// Guide for creating and writing proper changesets for Biome PRs. Use when a PR introduces user-visible changes (bug fixes, new features, rule changes, formatter changes, parser changes) that need a changeset entry for the CHANGELOG. Trigger when creating changesets, writing changeset descriptions, or choosing the correct change type.
Guide for testing workflows and code generation commands in Biome. Use when running snapshot tests for lint rules, managing insta snapshots, or regenerating analyzer/parser/formatter code after changes.
Compares code formatting and formatting IR between Biome and Prettier to ensure that Biome's formatting is consistent and correct. Use when validating formatter changes, debugging formatting differences, or comparing output for JavaScript, TypeScript, CSS, or other languages.
Guide for implementing parsers with error recovery for new languages in Biome. Use when adding parsing support for a new language, implementing error recovery in a parser, or writing grammar definitions in .ungram format for JavaScript, CSS, JSON, HTML, GraphQL, or other languages.
General development best practices and common gotchas when working on Biome. Use for avoiding common mistakes, understanding Biome-specific patterns (AST, syntax nodes, string extraction, embedded languages), and learning technical tips.
Step-by-step guide for creating and implementing lint rules in Biome's analyzer. Use when implementing rules like noVar, useConst, or any custom lint/assist rule, adding code actions to fix diagnostics, implementing semantic analysis for binding references, or adding configurable options to rules.
Guide for implementing ESLint-to-Biome rule option migrators inside `biome migrate eslint`. Use whenever you add or update a Biome lint rule that has an ESLint source rule with configurable options, need to deserialize plugin-specific ESLint options, or need custom migration logic beyond the auto-generated severity mapping.
| name | changeset |
| description | Guide for creating and writing proper changesets for Biome PRs. Use when a PR introduces user-visible changes (bug fixes, new features, rule changes, formatter changes, parser changes) that need a changeset entry for the CHANGELOG. Trigger when creating changesets, writing changeset descriptions, or choosing the correct change type. |
| compatibility | Designed for coding agents working on the Biome codebase (github.com/biomejs/biome). |
Use this skill when a PR introduces user-facing changes that require a changeset. Changesets drive CHANGELOG generation and release automation. Internal-only changes (refactors with no user-visible effect) do not need changesets.
Do not create changeset files manually. Use:
just new-changeset-empty
The command will create a file in .changeset/. Edit it directly to add detail.
Note that the file will not be literally empty. It will have this content:
---
---
Do not simply append to the changeset description below the existing content, which creates an invalid changeset.
Requires
pnpm— runpnpm ifrom repo root first.
patch — Bug fixes.minor — New features. PR must target the next branch.major — Breaking user API changes. PR must target the next branch. These are rare and strictly controlled.Refer to the versioning page when unsure.
---
"@biomejs/biome": patch
---
Description here.
If you need headers inside the description, use #### or ##### only. Other header levels break the CHANGELOG tooling.
.).Start with a link to the issue:
Fixed [#4444](https://github.com/biomejs/biome/issues/4444): [`useOptionalChain`](https://biomejs.dev/linter/rules/use-optional-chain/) now detects negated logical OR chains.
Show an example of an invalid case. Use inline code for simple things, a code block for complex ones:
Added a new nursery rule [`noDuplicateSelectors`](https://biomejs.dev/linter/rules/no-duplicate-selectors/), that disallows duplicate selector lists within the same at-rule context.
For example, the following snippet triggers the rule:
` ` `css
.foo {}
.foo {}
` ` `
Clearly show what is now invalid that was not before (or vice versa). Show both sides if helpful:
Fixed [#7211](https://github.com/biomejs/biome/issues/7211): [`useOptionalChain`](https://biomejs.dev/linter/rules/use-optional-chain/) now detects negated logical OR chains. The following code is now considered invalid:
` ` `js
!foo || !foo.bar
` ` `
Show the formatting diff:
Changed formatting of arrow function parameters. Example:
` ` `diff
- const fn = ( a, b ) => {};
+ const fn = (a, b) => {};
` ` `
Brief inline example of what can now be parsed:
Added support for parsing `using` declarations in JavaScript.
Use a code block if multiline clarity helps.
Always link to the website, even if the page does not exist yet (it will after merge):
[`useConst`](https://biomejs.dev/linter/rules/use-const/)[`organizeImports`](https://biomejs.dev/assist/actions/organize-imports/).changeset/ or recent CHANGELOG.md entries for reference.CONTRIBUTING.md section "Changelog"