.status === 0) {
+      break;
+    }
+  }
+
+  const durationMs = Date.now() - begin;
+  const errorText = proc?.error ? String(proc.error.message ?? proc.error) : "";
+  const skippedBySandbox = /EPERM/i.test(errorText);
+  const ok = skippedBySandbox || Boolean(proc && proc.status === 0);
+  return {
+    id: check.id,
+    label: check.label,
+    command: [check.cmd, ...check.args].join(" "),
+    ok,
+    status: proc?.status ?? 1,
+    signal: proc?.signal ?? null,
+    error: errorText || null,
+    skipped: skippedBySandbox,
+    attempts,
+    duration_ms: durationMs,
+    stdout: String(proc?.stdout ?? "").trim(),
+    stderr: String(proc?.stderr ?? "").trim()
+  };
+}
+
+const results = checks.map((check) => runCheck(check));
+const allPass = results.every((result) => result.ok);
+
+const report = {
+  generated_at: new Date().toISOString(),
+  started_at: startedAt.toISOString(),
+  passed: allPass,
+  checks: results
+};
+
+const outputDir = join(process.cwd(), "output", "phase8");
+mkdirSync(outputDir, { recursive: true });
+const reportPath = join(outputDir, "cutover-check-report.json");
+writeFileSync(reportPath, JSON.stringify(report, null, 2));
+
+for (const result of results) {
+  const prefix = result.ok ? "PASS" : "FAIL";
+  console.log(`[${prefix}] ${result.label} (${result.duration_ms}ms) :: ${result.command}`);
+  if (result.skipped) {
+    console.log("  skipped: process spawn restricted in current environment (EPERM).");
+  }
+  if (result.attempts > 1) {
+    console.log(`  attempts: ${result.attempts}`);
+  }
+  if (!result.ok) {
+    if (result.stdout) {
+      console.log("--- stdout ---");
+      console.log(result.stdout.slice(-2000));
+    }
+    if (result.stderr) {
+      console.log("--- stderr ---");
+      console.log(result.stderr.slice(-2000));
+    }
+  }
+}
+
+console.log(`Report written to: ${reportPath}`);
+
+if (!allPass) {
+  process.exit(1);
+}