Improve webpack loader error messages and handling #89698

sokra · 2026-02-09T06:50:57Z

What?

This PR improves error reporting for webpack loaders by:

Preserving loader paths in error messages: Modified the webpack message formatter to rewrite "Module build failed" headers to preserve the loader path information while removing the verbose prefix. This makes errors more readable in both CLI output and error overlays.
Better error wrapping for non-Error throws: When loaders throw non-Error values (strings, objects, etc.), the error is now wrapped with loader path information in the stack trace, making it easier to identify which loader caused the error.
Catching deferred loader errors: Added handling for errors that occur after loader completion (e.g., unhandled Promise rejections, setTimeout throws) by:
- Adding an unhandledRejection handler in the IPC layer
- Delaying resolution in webpack-loaders transform to allow error handlers to fire before sending the 'end' message
Comprehensive test coverage: Added e2e tests covering various error scenarios (Error throws, string throws, Promise rejections, setTimeout errors) with verification of both CLI output and error overlay display.

Why?

Webpack loader errors were previously difficult to debug because:

The verbose "Module build failed" prefix obscured the actual error message
Non-Error throws lost their context and loader information
Deferred errors (Promise rejections, async throws) were not properly surfaced
Error overlays didn't clearly show which loader caused the problem

How?

format-webpack-messages.ts: Changed from filtering out loader headers to rewriting them, preserving the (from ./loaders/...) information
webpack-loaders.ts: Added error wrapping for non-Error values and deferred resolution to catch async errors
ipc/index.ts: Added unhandledRejection handler to surface Promise-related errors
Test suite: Added comprehensive e2e tests with multiple loader error scenarios

Fixes #

https://claude.ai/code/session_01XnHv9KVqYRDLDti1q6vni4

nextjs-bot · 2026-02-09T07:00:44Z

Failing test suites

Commit: 79ad382 | About building and testing Next.js

pnpm test-dev test/development/app-dir/server-component-next-dynamic-ssr-false/server-component-next-dynamic-ssr-false.test.ts (job)

app-dir - server-component-next-dynamic-ssr-false > should error when use dynamic ssr:false in server component (DD)

Expand output

● app-dir - server-component-next-dynamic-ssr-false › should error when use dynamic ssr:false in server component

expect(received).toMatchInlineSnapshot(snapshot)

Snapshot name: `app-dir - server-component-next-dynamic-ssr-false should error when use dynamic ssr:false in server component 2`

- Snapshot  - 2
+ Received  + 0

@@ -7,7 +7,5 @@
     :                       ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
   4 | 
   5 | export default function Page() {
   6 |   return <DynamicClient />
     `----
-
-   (from ./node_modules/next/dist/build/webpack/loaders/next-swc-loader.js)

  62 |         `"  x \`ssr: false\` is not allowed with \`next/dynamic\` in Server Components. Please move it into a Client Component."`
  63 |       )
> 64 |       expect(redbox.source).toMatchInlineSnapshot(`
     |                             ^
  65 |          "./app/page.js
  66 |          Error:   x \`ssr: false\` is not allowed with \`next/dynamic\` in Server Components. Please move it into a Client Component.
  67 |             ,-[3:1]

  at Object.toMatchInlineSnapshot (development/app-dir/server-component-next-dynamic-ssr-false/server-component-next-dynamic-ssr-false.test.ts:64:29)

pnpm test-dev test/e2e/app-dir/error-on-next-codemod-comment/error-on-next-codemod-comment.test.ts (job)

Invalid SCSS in _document > should show a build error (DD)

Expand output

● Invalid SCSS in _document › should show a build error

expect(received).toMatchInlineSnapshot(snapshot)

Snapshot name: `Invalid SCSS in _document should show a build error 1`

- Snapshot  - 1
+ Received  + 0

  ./styles.module.scss
  CSS cannot be imported within pages/_document.js. Please move global styles to pages/_app.js.
  Location: pages/_document.js
-   (from ./node_modules/next/dist/build/webpack/loaders/error-loader.js)

