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Official agent skill

Testing Codegen

by biomejs in biomejs/biome

A skill your agent uses when selecting or running Biome test fixtures, quick tests, insta snapshot workflows, expectation comments, orphan checks, or required code generators.

OfficialApache-2.0Auto-check passedDevelopment

Install Testing Codegen

skills CLI
$ npx skills add biomejs/biome --skill testing-codegen -a claude-code

Project install by default; add -g for ~/.claude/skills/.

GitHub CLI
$ gh skill install biomejs/biome testing-codegen --agent claude-code

Project scope by default; add --scope user for a personal install. Needs GitHub CLI 2.90.0 or later (public preview).

Manual copy
$ git clone --depth 1 https://github.com/biomejs/biome.git skills-src && mkdir -p .claude/skills && cp -r skills-src/.agents/skills/testing-codegen .claude/skills/testing-codegen && rm -rf skills-src

Use ~/.claude/skills/ instead of .claude/skills for a personal install. The folder must contain SKILL.md.

Claude Code skills documentation · loads skills from .claude/skills/

Facts

Skill name
testing-codegen
GitHub stars
26k
Token cost
~1.8k tokens
SKILL.md length
862 words
Files
1
Skills in repo
12
Repo updated
First seen
Licence
Apache-2.0

At a glance

A skill your agent uses when selecting or running Biome test fixtures, quick tests, insta snapshot workflows, expectation comments, orphan checks, or required code generators.

  • Running Biome test fixtures
  • SKILL.md covers Test Selection, Snapshot Workflow, Biome CLI Behavior Tests and Analyzer Fixtures, plus 5 more sections
  • Calls just and cargo; reaches github.com
  • Insta snapshot workflows

What it does

Testing Codegen is an agent skill from biomejs/biome, published by the product's own GitHub organization. Use this skill when selecting or running Biome test fixtures, quick tests, insta snapshot workflows, expectation comments, orphan checks, or required code generators. Do not use for subsystem implementation design.

Its SKILL.md is about 1.8k tokens, which your agent loads only when the skill is triggered. It is a single SKILL.md file with no bundled scripts. Compatibility notes: Designed for coding agents working on the Biome codebase (github.com/biomejs/biome).

It sits in Development, covering Linting and formatting, Project scaffolding and Test data and fixtures. The repository describes itself as: A toolchain for web projects, aimed to provide functionalities to maintain them. Biome offers formatter and linter, usable via CLI and LSP. The licence is Apache-2.0.

When your agent uses it

  • Running Biome test fixtures
  • Insta snapshot workflows
  • Expectation comments
  • Required code generators

Example prompts

  • “/testing-codegen”

Requirements

  • Compatibility (from SKILL.md): Designed for coding agents working on the Biome codebase (github.com/biomejs/biome).

What it can do on your machine

Read from SKILL.md and the folder at commit c870caa. It shows what the files ask for, not the result of running them.

  • Tool permissions

    Pre-approves nothing: there is no allowed-tools line, so your agent's usual permission prompts apply.

    From allowed-tools in the SKILL.md frontmatter.

  • Runs code

    Shell commands in SKILL.md call:

    • just
    • cargo

    From the folder's file list and the shell code blocks in SKILL.md.

  • Network

    Hosts in commands or code, which the agent is likely to contact:

    • github.com

    From URLs in SKILL.md, links to its own repository left out.

  • Credentials

    Names no API keys, tokens, secrets or passwords.

    From names ending in _API_KEY, _TOKEN, _SECRET, _KEY or _PASSWORD in SKILL.md.

  • Compatibility

    Designed for coding agents working on the Biome codebase (github.com/biomejs/biome).

    From compatibility in the SKILL.md frontmatter.

Context cost

Testing Codegen loads about 1.8k tokens when it runs. Until then it costs about 58 tokens; SKILL.md has 862 words of instructions outside code blocks.

