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fix(build): prevent route handler manifests from inheriting unrelated client components #88419

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Merged
timneutkens merged 2 commits into from
Jan 14, 2026
Merged

fix(build): prevent route handler manifests from inheriting unrelated client components #88419

timneutkens merged 2 commits into from
Jan 14, 2026

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@timneutkens timneutkens commented Jan 12, 2026 •
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Summary

Fixes #88316

Route handlers (API routes) were generating bloated client-reference-manifest.js files containing unrelated client components from pages when using Webpack. This was caused by the manifest merging logic in flight-manifest-plugin.ts which inherited manifests from all parent path segments.

Changes

  • Modified flight-manifest-plugin.ts to detect route handlers (/route$ pattern)
  • Route handlers now only get their own group's manifest, skipping parent manifest inheritance
  • Pages continue to inherit manifests from parent layouts as before

Why this works

Route handlers:

  • Don't render React components
  • Return Response objects, not JSX
  • Don't need client component references from parent layouts/pages

Turbopack already handles this correctly by doing targeted module graph traversal per entry. This fix aligns Webpack's behavior.

Test Plan

Added test suite test/production/app-dir/route-handler-manifest-size/:

  • Verifies pure route handlers don't include unrelated page client components
  • Verifies pages still include their client components
  • Verifies route handlers with direct client imports don't get page components
  • Tests pass for both Webpack and Turbopack

@timneutkens
fix(build): prevent route handler manifests from inheriting unrelated…
7b0ec41 
… client components

Route handlers don't render React components and don't need client component
references from parent layouts/pages. This change modifies the webpack
flight-manifest-plugin to skip parent group manifest merging for route handlers,
significantly reducing manifest size for API routes.

Fixes #88316
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timneutkens commented Jan 12, 2026 •
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@timneutkens timneutkens marked this pull request as ready for review January 12, 2026 12:01
@timneutkens timneutkens marked this pull request as draft January 12, 2026 12:02
@timneutkens timneutkens mentioned this pull request Jan 12, 2026
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nextjs-bot commented Jan 12, 2026 •
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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) 456ms 455ms ✓ ▁▁▁▁█
Cold (Ready in log) 440ms 439ms ✓ ▇▇▆█▅
Cold (First Request) 1.148s 1.147s ✓ ▇███▃
Warm (Listen) 457ms 457ms ✓ ▁▁▁▁█
Warm (Ready in log) 444ms 450ms ✓ ▁▁▁▁█
Warm (First Request) 342ms 342ms ✓ █▂▂▁▆
📦 Dev Server (Webpack) (Legacy)

📦 Dev Server (Webpack)

Metric Canary PR Change Trend
Cold (Listen) 455ms 455ms ✓ ███▁▁
Cold (Ready in log) 438ms 439ms ✓ ██▇▁▇
Cold (First Request) 1.825s 1.828s ✓ █▅▅▁▄
Warm (Listen) 455ms 456ms ✓ ▅▅▁▁▅
Warm (Ready in log) 439ms 438ms ✓ ▇▇▇▁█
Warm (First Request) 1.860s 1.841s ✓ █▆▆▁▇

⚡ Production Builds

Metric Canary PR Change Trend
Fresh Build 4.095s 4.101s ✓ ▁▁▂▂█
Cached Build 4.089s 4.076s ✓ ▁▂▁▂█
📦 Production Builds (Webpack) (Legacy)

📦 Production Builds (Webpack)

Metric Canary PR Change Trend
Fresh Build 13.981s 14.015s ✓ █▁▂▁▂
Cached Build 14.168s 14.165s ✓ █▁▂▁▂
node_modules Size 458 MB 458 MB ✓ █████
📦 Bundle Sizes

Bundle Sizes

⚡ Turbopack

Client

Main Bundles: **431 kB** → **431 kB** ✅ -10 B

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

Server

Middleware
Canary PR Change
middleware-b..fest.js gzip 787 B 791 B ✓
Total 787 B 791 B ⚠️ +4 B
Build Details
Build Manifests
Canary PR Change
_buildManifest.js gzip 449 B 451 B ✓
Total 449 B 451 B ⚠️ +2 B

📦 Webpack

Client

Main Bundles
Canary PR Change
2086.HASH.js gzip 169 B N/A -
2161-HASH.js gzip 5.41 kB N/A -
2747-HASH.js gzip 4.48 kB N/A -
4322-HASH.js gzip 52.8 kB N/A -
ec793fe8-HASH.js gzip 62.3 kB N/A -
framework-HASH.js gzip 59.8 kB 59.8 kB ✓
main-app-HASH.js gzip 251 B 253 B ✓
main-HASH.js gzip 38.6 kB 39 kB ✓
webpack-HASH.js gzip 1.68 kB 1.68 kB ✓
1596.HASH.js gzip N/A 169 B -
2658-HASH.js gzip N/A 52.7 kB -
6349-HASH.js gzip N/A 4.46 kB -
7019-HASH.js gzip N/A 5.43 kB -
b17a3386-HASH.js gzip N/A 62.3 kB -
Total 225 kB 226 kB ⚠️ +244 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 193 B ✓
_error-HASH.js gzip 182 B 182 B ✓
css-HASH.js gzip 336 B 335 B ✓
dynamic-HASH.js gzip 1.8 kB 1.8 kB ✓
edge-ssr-HASH.js gzip 256 B 256 B ✓
head-HASH.js gzip 352 B 349 B ✓
hooks-HASH.js gzip 385 B 384 B ✓
image-HASH.js gzip 580 B 580 B ✓
index-HASH.js gzip 259 B 258 B ✓
link-HASH.js gzip 2.5 kB 2.51 kB ✓
routerDirect..HASH.js gzip 319 B 317 B ✓
script-HASH.js gzip 385 B 387 B ✓
withRouter-HASH.js gzip 316 B 315 B ✓
1afbb74e6ecf..834.css gzip 106 B 106 B ✓
Total 7.97 kB 7.96 kB ✅ -8 B