  50 |           `)
  51 |         } else {
> 52 |           expect(errorSource).toMatchInlineSnapshot(`
     |                               ^
  53 |            "./styles.module.scss
  54 |            CSS cannot be imported within pages/_document.js. Please move global styles to pages/_app.js.
  55 |            Location: pages/_document.js

  at Object.toMatchInlineSnapshot (e2e/app-dir/scss/invalid-module-document/invalid-module-document.test.ts:52:31)

pnpm test-dev test/e2e/app-dir/scss/invalid-global/invalid-global.test.ts (job)

Valid and Invalid Global CSS with Custom App > should show a build error (DD)

Expand output

● Valid and Invalid Global CSS with Custom App › should show a build error

expect(received).toMatchInlineSnapshot(snapshot)

Snapshot name: `Valid and Invalid Global CSS with Custom App should show a build error 1`

- Snapshot  - 1
+ Received  + 0

  ./styles/global.scss
  Global CSS cannot be imported from files other than your Custom <App>. Due to the Global nature of stylesheets, and to avoid conflicts, Please move all first-party global CSS imports to pages/_app.js. Or convert the import to Component-Level CSS (CSS Modules).
  Read more: https://nextjs.org/docs/messages/css-global
  Location: pages/index.js
-   (from ./node_modules/next/dist/build/webpack/loaders/error-loader.js)

  57 |         `)
  58 |       } else {
> 59 |         expect(errorSource).toMatchInlineSnapshot(`
     |                             ^
  60 |          "./styles/global.scss
  61 |          Global CSS cannot be imported from files other than your Custom <App>. Due to the Global nature of stylesheets, and to avoid conflicts, Please move all first-party global CSS imports to pages/_app.js. Or convert the import to Component-Level CSS (CSS Modules).
  62 |          Read more: https://nextjs.org/docs/messages/css-global

  at Object.toMatchInlineSnapshot (e2e/app-dir/scss/valid-and-invalid-global/valid-and-invalid-global.test.ts:59:29)

pnpm test-dev test/e2e/app-dir/use-cache-unknown-cache-kind/use-cache-unknown-cache-kind.test.ts (job)

use-cache-unknown-cache-kind > should show a build error (DD)

Expand output

● use-cache-unknown-cache-kind › should show a build error

expect(received).toMatchInlineSnapshot(snapshot)

Snapshot name: `use-cache-unknown-cache-kind should show a build error 2`

- Snapshot  - 1
+ Received  + 1

@@ -2,9 +2,9 @@
  Error:   x Unknown cache kind "custom". Please configure a cache handler for this kind in the `cacheHandlers` object in your Next.js config.

     ,-[1:1]
   1 | 'use cache: custom'
     : ^^^^^^^^^^^^^^^^^^^
-  2 |
+  2 | 
   3 | export default async function Page() {
   4 |   return <p>hello world</p>
     `----"

  157 |         `)
  158 |       } else {
> 159 |         expect(errorSource).toMatchInlineSnapshot(`
      |                             ^
  160 |          "./app/page.tsx
  161 |          Error:   x Unknown cache kind "custom". Please configure a cache handler for this kind in the \`cacheHandlers\` object in your Next.js config.
  162 | 

  at Object.toMatchInlineSnapshot (e2e/app-dir/use-cache-unknown-cache-kind/use-cache-unknown-cache-kind.test.ts:159:29)

pnpm test-dev test/e2e/app-dir/scss/invalid-module-document/invalid-module-document.test.ts (job)

Invalid SCSS in _document > should show a build error (DD)

Expand output

● Invalid SCSS in _document › should show a build error

expect(received).toMatchInlineSnapshot(snapshot)

Snapshot name: `Invalid SCSS in _document should show a build error 1`

- Snapshot  - 1
+ Received  + 0

  ./styles.module.scss
  CSS cannot be imported within pages/_document.js. Please move global styles to pages/_app.js.
  Location: pages/_document.js
-   (from ./node_modules/next/dist/build/webpack/loaders/error-loader.js)