Always · name and description, kept in context so the agent knows when to use it
~58
When it runs · the whole SKILL.md, loaded when a task matches
~1.8k

Estimates: characters ÷ 4, the usual rule of thumb; real counts depend on the model's tokenizer. Scripts and assets cost tokens only if the agent reads them.

Safety

Auto-check passed

The automated check found no risky patterns in SKILL.md.

Automated static check — not a guarantee. Review scripts before installing. It scans the text of SKILL.md for risky patterns (piping downloads into a shell, reading credential files, hidden Unicode, destructive commands); files beside SKILL.md are not scanned.

SKILL.md

The full file from biomejs/biome at commit c870caa, republished under its Apache-2.0 licence (© biomejs). 862 words, ~1,793 tokens.

Download SKILL.mdSave it as .claude/skills/testing-codegen/SKILL.md (or your agent's skills folder).
name
testing-codegen
description
Use this skill when selecting or running Biome test fixtures, quick tests, `insta` snapshot workflows, expectation comments, orphan checks, or required code generators. Do not use for subsystem implementation design.
compatibility
Designed for coding agents working on the Biome codebase (github.com/biomejs/biome).

Testing and Code Generation

Choose the narrowest test that exercises the changed behavior, then broaden only when shared infrastructure or integration risk justifies it.

Test Selection

ChangeStart with
Lint rulejust test-lintrule <ruleName>
One cratecargo test -p <crate> or the crate's focused test target
Parser or formatter investigationjust qt <package>
CLI migrationfocused biome_cli migration tests
Documentation codejust test-doc

Use -- --show-output or --nocapture only when the test's diagnostic output is needed.

Quick tests are scratch space for inspecting CST, formatter IR, or one analyzer query. Persistent behavior belongs in the subsystem's normal fixture directory before finishing.

Snapshot Workflow

Run the focused test to create pending snapshots. Treat unrelated test failures separately. Never update, add or delete snapshots manually. Let insta do that.

Agents must not run unfiltered cargo insta review; it needs a TTY. In a non-TTY agent shell, cargo-insta 1.44+ prints a filtered --snapshot diff without prompting:

shell
cargo insta pending-snapshots --workspace
cargo insta review --workspace --snapshot '<path-from-list>'

After inspecting the full diff, accept or reject only that snapshot:

shell
cargo insta accept --workspace --snapshot '<same-path>'
cargo insta reject --workspace --snapshot '<same-path>'

Repeat for each change, list pending snapshots again, then rerun the test. Review means checking behavior in the diff. A passing test proves only that output matches the snapshot.

Never use unfiltered cargo insta accept or cargo insta reject, cargo insta test --accept, cargo insta test --force-update-snapshots, or INSTA_UPDATE=always/force. They skip individual review and can modify unrelated work.

Orphaned Snapshots

Do not delete suspected orphan snapshots manually. Deletion is safe only after running the complete workspace snapshot suite without package, target, or test filters:

shell
cargo insta test --workspace --unreferenced delete

For a scoped run, use --unreferenced warn or --unreferenced reject; incomplete test selection cannot prove that a snapshot is orphaned. Inspect every deletion from a complete run.

Biome CLI Behavior Tests

Every new or changed test that calls run_cli or a variant must snapshot the full session. Use assert_cli_snapshot, assert_cli_snapshot_with_redactor, or an insta-based wrapper. Scalar assertions do not replace the snapshot. Unit and helper tests that do not execute the CLI are exempt.

Snapshot after other assertions to capture the final filesystem, configuration, output, and result. Stabilize ordering or redact unstable values; do not omit the snapshot.

Keep related cases with the same arguments, configuration, source kind, and filesystem state in one fixture and usually one snapshot. Include valid, invalid, and boundary cases together. Keep related multi-file cases in one test. Split only for incompatible inputs or test setup.

For one migration rule, keep compatible cases in one JSONC fixture. Prefer overrides when they preserve the starting configuration.

