Turbopack: Enable server HMR by default for app pages by wbinnssmith · Pull Request #91476 · vercel/next.js

wbinnssmith · 2026-03-16T22:28:20Z

This enables server HMR (formally, this is called server fast refresh) by default for app router pages. For now, it's still disabled for routes and middleware even with app router.

Replaces --experimental-server-fast-refresh with --no-server-fast-refresh to opt out
Updates telemetry to record feature.serverFastRefreshDisabled when the disable flag is passed
Verify next dev enables server HMR by default (Turbopack, app dir, non-edge routes)
Verify next dev --no-server-fast-refresh disables it
CI

…ext dev` with Turbopack. Previously it required opting in with `--experimental-server-fast-refresh`. - Replaces `--experimental-server-fast-refresh` with `--no-server-fast-refresh` to opt out - Updates telemetry to record `feature.serverFastRefreshDisabled` when the disable flag is passed - [ ] Verify `next dev` enables server HMR by default (Turbopack, app dir, non-edge routes) - [ ] Verify `next dev --no-server-fast-refresh` disables it - CI

nextjs-bot · 2026-03-16T22:45:28Z

Failing test suites

Commit: c6017da | About building and testing Next.js

pnpm test-start-turbo test/production/graceful-shutdown/index.test.ts (turbopack) (job)

Graceful Shutdown > production (standalone mode) > should not accept new requests during shutdown cleanup > should stop accepting new requests when shutting down (DD)

Expand output

● Graceful Shutdown › production (standalone mode) › should not accept new requests during shutdown cleanup › should stop accepting new requests when shutting down

expect(received).toEqual(expected) // deep equality

- Expected  - 1
+ Received  + 1

  Array [
-   143,
    null,
+   "SIGTERM",
  ]

  227 |
  228 |         // App finally shuts down with signal-based exit code (128 + 15 for SIGTERM)
> 229 |         expect(await appKilledPromise).toEqual([143, null])
      |                                        ^
  230 |         expect(app.exitCode).toBe(143)
  231 |       })
  232 |     })

  at Object.toEqual (production/graceful-shutdown/index.test.ts:229:40)

pnpm test-start test/production/app-dir/metadata-static-route-cache/metadata-static-route-cache.test.ts (job)

app dir - metadata static routes cache > should generate different content after replace the static metadata file (DD)

Expand output

● app dir - metadata static routes cache › should generate different content after replace the static metadata file

next already started

  63 |   public async start(options: { skipBuild?: boolean } = {}) {
  64 |     if (this.childProcess) {
> 65 |       throw new Error('next already started')
     |             ^
  66 |     }
  67 |
  68 |     this._cliOutput = ''

  at NextStartInstance.start (lib/next-modes/next-start.ts:65:13)
  at Object.start (production/app-dir/metadata-static-route-cache/metadata-static-route-cache.test.ts:17:16)

pnpm test-start-turbo test/production/deterministic-build/deployment-id.test.ts (turbopack) (job)

deterministic build - changing deployment id > build output API - standard builder > should produce identical build outputs even when changing deployment id (DD)

Expand output

● deterministic build - changing deployment id › build output API - standard builder › should produce identical build outputs even when changing deployment id

can not run export while server is running, use next.stop() first

  251 |   ) {
  252 |     if (this.childProcess) {
> 253 |       throw new Error(
      |             ^
  254 |         `can not run export while server is running, use next.stop() first`
  255 |       )
  256 |     }

  at NextStartInstance.build (lib/next-modes/next-start.ts:253:13)
  at build (production/deterministic-build/deployment-id.test.ts:125:22)
  at Object.runTest (production/deterministic-build/deployment-id.test.ts:262:36)

codspeed-hq · 2026-03-16T22:48:15Z

Merging this PR will not alter performance

✅ 17 untouched benchmarks
⏩ 3 skipped benchmarks¹

_{Comparing wbinnssmith/server-hmr-default (c6017da) with canary (672b02b)}

3 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

nextjs-bot · 2026-03-16T22:51:52Z

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)	437ms	439ms	✓	█▁▁▁▂
Cold (First Request)	1.191s	1.176s	✓	█▂▂▁▃
Warm (Listen)	456ms	457ms	✓	█▁▁▁▁
Warm (Ready in log)	444ms	443ms	✓	█▁▁▁▁
Warm (First Request)	350ms	344ms	✓	█▁▁▁▁

📦 Dev Server (Webpack) (Legacy)

📦 Dev Server (Webpack)

