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Start with the right debug command
Use the command that matches your shell and the test mode you are running.
macOS and Linux
DEBUG=cypress:server:preprocessor,cypress:webpack,cypress:webpack:stats npx cypress runTo focus only on Webpack bundle output:
DEBUG=cypress:webpack:stats npx cypress runFor broader preprocessor messages without bundle statistics:
DEBUG=cypress:webpack npx cypress runCypress accepts comma-separated debug namespaces, so combining them gives you the broadest view of a default Webpack-preprocessed end-to-end run.
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Windows Command Prompt
set DEBUG=cypress:server:preprocessor,cypress:webpack,cypress:webpack:stats&& npx cypress runWindows PowerShell
$env:DEBUG="cypress:server:preprocessor,cypress:webpack,cypress:webpack:stats"; npx cypress runRun the command from the project directory so Cypress uses the same configuration, dependencies, and lockfile as your normal test command. Save the terminal output to a file when a CI log is difficult to search:
DEBUG=cypress:server:preprocessor,cypress:webpack,cypress:webpack:stats npx cypress run 2>cypress-debug.log | tee cypress-output.logWhat each namespace reveals
| Namespace | Scope | Useful output |
|---|---|---|
cypress:webpack:stats |
Webpack compilation statistics from @cypress/webpack-preprocessor |
Bundle timings, chunks, and sizes |
cypress:webpack |
Preprocessor-level Webpack diagnostics | Broader module and compilation messages |
cypress:server:preprocessor |
Cypress’s file-preprocessing lifecycle | Which files are sent for preprocessing and when that work starts or finishes |
Statistics are not a substitute for the first actual error. A large chunk or a long timing can explain performance, but it does not identify a missing import by itself. Read the first meaningful error naming a file, loader, module, or source position; later messages are often consequences.
Identify which build is failing
The message “We found an error preparing your test file” means Cypress could not compile or bundle a spec or support file. Typical causes documented by Cypress include a missing file, invalid syntax in the file or one of its dependencies, and a missing dependency.
End-to-end spec or support preprocessing
In end-to-end mode, Cypress packages a default Webpack preprocessor when you have not supplied a custom file:preprocessor. That path handles common TypeScript and JSX cases through its bundled configuration. The debug namespaces above are intended for this process.
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Component testing
Component tests are compiled by the configured dev server. Depending on your project, that server is Vite or Webpack. A component-test alias or loader must therefore be fixed in that dev-server configuration, not assumed to be controlled by the default end-to-end preprocessor.
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An application build outside Cypress
If your test starts a separately managed application build, its compiler output comes from that build command. Cypress’s preprocessor debug variables will not make an unrelated application bundler verbose. Run the application’s own build or development command with its supported diagnostics as well.
A reliable diagnostic sequence
- Reproduce one failing file. Run the smallest spec or test group that still fails. This reduces unrelated bundle statistics and makes the first error easier to locate.
- Turn on all three namespaces. Start with
cypress:server:preprocessor,cypress:webpack,cypress:webpack:stats. If the output is too large, remove namespaces one at a time, retaining the one that shows the missing module or loader. - Locate the first root error. Note the exact path and imported module. Check that the file exists with the same capitalization used by the import, that the package is installed, and that the syntax is supported by the active loader.
- Inspect imported files. The reported spec can be valid while an imported helper, fixture, TypeScript file, or package contains the syntax error.
- Confirm the active preprocessor. Review
setupNodeEventsfor a customon('file:preprocessor', ...)registration. A custom implementation may use different debug namespaces and options. - Check aliases. Test the exact import path from the error, not merely a relative equivalent. Configure aliases in Webpack when needed.
- Enable source maps. Add inline source maps so Cypress can show source-level locations and code frames.
- Re-run without unrelated changes. Once the first error is fixed, run the same command again; a second error may have been hidden behind the first compilation failure.
Fix path aliases explicitly
The default Webpack preprocessor does not automatically inherit compilerOptions.paths from tsconfig.json or _moduleAliases from package.json. An import such as @/support/commands can therefore fail even though an editor resolves it.
Webpack alias configuration
Register a custom preprocessor and add the alias to Webpack’s resolve.alias. A typical shape is:
const webpack = require('@cypress/webpack-preprocessor')
const path = require('path')
module.exports = {
e2e: {
setupNodeEvents(on) {
on('file:preprocessor', webpack({
webpackOptions: {
resolve: {
alias: {
'@': path.resolve(__dirname, 'src')
}
}
}
}))
}
}
}Merge this with your existing loader and resolve settings rather than replacing them blindly. The directory must match the alias target used by your project.