Analyzer Fixtures

Place rule fixtures under the language analyzer's current tests/specs/<group>/<rule>/ hierarchy. The directory group must match the rule declaration.

Use focused source files for parser-dependent cases. Use .jsonc arrays when multiple independent script snippets share the same configuration and module semantics are not required. Use options.json in a subdirectory when tests need different rule configuration.

Show full SKILL.md (401 more words)Show less
Diagnostic Expectation Comments

The test utilities recognize these marker texts in source comments:

text
should generate diagnostics
should not generate diagnostics

Current enforcement:

  • a filename containing valid but not invalid must contain one of the expectation markers;
  • when either marker is present, actual diagnostics must match it;
  • an invalid or neutral filename without a marker is accepted, but adding the correct marker makes intent explicit;
  • .snap, .json, .jsonc, and .md are exempt from the mandatory valid-file marker check;
  • HTML-family workspace fixtures are checked from raw file content and should use a top-level HTML comment.

Put the marker at the top of a primary fixture. Sidecar files with neutral names do not need a marker unless they are independently analyzed as cases.

Parser Fixtures

Use the parser crate's established ok/ and error/ directories. A recovery regression should include valid syntax after the malformed construct to prove the parser resumes at the intended boundary.

Use just qt <parser-package> to inspect a CST during development; do not leave the quick test as the only regression coverage.

Formatter Fixtures

Use internal specs for behavior introduced or fixed by the change. External Prettier snapshots record comparison results but do not replace focused internal coverage.

The formatter harness performs its idempotency reformat during one test invocation for eligible files. Inspect both formatted output and any IR shown for a mismatch.

Required Code Generation

Changed sourceCommand
.ungram grammarjust gen-grammar <lang>
Formatter sourcejust gen-formatter <lang>
Lint rule or assistjust gen-rules and just gen-configuration
Bindings needed locallyjust gen-bindings

Root AGENTS.md is canonical for which artifacts must be committed and which full outputs CI Autofix may provide.

Do not run just ready in a dirty working tree: the recipe checks for a clean diff before and after its full verification sequence. Use the focused commands required by the current task, then just f and just l.

Completion Checklist

  • A code change has focused persistent coverage.
  • A bug fixture fails without the fix.
  • Every changed snapshot was individually diffed before a filtered accept or reject.
  • Every new or modified CLI behavior test creates a session snapshot.
  • Related CLI cases are consolidated instead of fragmented across tiny snapshots.
  • Expectation comments match fixture intent.
  • Orphan snapshots were pruned through insta.
  • Required generated artifacts are present.
  • Narrow tests pass before broader checks run.

References

  • Main test guide: CONTRIBUTING.md#testing
  • Insta non-interactive review: https://github.com/mitsuhiko/insta/blob/1.48.0/CHANGELOG.md#1440
  • Analyzer guide: crates/biome_analyze/CONTRIBUTING.md
  • CLI snapshot harness: crates/biome_cli/tests/snap_test.rs
  • Expectation enforcement: crates/biome_test_utils/src/lib.rs
  • Formatter harness: crates/biome_formatter_test/src/spec.rs
  • Generator recipes: justfile

© biomejs, Apache-2.0. Rendered from Markdown: HTML in the file is shown as text, images as links, and headings moved down two levels. Raw file

Files

Just SKILL.md in .agents/skills/testing-codegen of biomejs/biome.

Open the folder on GitHubat commit c870caa

Compare with similar skills

Testing Codegen next to the 5 skills that share the most tags, products or categories with it. Stars are the repository's; “used in” counts other GitHub owners with a copy.

