Turbopack: Add --experimental-server-fast-refresh CLI flag #89274

wbinnssmith · 2026-01-30T03:55:04Z

This adds --experimental-server-fast-refresh CLI flag to enable experimental server-side Fast Refresh (HMR) during development. It's disabled by default.

We call HMR "fast refresh" in public docs so let's keep the flag consistent with that. We use the HMR naming past this outer layer in the codebase.

Test Plan:

next dev --experimental-server-fast-refresh

wbinnssmith · 2026-01-30T04:01:35Z

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nextjs-bot · 2026-01-30T04:12:01Z

Failing test suites

Commit: 3c45ded | About building and testing Next.js

pnpm test-start-turbo test/e2e/postcss-config-ts/index.test.ts (turbopack) (job)

Expand output

● Test suite failed to run

Jest encountered an unexpected token

Jest failed to parse a file. This happens e.g. when your code or its dependencies use non-standard JavaScript syntax, or when Jest is not configured to support such syntax.

Out of the box Jest supports Babel, which will be used to transform your files into valid JS based on your Babel configuration.

By default "node_modules" folder is ignored by transformers.

Here's what you can do:
 • If you are trying to use ECMAScript Modules, see https://jestjs.io/docs/ecmascript-modules for how to enable it.
 • If you are trying to use TypeScript, see https://jestjs.io/docs/getting-started#using-typescript
 • To have some of your "node_modules" files transformed, you can specify a custom "transformIgnorePatterns" in your config.
 • If you need a custom transformation specify a "transform" option in your config.
 • If you simply want to mock your non-JS modules (e.g. binary assets) you can stub them out with the "moduleNameMapper" config option.

You'll find more details and examples of these config options in the docs:
https://jestjs.io/docs/configuration
For information about custom transformations, see:
https://jestjs.io/docs/code-transformation

Details:

/root/actions-runner/_work/next.js/next.js/test/e2e/postcss-config-ts/index.test.ts:1
({"Object.<anonymous>":function(module,exports,require,__dirname,__filename,jest){import { FileRef, nextTestSetup } from 'e2e-utils';
                                                                                  ^^^^^^

SyntaxError: Cannot use import statement outside a module

  at Runtime.createScriptFromCode (../node_modules/.pnpm/jest-runtime@29.7.0/node_modules/jest-runtime/build/index.js:1505:14)

pnpm test-start-turbo test/e2e/app-dir/segment-cache/prefetch-runtime/prefetch-runtime.test.ts (turbopack) (job)

runtime prefetching > passed to a public cache > can completely prefetch a page that uses cookies and no uncached IO (DD)

Expand output

● runtime prefetching › passed to a public cache › can completely prefetch a page that uses cookies and no uncached IO

apiRequestContext.fetch: read ECONNRESET
Call log:
  - → GET http://localhost:34569/passed-to-public-cache/cookies-only?_rsc=gtqjj
  -   user-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) HeadlessChrome/130.0.6723.31 Safari/537.36
  -   accept: */*
  -   accept-encoding: gzip,deflate,br
  -   cookie: testCookie=initialValue
  -   next-test-fetch-priority: low
  -   referer: http://localhost:34569/
  -   next-router-prefetch: 1
  -   next-router-segment-prefetch: /!KGRlZmF1bHQp/passed-to-public-cache
  -   next-url: /
  -   rsc: 1
  -   sec-ch-ua: "Chromium";v="130", "HeadlessChrome";v="130", "Not?A_Brand";v="99"
  -   sec-ch-ua-mobile: ?0
  -   sec-ch-ua-platform: "Linux"

  225 |             // server; we pass the request to the server the immediately.
  226 |             result: (async () => {
> 227 |               const originalResponse = await page.request.fetch(request, {
      |                                                           ^
  228 |                 maxRedirects: 0,
  229 |               })
  230 |

  at fetch (lib/router-act.ts:227:59)
  at lib/router-act.ts:245:13
  at routeHandler (lib/router-act.ts:257:7)

● runtime prefetching › passed to a public cache › can completely prefetch a page that uses cookies and no uncached IO

apiRequestContext.fetch: read ECONNRESET
Call log:
  - → GET http://localhost:34569/passed-to-public-cache/cookies-only?_rsc=gtqjj
  -   user-agent: Mozilla/5.0 (X11; Linux x86_64) AppleWebKit/537.36 (KHTML, like Gecko) HeadlessChrome/130.0.6723.31 Safari/537.36
  -   accept: */*
  -   accept-encoding: gzip,deflate,br
  -   cookie: testCookie=initialValue
  -   next-test-fetch-priority: low
  -   referer: http://localhost:34569/
  -   next-router-prefetch: 1
  -   next-router-segment-prefetch: /!KGRlZmF1bHQp/passed-to-public-cache
  -   next-url: /
  -   rsc: 1
  -   sec-ch-ua: "Chromium";v="130", "HeadlessChrome";v="130", "Not?A_Brand";v="99"
  -   sec-ch-ua-mobile: ?0
  -   sec-ch-ua-platform: "Linux"