Using TypeScript path mappings
When your project maintains aliases in tsconfig.json, use a Webpack path-resolution plugin such as tsconfig-paths-webpack-plugin where appropriate, or duplicate the required mappings in resolve.alias. The important point is that the bundler, not only the TypeScript editor, must know the mapping.
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Make source locations readable with inline maps
Webpack compilation statistics and source maps solve different problems. Statistics show bundle-level information; source maps let Cypress map generated code back to the original TypeScript, JavaScript, or JSX and display code frames. For the Webpack preprocessor, Cypress documents:
module.exports = {
devtool: 'inline-source-map'
}Place this in the Webpack options used by the Cypress preprocessor. Without inline source maps, Cypress says code frames will not appear. Source maps can make a stack trace actionable, but they do not add missing-module details or replace the debug namespaces.
Common errors and targeted fixes
“Module not found: Can’t resolve …”
Verify the import spelling and case, confirm the target exists, and install the named dependency in the package that runs Cypress. If the path uses an alias, configure that alias in the active bundler.
“You may need an appropriate loader”
The file extension is reaching Webpack without a matching loader. Check whether the failing file is TypeScript, JSX, CSS, or another asset type, then add or restore the corresponding rule in the custom preprocessor configuration. Do not add a loader to a component dev server configuration when the error came from end-to-end preprocessing.
Unexpected token or syntax error
Read the file and line named in the first error, then inspect its imports. A dependency may use syntax your current loader or target does not transform. Confirm that the file is included by the intended rule and that the syntax is valid for the configured parser.
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Alias works in the editor but fails in Cypress
Editor resolution is independent of Webpack resolution. Add resolve.alias or a suitable TypeScript-path plugin to the Cypress bundler, then rerun with DEBUG=cypress:webpack to verify the custom preprocessor is active.
No Webpack debug output appears
The failing process may not be @cypress/webpack-preprocessor, the variable may not have been exported in the shell, or a wrapper command may be clearing the environment. Print the variable in the same shell, run npx cypress run directly, and check whether component testing uses Vite or another dev server.
The log is huge or slow
Run one spec, start with cypress:webpack:stats, and add the other namespaces only when needed. Statistics can expose expensive bundles; they do not require you to keep verbose logging enabled in every CI run.
Custom preprocessor configuration checklist
- Register the package through
on('file:preprocessor', ...)insetupNodeEvents. - Pass Webpack options through the preprocessor configuration rather than editing files Cypress does not use.
- Keep the loader rules required by your specs and support files.
- Define aliases in Webpack or connect Webpack to your TypeScript path mappings.
- Set
devtool: 'inline-source-map'when code frames are needed. - Retest both a minimal spec and a representative spec after changing the configuration.
Performance and CI practice
Use verbose namespaces interactively or for a single failing CI job. Keep normal runs quieter, but preserve the command in your team troubleshooting notes. Bundle statistics can reveal whether a failure is accompanied by unusually long compilation or oversized chunks; they are diagnostic context, not a pass/fail threshold.
For reproducibility, collect the Cypress mode, active preprocessor, Node package lock state, exact failing file, first error, and relevant alias or loader configuration. This prevents a generic “Webpack failed” report from losing the details needed to fix it.
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Frequently asked questions
Do debug statistics fix a compilation error?
No. They expose evidence about the compilation; you still correct the missing file, dependency, syntax, alias, or loader that caused the failure.
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No. Enable the combined command while diagnosing, then narrow logging for routine runs and CI output.
Why do I see a source line but no code frame?
Configure Webpack with devtool: 'inline-source-map' in the preprocessor options used by Cypress.
Are component-test errors covered by these variables?
Only if the component setup actually uses the Webpack preprocessor. Otherwise inspect the configured Vite or Webpack dev-server diagnostics.
Frequently Asked Questions
Can I combine Cypress debug namespaces?
Yes. Separate namespaces with commas, for example DEBUG=cypress:server:preprocessor,cypress:webpack,cypress:webpack:stats.
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What should I capture for a bug report?
Include the Cypress mode, active preprocessor, exact command, first compilation error, named file or module, and the relevant alias or loader configuration.
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