Testing Codegen compared with similar skills
SkillStarsUsed inTokensAuto-checkLicenceRepo updated
Testing Codegen this skillbiomejs/biome26k—~1.8kAutomated safety check: PassApache-2.0
Unslop CodeJCarterJohnson/vibecoded-design-tells511—~2.5kAutomated safety check: PassCustom licence
Rust Hygiene Audittsz-org/tsz577—~1.5kAutomated safety check: PassApache-2.0
Qwen Agentthananon/9arm-skills3.2k—~1.5kAutomated safety check: PassNone
Create Saleor Packagesaleor/apps162—~608Automated safety check: PassCustom licence
Deno Runtime and Package Managerdenoland/skills100—~2.7kAutomated safety check: PassMIT

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Categories

Questions about Testing Codegen

What does Testing Codegen do?

A skill your agent uses when selecting or running Biome test fixtures, quick tests, insta snapshot workflows, expectation comments, orphan checks, or required code generators. Testing Codegen is an agent skill from biomejs/biome, published by the product's own GitHub organization. Use this skill when selecting or running Biome test fixtures, quick tests, insta snapshot workflows, expectation comments, orphan checks, or required code generators.

When should I use Testing Codegen?

Testing Codegen fits situations like: running Biome test fixtures; insta snapshot workflows; expectation comments; required code generators.

How do I install Testing Codegen in Claude Code?

Run `npx skills add biomejs/biome --skill testing-codegen -a claude-code`. Or copy the skill folder (.agents/skills/testing-codegen in biomejs/biome) into .claude/skills/testing-codegen in your project. Claude Code loads it when a task matches its description.

How do I install Testing Codegen in Codex?

Run `npx skills add biomejs/biome --skill testing-codegen -a codex`. Or copy the skill folder (.agents/skills/testing-codegen in biomejs/biome) into .agents/skills/testing-codegen in your project. Codex loads it when a task matches its description.

Can I use Testing Codegen in Cursor, Gemini CLI or GitHub Copilot?

Cursor, Gemini CLI, GitHub Copilot and OpenCode also load SKILL.md folders. With the skills CLI, run `npx skills add biomejs/biome --skill testing-codegen -a cursor` (or -a gemini-cli, github-copilot or opencode for the others). To copy it by hand, put the folder in .cursor/skills/testing-codegen, .gemini/skills/testing-codegen, .github/skills/testing-codegen and .opencode/skills/testing-codegen in your project.

What does Testing Codegen need to run?

Going by SKILL.md and its folder, Testing Codegen needs the command-line tools its instructions call (just and cargo). Compatibility (from SKILL.md): Designed for coding agents working on the Biome codebase (github.com/biomejs/biome)..

Does Testing Codegen access the network?

SKILL.md names 1 domain. In commands or code: github.com; the agent is likely to contact it when it follows the instructions. This is read from the text; nothing was executed.

Is Testing Codegen safe to install?

Our automated static check of SKILL.md found no risky patterns, such as piping downloads into a shell, reading credential files or hidden Unicode. It is not a guarantee. Review the folder before installing.

What licence does Testing Codegen use?

Testing Codegen is published under the Apache-2.0 licence (the repository's licence). It allows redistribution, so the full SKILL.md is shown on this page.

How many tokens does Testing Codegen use?

About 1.8k tokens (SKILL.md is roughly 7.2k characters). Agents keep only the skill's name and description in context until a task matches; then they load SKILL.md in full.

What are the alternatives to Testing Codegen?

Skills that share tags, products or a category with Testing Codegen: Unslop Code (JCarterJohnson/vibecoded-design-tells, 511 stars), Rust Hygiene Audit (tsz-org/tsz, 577 stars), Qwen Agent (thananon/9arm-skills, 3.2k stars) and Create Saleor Package (saleor/apps, 162 stars). The comparison table on this page puts their stars, adoption, token cost, safety result and licence side by side.

Who maintains Testing Codegen?

biomejs (a GitHub organization, an official publisher) maintains it in biomejs/biome, which has 25,910 GitHub stars. The repository holds 12 skills in this directory. The repository was last updated on October 8, 2026.

Source: biomejs/biome on GitHub. Facts on this page come from the repository at the commit we read; the author's words are quoted as theirs.