  50 |           `)
  51 |         } else {
> 52 |           expect(errorSource).toMatchInlineSnapshot(`
     |                               ^
  53 |            "./styles.module.scss
  54 |            CSS cannot be imported within pages/_document.js. Please move global styles to pages/_app.js.
  55 |            Location: pages/_document.js

  at Object.toMatchInlineSnapshot (e2e/app-dir/scss/invalid-module-document/invalid-module-document.test.ts:52:31)

pnpm test-start test/e2e/app-dir/use-cache-without-experimental-flag/use-cache-without-experimental-flag.test.ts (job)

use-cache-without-experimental-flag > should fail the build with an error (DD)

Expand output

● use-cache-without-experimental-flag › should fail the build with an error

expect(received).toMatchInlineSnapshot(snapshot)

Snapshot name: `use-cache-without-experimental-flag should fail the build with an error 1`

- Snapshot  - 1
+ Received  + 0

@@ -12,10 +12,9 @@
   4 |   return <p>hello world</p>
     `----

  Import trace for requested module:
  ./app/page.tsx
-   (from ./node_modules/next/dist/build/webpack/loaders/next-swc-loader.js)


  > Build failed because of webpack errors
  "

  75 |         `)
  76 |       } else {
> 77 |         expect(buildOutput).toMatchInlineSnapshot(`
     |                             ^
  78 |          "
  79 |          ./app/page.tsx
  80 |          Error:   x To use "use cache", please enable the feature flag \`cacheComponents\` in your Next.js config.

  at Object.toMatchInlineSnapshot (e2e/app-dir/use-cache-without-experimental-flag/use-cache-without-experimental-flag.test.ts:77:29)

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

app dir - prefetching > should immediately render the loading state for a dynamic segment when fetched from higher up in the tree (DD)

Expand output

● app dir - prefetching › should immediately render the loading state for a dynamic segment when fetched from higher up in the tree

thrown: "Exceeded timeout of 120000 ms for a test.
Add a timeout value to this test to increase the timeout, if this is a long-running test. See https://jestjs.io/docs/api#testname-fn-timeout."

  302 |   })
  303 |
> 304 |   it('should immediately render the loading state for a dynamic segment when fetched from higher up in the tree', async () => {
      |   ^
  305 |     let act: ReturnType<typeof createRouterAct>
  306 |     const browser = await next.browser('/', {
  307 |       beforePageLoad(page) {

  at it (e2e/app-dir/app-prefetch/prefetching.test.ts:304:3)
  at Object.describe (e2e/app-dir/app-prefetch/prefetching.test.ts:11:1)

pnpm test-dev test/development/app-dir/ssr-in-rsc/ssr-in-rsc.test.ts (job)

react-dom/server in React Server environment > implicit react-dom/server.edge usage in app code (DD)

Expand output

● react-dom/server in React Server environment › implicit react-dom/server.edge usage in app code

expect(received).toMatchInlineSnapshot(snapshot)

Snapshot name: `react-dom/server in React Server environment implicit react-dom/server.edge usage in app code 1`

- Snapshot  - 1
+ Received  + 1

@@ -1,10 +1,10 @@
  {
    "description": "  x You're importing a component that imports react-dom/server. To fix it, render or return the content directly as a Server Component instead for perf and security.",
    "environmentLabel": null,
    "label": "Build Error",
-   "source": "<FIXME-nextjs-internal-source>
+   "source": "./node_modules/next/dist/build/webpack/loaders/next-app-loader/index.js?name=app%2Fexports%2Fapp-code%2Freact-dom-server-edge-implicit%2Fpage&page=%2Fexports%2Fapp-code%2Freact-dom-server-edge-implicit%2Fpage&appPaths=%2Fexports%2Fapp-code%2Freact-dom-server-edge-implicit%2Fpage&allNormalizedAppPaths=&pagePath=private-next-app-dir%2Fexports%2Fapp-code%2Freact-dom-server-edge-implicit%2Fpage.js&appDir=%2Ftmp%2Fnext-install-d0dbfdb3e407acb0fff083d784765273e7781766558a998d5e8eb9b544fcdc8f%2Fapp&pageExtensions=tsx&pageExtensions=ts&pageExtensions=jsx&pageExtensions=js&rootDir=%2Ftmp%2Fnext-install-d0dbfdb3e407acb0fff083d784765273e7781766558a998d5e8eb9b544fcdc8f&isDev=true&tsconfigPath=&basePath=&assetPrefix=&nextConfigOutput=&preferredRegion=&middlewareConfig=e30%3D&isGlobalNotFoundEnabled=!./app/exports/app-code/react-dom-server-edge-implicit/page.js?__next_edge_ssr_entry__
  Error:   x You're importing a component that imports react-dom/server. To fix it, render or return the content directly as a Server Component instead for perf and security.
    | Learn more: https://nextjs.org/docs/app/building-your-application/rendering
     ,-[1:1]
   1 | import * as ReactDOMServerEdge from 'react-dom/server'
     : ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^