Metric	Canary	PR	Change	Trend
Cold (Listen)	455ms	455ms	✓	▁█▁▁▃
Cold (Ready in log)	439ms	439ms	✓	▃█▃▁▃
Cold (First Request)	1.976s	1.960s	✓	▂█▂▁▂
Warm (Listen)	456ms	456ms	✓	▁█▁▁▁
Warm (Ready in log)	439ms	438ms	✓	▃█▃▁▃
Warm (First Request)	1.964s	1.969s	✓	▂█▂▁▂

⚡ Production Builds

Metric	Canary	PR	Change	Trend
Fresh Build	3.753s	3.800s	✓	█▃▃▂▁
Cached Build	3.744s	3.788s	✓	█▃▂▂▁

📦 Production Builds (Webpack) (Legacy)

📦 Production Builds (Webpack)

Metric	Canary	PR	Change	Trend
Fresh Build	14.335s	14.350s	✓	▁█▁▁▁
Cached Build	14.597s	14.668s	✓	▁█▁▁▂
node_modules Size	483 MB	483 MB	✓	█████

📦 Bundle Sizes

Bundle Sizes

⚡ Turbopack

Client

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Server

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	712 B	711 B	✓
Total	712 B	711 B	✅ -1 B

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	433 B	432 B	✓
Total	433 B	432 B	✅ -1 B

📦 Webpack

Client

Main Bundles

	Canary	PR	Change
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6280-HASH.js gzip	60.3 kB	N/A	-
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Polyfills

	Canary	PR	Change
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Total	39.4 kB	39.4 kB	✓

Pages

	Canary	PR	Change
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hooks-HASH.js gzip	384 B	383 B	✓
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Build Details

Build Manifests

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Build Cache

	Canary	PR	Change
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🔄 Shared (bundler-independent)

Runtimes

	Canary	PR	Change
app-page-exp...dev.js gzip	333 kB	333 kB	✓
app-page-exp..prod.js gzip	181 kB	181 kB	✓
app-page-tur...dev.js gzip	332 kB	332 kB	✓
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dist_client_...dev.js gzip	324 B	324 B	✓
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pages-api.ru...dev.js gzip	43.3 kB	43.3 kB	✓
pages-api.ru..prod.js gzip	33 kB	33 kB	✓
pages-turbo....dev.js gzip	52.7 kB	52.7 kB	✓
pages-turbo...prod.js gzip	38.6 kB	38.6 kB	✓
pages.runtim...dev.js gzip	52.7 kB	52.7 kB	✓
pages.runtim..prod.js gzip	38.6 kB	38.6 kB	✓
server.runti..prod.js gzip	62.4 kB	62.4 kB	✓
Total	2.95 MB	2.95 MB	⚠️ +2 B

📎 Tarball URL

https://vercel-packages.vercel.app/next/commits/267a1b2f1e7202036ebb3fc5e40df2f8e9bacae4/next

andrewimm

spent forever trying to figure out how the option was getting sent to the actual dev server, before I realized that Commander has special casing for options that start with "no-"

…ext.js 16.2+ compatibility (#16074) [Next.js 16.2 enables server fast refresh by default](vercel/next.js#91476), which breaks Payload's HMR - config changes don't propagate until a full server restart. Adding `--no-server-fast-refresh` to the dev command fixes it. This is a temporary workaround until the upstream issue is resolved. --- - To see the specific tasks where the Asana app for GitHub is being used, see below: - https://app.asana.com/0/0/1213814287069601

wbinnssmith changed the title ~~Server-side Fast Refresh (server HMR) is now enabled by default in next dev with Turbopack. Previously it required opting in with --experimental-server-fast-refresh.~~ Turbopack: Enable server HMR by default Mar 16, 2026

wbinnssmith changed the title ~~Turbopack: Enable server HMR by default~~ Turbopack: Enable server HMR by default for app pages Mar 16, 2026

wbinnssmith requested review from andrewimm, lukesandberg, sokra and timneutkens March 16, 2026 23:21

wbinnssmith marked this pull request as ready for review March 16, 2026 23:21

andrewimm approved these changes Mar 16, 2026

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wbinnssmith merged commit 0f867bb into canary Mar 16, 2026
282 of 287 checks passed

wbinnssmith deleted the wbinnssmith/server-hmr-default branch March 16, 2026 23:37

AlessioGr mentioned this pull request Mar 26, 2026

fix(templates): add --no-server-fast-refresh to all dev scripts for Next.js 16.2+ compatibility payloadcms/payload#16074

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Turbopack: Enable server HMR by default for app pages#91476

Turbopack: Enable server HMR by default for app pages#91476
wbinnssmith merged 1 commit into
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Merging this PR will not alter performance

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Stats from current PR

✅ No significant changes detected

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wbinnssmith commented Mar 16, 2026 •

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