  225 |             // server; we pass the request to the server the immediately.
  226 |             result: (async () => {
> 227 |               const originalResponse = await page.request.fetch(request, {
      |                                                           ^
  228 |                 maxRedirects: 0,
  229 |               })
  230 |

  at fetch (lib/router-act.ts:227:59)
  at lib/router-act.ts:245:13
  at routeHandler (lib/router-act.ts:257:7)

nextjs-bot · 2026-01-30T04:19:30Z

Stats from current PR

✅ No significant changes detected

📊 All Metrics

📖 Metrics Glossary

Dev Server Metrics:

Listen = TCP port starts accepting connections
First Request = HTTP server returns successful response
Cold = Fresh build (no cache)
Warm = With cached build artifacts

Build Metrics:

Fresh = Clean build (no .next directory)
Cached = With existing .next directory

Change Thresholds:

Time: Changes < 50ms AND < 10%, OR < 2% are insignificant
Size: Changes < 1KB AND < 1% are insignificant
All other changes are flagged to catch regressions

⚡ Dev Server

Metric	Canary	PR	Change	Trend
Cold (Listen)	455ms	455ms	✓	▁▁█▁▁
Cold (Ready in log)	436ms	437ms	✓	██▆▆▇
Cold (First Request)	1.121s	1.187s	✓	▇▆▇▇▇
Warm (Listen)	456ms	456ms	✓	▁█▁▁█
Warm (Ready in log)	441ms	442ms	✓	▂▁▃▁▂
Warm (First Request)	337ms	338ms	✓	▅▁▅▁█

📦 Dev Server (Webpack) (Legacy)

📦 Dev Server (Webpack)

Metric	Canary	PR	Change	Trend
Cold (Listen)	456ms	456ms	✓	▁▁▁▁▁
Cold (Ready in log)	445ms	445ms	✓	▁▁▁▁▁
Cold (First Request)	1.873s	1.857s	✓	▁▁▁▁▁
Warm (Listen)	457ms	456ms	✓	▁▁▁▁▁
Warm (Ready in log)	445ms	442ms	✓	▁▁▁▁▁
Warm (First Request)	1.869s	1.860s	✓	▁▁▁▁▁

⚡ Production Builds

Metric	Canary	PR	Change	Trend
Fresh Build	3.867s	3.898s	✓	▂▁▁▁▁
Cached Build	3.879s	3.901s	✓	▁▁▁▁▁

📦 Production Builds (Webpack) (Legacy)

📦 Production Builds (Webpack)

Metric	Canary	PR	Change	Trend
Fresh Build	13.965s	14.001s	✓	▂▁▁▁▁
Cached Build	14.013s	14.040s	✓	▁▁▁▁▁
node_modules Size	463 MB	463 MB	✓	▁▁▁▁▁

📦 Bundle Sizes

Bundle Sizes

⚡ Turbopack

Client

Main Bundles: **434 kB** → **434 kB** ✅ -2 B

81 files with content-based hashes (individual files not comparable between builds)

Server

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	765 B	765 B	✓
Total	765 B	765 B	✓

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	451 B	451 B	✓
Total	451 B	451 B	✓

📦 Webpack

Client

Main Bundles

	Canary	PR	Change
5528-HASH.js gzip	5.47 kB	N/A	-
6280-HASH.js gzip	54.4 kB	N/A	-
6335.HASH.js gzip	169 B	N/A	-
912-HASH.js gzip	4.53 kB	N/A	-
e8aec2e4-HASH.js gzip	62.5 kB	N/A	-
framework-HASH.js gzip	59.7 kB	59.7 kB	✓
main-app-HASH.js gzip	256 B	254 B	✓
main-HASH.js gzip	39 kB	39 kB	✓
webpack-HASH.js gzip	1.68 kB	1.68 kB	✓
262-HASH.js gzip	N/A	4.52 kB	-
2889.HASH.js gzip	N/A	169 B	-
5602-HASH.js gzip	N/A	5.48 kB	-
6948ada0-HASH.js gzip	N/A	62.5 kB	-
9544-HASH.js gzip	N/A	55.1 kB	-
Total	228 kB	228 kB	⚠️ +692 B

Polyfills

	Canary	PR	Change
polyfills-HASH.js gzip	39.4 kB	39.4 kB	✓
Total	39.4 kB	39.4 kB	✓