  223 |       // Expected: `./app/exports/app-code/react-dom-server-edge-implicit/page.js`
  224 |       // Observed: `./node_modules/.pnpm/next@file+..+next-repo.../page.js?__next_edge_ssr_entry__
> 225 |       await expect(browser).toDisplayRedbox(`
      |                             ^
  226 |        {
  227 |          "description": "  x You're importing a component that imports react-dom/server. To fix it, render or return the content directly as a Server Component instead for perf and security.",
  228 |          "environmentLabel": null,

  at Object.toDisplayRedbox (development/app-dir/ssr-in-rsc/ssr-in-rsc.test.ts:225:29)

codspeed-hq · 2026-02-09T07:11:22Z

CodSpeed Performance Report

Merging this PR will not alter performance

_{Comparing claude/test-webpack-loader-errors-JQxSZ (79ad382) with canary (c76b0fe)}

Summary

✅ 17 untouched benchmarks
⏩ 3 skipped benchmarks¹

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 · 2026-02-09T07:13:27Z

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)	507ms	507ms	✓	▅█▅▁█
Cold (Ready in log)	484ms	484ms	✓	▇▇▇▆▇
Cold (First Request)	935ms	948ms	✓	▇██▇▇
Warm (Listen)	508ms	508ms	✓	▁█▁▁▁
Warm (Ready in log)	482ms	480ms	✓	▃▇▄▁▆
Warm (First Request)	367ms	365ms	✓	▂▂█▁▂

📦 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)	1.860s	1.846s	✓	▁▁▁█▁
Warm (Listen)	455ms	455ms	✓	▁▁▁█▁
Warm (Ready in log)	438ms	437ms	✓	▂▁▂█▁
Warm (First Request)	1.865s	1.867s	✓	▁▁▁█▁

⚡ Production Builds

Metric	Canary	PR	Change	Trend
Fresh Build	4.567s	4.595s	✓	▄▃▄▂▂
Cached Build	4.519s	4.567s	✓	▂▁▂▁▁

📦 Production Builds (Webpack) (Legacy)

📦 Production Builds (Webpack)

Metric	Canary	PR	Change	Trend
Fresh Build	13.887s	13.876s	✓	▁▁▁█▁
Cached Build	14.097s	14.077s	✓	▁▁▁█▁
node_modules Size	467 MB	467 MB	✓	▁▁▁▁▁

📦 Bundle Sizes

Bundle Sizes

⚡ Turbopack

Client

Main Bundles: **437 kB** → **437 kB** ⚠️ +11 B

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

Server

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	756 B	759 B	✓
Total	756 B	759 B	⚠️ +3 B

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	451 B	452 B	✓
Total	451 B	452 B	⚠️ +1 B

📦 Webpack

Client

Main Bundles

	Canary	PR	Change
5528-HASH.js gzip	5.47 kB	N/A	-
6280-HASH.js gzip	57 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	39.1 kB	39.1 kB	✓
webpack-HASH.js gzip	1.68 kB	1.68 kB	✓
262-HASH.js gzip	N/A	4.53 kB	-
2889.HASH.js gzip	N/A	169 B	-
5602-HASH.js gzip	N/A	5.49 kB	-
6948ada0-HASH.js gzip	N/A	62.5 kB	-
9544-HASH.js gzip	N/A	57.6 kB	-
Total	230 kB	231 kB	⚠️ +612 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	✓
Total	7.97 kB	7.97 kB	✅ -1 B

