Core: Switch from mlly to exsolve

[mrginglymus]

Closes #32364

What I did

mlly resolve utils are deprecated
(unjs/mlly#324 (comment)), and it is recommended to use exsolve instead.

The old strip-abs-node-modules-path utility is reintroduced as there is no equivalent in exsolve, and the mlly implementation was more or less the same anyway (except using regex instead of split)

Checklist for Contributors

Testing

The changes in this PR are covered in the following automated tests:

• stories
• unit tests
• integration tests
• end-to-end tests

Manual testing

This section is mandatory for all contributions. If you believe no manual test is necessary, please state so explicitly. Thanks!

Documentation

• Add or update documentation reflecting your changes
• If you are deprecating/removing a feature, make sure to update
  MIGRATION.MD

Checklist for Maintainers

• When this PR is ready for testing, make sure to add ci:normal, ci:merged or ci:daily GH label to it to run a specific set of sandboxes. The particular set of sandboxes can be found in code/lib/cli-storybook/src/sandbox-templates.ts

• Make sure this PR contains one of the labels below:

  Available labels

  • bug: Internal changes that fixes incorrect behavior.
  • maintenance: User-facing maintenance tasks.
  • dependencies: Upgrading (sometimes downgrading) dependencies.
  • build: Internal-facing build tooling & test updates. Will not show up in release changelog.
  • cleanup: Minor cleanup style change. Will not show up in release changelog.
  • documentation: Documentation only changes. Will not show up in release changelog.
  • feature request: Introducing a new feature.
  • BREAKING CHANGE: Changes that break compatibility in some way with current major version.
  • other: Changes that don't fit in the above categories.

🦋 Canary release

This PR does not have a canary release associated. You can request a canary release of this pull request by mentioning the @storybookjs/core team here.

core team members can create a canary release here or locally with gh workflow run --repo storybookjs/storybook canary-release-pr.yml --field pr=<PR_NUMBER>

Greptile Summary

Updated On: 2025-09-03 09:05:50 UTC

This PR migrates Storybook from the deprecated mlly library to exsolve for module resolution utilities across the core package. The main changes include:

1. Dependency Migration: Removes mlly (^1.7.4) and adds exsolve (^1.0.7) in the core package.json
2. API Updates: Replaces resolvePathSync from mlly with resolveModulePath from exsolve in multiple files, with minor parameter changes (e.g., url becomes from)
3. Utility Reintroduction: Adds a new stripAbsNodeModulesPath utility function since exsolve doesn't provide an equivalent to mlly's parseNodeModulePath
4. Cross-Platform Compatibility: The new utility uses posix.normalize and slash for better Windows path handling

The changes affect module resolution in framework validation (validate-config.ts), manager building (builder-manager/index.ts), and preset loading (presets.ts). This migration addresses a critical Windows compatibility issue where mlly's path parsing was causing Storybook to fail on Windows systems. The exsolve library provides the same core functionality with better cross-platform support and is the officially recommended replacement by the mlly maintainers.

Confidence score: 3/5

• This PR addresses a legitimate deprecation issue but has some implementation concerns that could cause problems
• Score reflects the presence of unused imports and incomplete migration patterns that suggest hasty implementation
• Pay close attention to code/core/src/common/presets.ts which contains unused imports and potential logic issues

[greptile-apps]

_{5 files reviewed, 1 comment}

_{Edit Code Review Bot Settings | Greptile}

[nx-cloud]

View your CI Pipeline Execution ↗ for commit ea94248

Command	Status	Duration	Result
nx run-many -t build --parallel=3	✅ Succeeded	44s	View ↗

---

☁️ Nx Cloud last updated this comment at 2025-09-11 11:18:35 UTC

[pi0]

Perf/Stability: Please make sure configDir ends with a trailing slash to indicate it is a dir otherwise resolution goes to a slow path to stat the path.

[pi0]

mlly is not deprecated. Only resolution util, you can keep using mlly for parseNodeModulePath :)

[mrginglymus]

I switched it on a one-in-one-out basis - doesn't make a huge difference to end users as its bundled, but given that all this is doing is splitting by node_modules, it doesn't seem worth the extra dep.

The function is also defined in the resolve file of mlly, so might presumably get swept up in a future clean out of the deprecated resolve utils.

[storybook-app-bot]

Package Benchmarks

^{Commit: ea94248, ran on 11 September 2025 at 10:31:14 UTC}

The following packages have significant changes to their size or dependencies:

storybook

	Before	After	Difference
Dependency count	48	48	0
Self size	30.74 MB	30.41 MB	🎉 -332 KB 🎉
Dependency size	17.61 MB	17.61 MB	0 B
Bundle Size Analyzer	Link	Link

@storybook/cli

	Before	After	Difference
Dependency count	204	204	0
Self size	879 KB	879 KB	0 B
Dependency size	81.72 MB	81.38 MB	🎉 -332 KB 🎉
Bundle Size Analyzer	Link	Link

@storybook/codemod

	Before	After	Difference
Dependency count	173	173	0
Self size	35 KB	35 KB	0 B
Dependency size	76.79 MB	76.46 MB	🎉 -332 KB 🎉
Bundle Size Analyzer	Link	Link

create-storybook

	Before	After	Difference
Dependency count	49	49	0
Self size	1.52 MB	1.52 MB	0 B
Dependency size	48.35 MB	48.02 MB	🎉 -332 KB 🎉
Bundle Size Analyzer	node	node

[pi0]

Please check comments above 😄

[ndelangen]

@JReinhold do you have a preference regarding the above?