Pages

	Canary	PR	Change
_app-HASH.js gzip	194 B	194 B	✓
_error-HASH.js gzip	183 B	180 B	🟢 3 B (-2%)
css-HASH.js gzip	331 B	330 B	✓
dynamic-HASH.js gzip	1.81 kB	1.81 kB	✓
edge-ssr-HASH.js gzip	256 B	256 B	✓
head-HASH.js gzip	351 B	352 B	✓
hooks-HASH.js gzip	384 B	383 B	✓
image-HASH.js gzip	580 B	581 B	✓
index-HASH.js gzip	260 B	260 B	✓
link-HASH.js gzip	2.49 kB	2.49 kB	✓
routerDirect..HASH.js gzip	320 B	319 B	✓
script-HASH.js gzip	386 B	386 B	✓
withRouter-HASH.js gzip	315 B	315 B	✓
1afbb74e6ecf..834.css gzip	106 B	106 B	✓
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Server

Edge SSR

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edge-ssr.js gzip	126 kB	126 kB	✓
page.js gzip	247 kB	248 kB	✓
Total	374 kB	374 kB	⚠️ +319 B

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	616 B	615 B	✓
middleware-r..fest.js gzip	156 B	155 B	✓
middleware.js gzip	33 kB	33.2 kB	✓
edge-runtime..pack.js gzip	842 B	842 B	✓
Total	34.6 kB	34.8 kB	⚠️ +239 B

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	732 B	736 B	✓
Total	732 B	736 B	⚠️ +4 B

Build Cache

	Canary	PR	Change
0.pack gzip	3.76 MB	3.77 MB	🔴 +10.1 kB (+0%)
index.pack gzip	102 kB	101 kB	🟢 1.22 kB (-1%)
index.pack.old gzip	102 kB	101 kB	✓
Total	3.97 MB	3.98 MB	⚠️ +7.87 kB

🔄 Shared (bundler-independent)

Runtimes

	Canary	PR	Change
app-page-exp...dev.js gzip	311 kB	311 kB	✓
app-page-exp..prod.js gzip	166 kB	166 kB	✓
app-page-tur...dev.js gzip	311 kB	311 kB	✓
app-page-tur..prod.js gzip	166 kB	166 kB	✓
app-page-tur...dev.js gzip	307 kB	307 kB	✓
app-page-tur..prod.js gzip	164 kB	164 kB	✓
app-page.run...dev.js gzip	307 kB	307 kB	✓
app-page.run..prod.js gzip	164 kB	164 kB	✓
app-route-ex...dev.js gzip	70.2 kB	70.2 kB	✓
app-route-ex..prod.js gzip	48.8 kB	48.8 kB	✓
app-route-tu...dev.js gzip	70.2 kB	70.2 kB	✓
app-route-tu..prod.js gzip	48.8 kB	48.8 kB	✓
app-route-tu...dev.js gzip	69.8 kB	69.8 kB	✓
app-route-tu..prod.js gzip	48.6 kB	48.6 kB	✓
app-route.ru...dev.js gzip	69.8 kB	69.8 kB	✓
app-route.ru..prod.js gzip	48.6 kB	48.6 kB	✓
dist_client_...dev.js gzip	324 B	324 B	✓
dist_client_...dev.js gzip	326 B	326 B	✓
dist_client_...dev.js gzip	318 B	318 B	✓
dist_client_...dev.js gzip	317 B	317 B	✓
pages-api-tu...dev.js gzip	43.1 kB	43.1 kB	✓
pages-api-tu..prod.js gzip	32.8 kB	32.8 kB	✓
pages-api.ru...dev.js gzip	43.1 kB	43.1 kB	✓
pages-api.ru..prod.js gzip	32.8 kB	32.8 kB	✓
pages-turbo....dev.js gzip	52.4 kB	52.4 kB	✓
pages-turbo...prod.js gzip	39.3 kB	39.3 kB	✓
pages.runtim...dev.js gzip	52.4 kB	52.4 kB	✓
pages.runtim..prod.js gzip	39.3 kB	39.3 kB	✓
server.runti..prod.js gzip	62.5 kB	62.5 kB	✓
Total	2.77 MB	2.77 MB	⚠️ +1 B

bgw · 2026-01-30T06:38:29Z

nextjs-bot added created-by: Turbopack team PRs by the Turbopack team. type: next labels Jan 30, 2026

wbinnssmith changed the title ~~Add --experimental-server-fast-refresh CLI flag~~ Jan 30, 2026

Add --experimental-server-fast-refresh CLI flag

3c45ded

wbinnssmith force-pushed the wbinnssmith/server-hmr-runtime-gate branch from 77b082c to 3c45ded Compare January 30, 2026 04:01

This was referenced Jan 30, 2026

Turbopack: Server HMR infrastructure #88870

Open

Turbopack server hmr: extract core hmr client logic #88912

Draft

Turbopack: Implement server hmr in nodejs dev runtime #89130

Draft

wbinnssmith requested review from bgw and lukesandberg January 30, 2026 04:05

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