Server

Edge SSR

	Canary	PR	Change
edge-ssr.js gzip	126 kB	126 kB	✓
page.js gzip	249 kB	249 kB	✓
Total	375 kB	376 kB	⚠️ +446 B

Middleware

	Canary	PR	Change
middleware-b..fest.js gzip	614 B	615 B	✓
middleware-r..fest.js gzip	156 B	155 B	✓
middleware.js gzip	33.3 kB	33.2 kB	✓
edge-runtime..pack.js gzip	842 B	842 B	✓
Total	34.9 kB	34.8 kB	✅ -158 B

Build Details

Build Manifests

	Canary	PR	Change
_buildManifest.js gzip	733 B	735 B	✓
Total	733 B	735 B	⚠️ +2 B

Build Cache

	Canary	PR	Change
0.pack gzip	3.84 MB	3.85 MB	🔴 +8.53 kB (+0%)
index.pack gzip	103 kB	103 kB	✓
index.pack.old gzip	103 kB	103 kB	✓
Total	4.05 MB	4.05 MB	⚠️ +8.05 kB

🔄 Shared (bundler-independent)

Runtimes

	Canary	PR	Change
app-page-exp...dev.js gzip	315 kB	315 kB	✓
app-page-exp..prod.js gzip	167 kB	167 kB	✓
app-page-tur...dev.js gzip	315 kB	315 kB	✓
app-page-tur..prod.js gzip	167 kB	167 kB	✓
app-page-tur...dev.js gzip	312 kB	312 kB	✓
app-page-tur..prod.js gzip	166 kB	166 kB	✓
app-page.run...dev.js gzip	312 kB	312 kB	✓
app-page.run..prod.js gzip	166 kB	166 kB	✓
app-route-ex...dev.js gzip	70.5 kB	70.5 kB	✓
app-route-ex..prod.js gzip	49 kB	49 kB	✓
app-route-tu...dev.js gzip	70.5 kB	70.5 kB	✓
app-route-tu..prod.js gzip	49 kB	49 kB	✓
app-route-tu...dev.js gzip	70.1 kB	70.1 kB	✓
app-route-tu..prod.js gzip	48.8 kB	48.8 kB	✓
app-route.ru...dev.js gzip	70.1 kB	70.1 kB	✓
app-route.ru..prod.js gzip	48.7 kB	48.7 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	43.2 kB	43.2 kB	✓
pages-api-tu..prod.js gzip	32.9 kB	32.9 kB	✓
pages-api.ru...dev.js gzip	43.2 kB	43.2 kB	✓
pages-api.ru..prod.js gzip	32.8 kB	32.8 kB	✓
pages-turbo....dev.js gzip	52.5 kB	52.5 kB	✓
pages-turbo...prod.js gzip	39.4 kB	39.4 kB	✓
pages.runtim...dev.js gzip	52.5 kB	52.5 kB	✓
pages.runtim..prod.js gzip	39.4 kB	39.4 kB	✓
server.runti..prod.js gzip	62.7 kB	62.7 kB	✓
Total	2.8 MB	2.8 MB	⚠️ +342 B

📝 Changed Files (4 files)

Files with changes:

app-page-exp..ntime.dev.js
app-page-tur..ntime.dev.js
app-page-tur..ntime.dev.js
app-page.runtime.dev.js

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sokra · 2026-02-09T10:43:49Z

Improve webpack loader error messages and handling #89698 👈 (View in Graphite)
canary

This stack of pull requests is managed by Graphite. Learn more about stacking.

