fix: keep multi-compiler options when --config-name or extends rebuild the configuration array

[alexander-akait]

Summary

Options for the whole MultiCompiler are set on the exported configuration array itself: parallelism, and plugins since webpack/webpack#22702 (which lets a plugin such as webpack-dev-server be applied once to the MultiCompiler). loadConfig rebuilds the array for --config-name, --extends and configurations using extends, and the rebuilt array lost those properties, so webpack silently fell back to the defaults. They are now copied over to the rebuilt array.

// webpack.config.js
module.exports = Object.assign([first, second], { parallelism: 1 });

Command	Before	After
webpack	1 compiler at a time	1 compiler at a time
webpack --config-name first --config-name second	2 at once	1 at a time
webpack -c ./extends.config.js (children use extends)	2 at once	1 at a time
webpack --extends ./base.config.js	2 at once	1 at a time

Checked against webpack/webpack#22702 too: on all those paths, plus arrays of config functions and a promise, a top-level plugin is applied once to the MultiCompiler.

Not covered: several -c files are concatenated into one new array, and there is no obvious rule to merge their array options, so they are still not carried over.

What kind of change does this PR introduce?

fix

Did you add tests for your changes?

Yes: test/build/multi-compiler-options/multi-compiler-options.test.js. A plugin in each child counts how many compilers run at once; parallelism is used because no released webpack has top-level plugins yet, and both take the same path. Three of the four cases fail without the fix.

Does this PR introduce a breaking change?

No.

If relevant, what needs to be documented once your changes are merged or what have you already documented?

n/a

Use of AI

Written with Claude Code: it reproduced the dropped options with the real CLI, wrote the fix and the test, and checked it against the webpack branch, directed and reviewed by the author.

🤖 Generated with Claude Code

https://claude.ai/code/session_01ML8tLtqrJohECUzhdBwqsw

---

Generated by Claude Code

Summary by CodeRabbit

• Bug Fixes
  • Preserved multi-compiler options, including parallelism and plugins, when selecting named configurations or resolving extended configurations.

[changeset-bot]

🦋 Changeset detected

Latest commit: db8378d

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
webpack-cli	Patch

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[github-actions]

This PR is packaged and the instant preview is available (5d4e9e4).

Install it locally:

• npm

npm i -D webpack-cli@https://pkg.pr.new/webpack-cli@5d4e9e4

• yarn

yarn add -D webpack-cli@https://pkg.pr.new/webpack-cli@5d4e9e4

• pnpm

pnpm add -D webpack-cli@https://pkg.pr.new/webpack-cli@5d4e9e4

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 94.84%. Comparing base (430bdd6) to head (db8378d).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #4871      +/-   ##
==========================================
- Coverage   94.98%   94.84%   -0.14%     
==========================================
  Files          14       14              
  Lines        5598     5626      +28     
  Branches      840      841       +1     
==========================================
+ Hits         5317     5336      +19     
- Misses        280      289       +9     
  Partials        1        1              

Files with missing lines	Coverage Δ
packages/webpack-cli/src/webpack-cli.ts	96.73% <100.00%> (+0.02%)	⬆️

... and 2 files with indirect coverage changes

---

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[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

• Configuration used: Organization UI
• Review profile: CHILL
• Plan: Advanced
• Run ID: 91b7fe6b-91b4-4483-9f8b-110920b93726

📥 Commits

Reviewing files that changed from the base of the PR and between 430bdd6 and db8378d.

📒 Files selected for processing: 9

• .changeset/keep-multi-compiler-options.md
• packages/webpack-cli/src/webpack-cli.ts
• test/build/multi-compiler-options/base.config.js
• test/build/multi-compiler-options/concurrency-plugin.js
• test/build/multi-compiler-options/extends.config.js
• test/build/multi-compiler-options/multi-compiler-options.test.js
• test/build/multi-compiler-options/src/first.js
• test/build/multi-compiler-options/src/second.js
• test/build/multi-compiler-options/webpack.config.js

Included review availability: This review used your included allowance. Your plan provides up to 8 included reviews per hour; 7 remain after this review.

Walkthrough

loadConfig now preserves non-index properties when it rebuilds multi-compiler configuration arrays for named-configuration selection and extends. New integration tests check that the array-level parallelism option remains effective across default loading, repeated --config-name, configuration-level extends, and CLI --extends.

Priority: ➖ Normal

Merge Risk: ⚪ Minimal · up to db837

This patch preserves array-level options such as parallelism when webpack-cli rebuilds multi-compiler configurations. No concrete merge-blocking risk was identified.

Pre-merge checks: 5

Passed checks: 5 passed

Check name	Status	Explanation
Description Check		Check skipped - CodeRabbit’s high-level summary is enabled.
Title check		The title clearly and concisely describes the main change: preserving multi-compiler options when --config-name or extends rebuilds the configuration array.
Docstring Coverage		No functions found in the changed files to evaluate docstring coverage. Skipping docstring coverage check. Docstring coverage is scoped to functions touched by this diff. Analyzed 0 functions across 8…
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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