Server

Edge SSR
Canary PR Change
edge-ssr.js gzip 125 kB 125 kB ✓
page.js gzip 241 kB 242 kB ✓
Total 366 kB 367 kB ⚠️ +491 B
Middleware
Canary PR Change
middleware-b..fest.js gzip 655 B 652 B ✓
middleware-r..fest.js gzip 155 B 156 B ✓
middleware.js gzip 33.2 kB 33.2 kB ✓
edge-runtime..pack.js gzip 842 B 842 B ✓
Total 34.8 kB 34.9 kB ⚠️ +75 B
Build Details
Build Manifests
Canary PR Change
_buildManifest.js gzip 738 B 738 B ✓
Total 738 B 738 B ✓
Build Cache
Canary PR Change
0.pack gzip 3.64 MB 3.65 MB 🔴 +7.45 kB (+0%)
index.pack gzip 99.7 kB 100 kB ✓
index.pack.old gzip 98.4 kB 101 kB 🔴 +2.47 kB (+3%)
Total 3.84 MB 3.85 MB ⚠️ +10.5 kB

🔄 Shared (bundler-independent)

Runtimes
Canary PR Change
app-page-exp...dev.js gzip 304 kB 304 kB ✓
app-page-exp..prod.js gzip 158 kB 158 kB ✓
app-page-tur...dev.js gzip 304 kB 304 kB ✓
app-page-tur..prod.js gzip 158 kB 158 kB ✓
app-page-tur...dev.js gzip 300 kB 300 kB ✓
app-page-tur..prod.js gzip 156 kB 156 kB ✓
app-page.run...dev.js gzip 300 kB 300 kB ✓
app-page.run..prod.js gzip 156 kB 156 kB ✓
app-route-ex...dev.js gzip 68.7 kB 68.7 kB ✓
app-route-ex..prod.js gzip 47.5 kB 47.5 kB ✓
app-route-tu...dev.js gzip 68.8 kB 68.8 kB ✓
app-route-tu..prod.js gzip 47.6 kB 47.6 kB ✓
app-route-tu...dev.js gzip 68.4 kB 68.4 kB ✓
app-route-tu..prod.js gzip 47.3 kB 47.3 kB ✓
app-route.ru...dev.js gzip 68.3 kB 68.3 kB ✓
app-route.ru..prod.js gzip 47.3 kB 47.3 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 41.1 kB 41.1 kB ✓
pages-api-tu..prod.js gzip 31.2 kB 31.2 kB ✓
pages-api.ru...dev.js gzip 41.1 kB 41.1 kB ✓
pages-api.ru..prod.js gzip 31.2 kB 31.2 kB ✓
pages-turbo....dev.js gzip 50.8 kB 50.8 kB ✓
pages-turbo...prod.js gzip 38.2 kB 38.2 kB ✓
pages.runtim...dev.js gzip 50.8 kB 50.8 kB ✓
pages.runtim..prod.js gzip 38.2 kB 38.2 kB ✓
server.runti..prod.js gzip 62.2 kB 62.2 kB ✓
Total 2.69 MB 2.69 MB ✅ -1 B

@timneutkens timneutkens requested review from mischnic and sokra January 12, 2026 15:00
@timneutkens timneutkens marked this pull request as ready for review January 12, 2026 15:00
@timneutkens
docs(AGENTS.md): add rule to not mark PRs as ready for review (#88420)
d216b96 
### What?

Added a rule to AGENTS.md instructing AI agents to not mark PRs as
"ready for review" using `gh pr ready`.

### Why?

AI agents should leave PRs in draft mode and let users decide when to
mark them ready. This ensures users maintain control over when their PRs
are published for review.

### How?

Added a single line to the "Commit and PR Style" section in AGENTS.md.
lukesandberg
lukesandberg reviewed Jan 12, 2026
View reviewed changes
@timneutkens timneutkens merged commit a93c03b into canary Jan 14, 2026
161 checks passed
@timneutkens timneutkens deleted the fix/route-handler-manifest-bloat branch January 14, 2026 10:19
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@mischnic mischnic mischnic approved these changes

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[Webpack] Route Handlers generate client-reference-manifest.js containing unrelated client components

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@timneutkens @nextjs-bot @lukesandberg @mischnic