Test that errors thrown in webpack loaders surface the source file path and error message in the CLI output. Covers four cases: - Loader throws an Error object - Loader throws a plain string (NonErrorEmittedError) - Loader has an unhandled rejected Promise - Loader has a setTimeout that throws after completion https://claude.ai/code/session_01XnHv9KVqYRDLDti1q6vni4

Add turbopack.rules config mapping each .data file to its corresponding loader, and remove the IS_TURBOPACK_TEST skip so the tests run for both webpack and Turbopack. https://claude.ai/code/session_01XnHv9KVqYRDLDti1q6vni4

Turbopack handles unhandled rejections and setTimeout errors differently from webpack when running loaders: - Unhandled promise rejections surface later attributed to a different file - setTimeout errors thrown after loader completion are swallowed entirely https://claude.ai/code/session_01XnHv9KVqYRDLDti1q6vni4

- Add loaders/error-loader path assertion for Turbopack (which preserves loader paths in stack traces) - Use loaders/ prefix for promise-error and timeout-error assertions to verify the loader path is actually present, not just matching the name in the error message - Document that string-error case loses loader path in both modes: webpack strips it via format-webpack-messages, turbopack only shows internal frames for string throws - Document that webpack strips loader path for Error throws via format-webpack-messages filtering "Module build failed (from ...)" lines https://claude.ai/code/session_01XnHv9KVqYRDLDti1q6vni4

- Preserve loader paths in format-webpack-messages.ts instead of stripping "Module build failed (from ...)" lines entirely - Add unhandledRejection handler in turbopack IPC to catch Promise.reject errors from loaders - Wrap non-Error throws (strings) with synthetic stack containing loader paths in turbopack webpack-loaders.ts - Delay loader resolution by one event loop turn to catch deferred errors (setTimeout throws, unhandled rejections) before sending IPC end message - Migrate webpack-loader-errors tests from test/development/ to test/e2e/ with proper e2e patterns (isNextDev guard, skipDeployment, error overlay) https://claude.ai/code/session_01XnHv9KVqYRDLDti1q6vni4

Instead of placing the `(from ./loader.js)` line where the original "Module build failed" header was, extract it and append at the end. This avoids shifting line positions which broke sass error detection and snapshot tests.

…Turbopack loader errors - Add no-stack-error-loader (throws Error with stack=undefined) and fs-error-loader (throws ENOENT via fs.readFileSync) test cases - Turbopack webpack-loaders.ts: append "(from <loader-path>)" to all error messages (both Error instances and string throws), matching webpack's format-webpack-messages style - Restructure tests: CLI tests for edge cases are Turbopack-only since webpack only logs errors[0] per compilation; overlay tests use generic assertions since build errors accumulate globally

Webpack error messages now include `(from .../loader.js)` attribution lines with dynamic pnpm content-addressable store hashes. Normalize these paths in test assertions and update inline snapshots to include the attribution lines with clean paths.

Filter out `(from ...)` loader attribution when the loader path is inside next/dist/. Internal loaders like next-swc-loader and sass-loader are noise for users. Third-party loader paths are still shown. Revert snapshot additions that included internal loader paths.

nextjs-bot added created-by: Turbopack team PRs by the Turbopack team. tests Turbopack Related to Turbopack with Next.js. type: next labels Feb 9, 2026

sokra force-pushed the claude/test-webpack-loader-errors-JQxSZ branch from 7bc5926 to 93cd56a Compare February 9, 2026 10:43

sokra force-pushed the claude/test-webpack-loader-errors-JQxSZ branch from 93cd56a to 2bdeafb Compare February 9, 2026 12:34

claude and others added 9 commits February 10, 2026 11:09

Enable webpack loader error tests for Turbopack

c77f095

Add turbopack.rules config mapping each .data file to its corresponding loader, and remove the IS_TURBOPACK_TEST skip so the tests run for both webpack and Turbopack. https://claude.ai/code/session_01XnHv9KVqYRDLDti1q6vni4

Move loader path to end of formatted webpack error message

b832e61

Instead of placing the `(from ./loader.js)` line where the original "Module build failed" header was, extract it and append at the end. This avoids shifting line positions which broke sass error detection and snapshot tests.

sokra force-pushed the claude/test-webpack-loader-errors-JQxSZ branch from 2bdeafb to 79ad382 Compare February 10, 2026 10:09

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Improve webpack loader error messages and handling #89698

Improve webpack loader error messages and handling #89698

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Improve webpack loader error messages and handling #89698

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