Biome is a web-development toolchain that formats and checks source code for languages such as JavaScript, TypeScript, JSX, and CSS. Developers use its command-line and language-server interfaces to maintain code quality in web projects. Catalogue add-ons extend workflows around the toolchain.
Borrowing it
Nothing to install: this file belongs to biomejs/biome. Take a copy, put it at the same path in your own repository, and replace the rules that are about this project with yours.
curl -O https://raw.githubusercontent.com/biomejs/biome/main/.agents/skills/testing-codegen/SKILL.mdgit clone --depth 1 https://github.com/biomejs/biomeWrote this? Show the measurements
A badge with what this costs and how it scanned, read live from this page, so it follows the numbers instead of freezing them. Markdown for a README, HTML for a documentation site or a project page.
[](https://agentmods.dev/skills/biomejs/biome/testing-codegen)<a href="https://agentmods.dev/skills/biomejs/biome/testing-codegen"><img src="https://agentmods.dev/badge/skills/biomejs/biome/testing-codegen/github.svg" alt="Measured on agentmods" height="20"></a>Or the 80×15 button, for a site that already has a row of RSS and ATOM ones. Only the verdict fits; the numbers stay here.
<a href="https://agentmods.dev/skills/biomejs/biome/testing-codegen"><img src="https://agentmods.dev/badge/skills/biomejs/biome/testing-codegen.svg" alt="Reviewed on agentmods" width="80" height="20"></a>- Snyk pass
- NVIDIA SkillSpector pass
What it costs to keep this loaded
Counted locally with the o200k_base tokenizer, which is exact for GPT models; Claude uses its own tokenizer and its counts differ. Treat this as one consistent yardstick across the catalogue rather than a bill. Prices are per million input tokens.
| Model | Per session | Once invoked |
|---|---|---|
| Fable 5.1 | $0.00045 | $0.01511 |
| Opus 5.5 | $0.00018 | $0.00604 |
| Sonnet 5.5 | $0.00009 | $0.00302 |
| Haiku 4.5 | $0.00005 | $0.00151 |
Grade A, and why
testing-codegen scanned grade A with 0 findings against 26 rules in 11 categories — prompt injection, anti-refusal, data exfiltration, privilege escalation, supply chain, agent snooping, system-prompt leakage, SSRF and excessive agency — measured 1mo ago.
A static scan of the body, not an audit. Every finding is printed with the line that produced it so you can judge whether it matters here. A mod is markdown that instructs an agent; that is exactly why what it instructs is worth reading.
Nothing flagged
None of the 26 patterns this scan looks for appear in this file: no shell pipes, no recursive deletes, no credential paths, no hidden text, no instruction-override or anti-refusal phrasing, no agent-config snooping. That is not a guarantee, it is the absence of the things that are checkable.
How it starts
The opening of the file, as written. The whole thing — 138 lines — stays where its author put it; the contents beside it link to each section on GitHub.
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
| Change | Start with |
|---|---|
| Lint rule | just test-lintrule <ruleName> |
| One crate | cargo test -p <crate> or the crate's focused test target |
| Parser or formatter investigation | just qt <package> |
| CLI migration | focused biome_cli migration tests |
| Documentation code | just 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:
cargo insta pending-snapshots --workspace
cargo insta review --workspace --snapshot '<path-from-list>'
After inspecting the full diff, accept or reject only that snapshot:
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:
What this file has done since we first saw it
Hashed on every crawl. A supply-chain change to an agent config is a question of when, not whether, so the history is kept rather than the latest state alone.
- 1mo ago Changed 7ffb53029932
- 1mo ago Changed · +26 lines 89ccebcdbce7
- 1mo ago First seen · 112 lines · 45 tokens per session scan A f15a331e9c73
testing-codegen is a skill published in the GitHub repository biomejs/biome (26,016 stars, last pushed today), licensed Apache-2.0. It adds 45 tokens to every session and 1,511 once invoked, about $0.0002 per session on Opus 5.5. A static security scan graded it A with 0 findings. No closer match exists in the catalogue, so it is treated as the original; first seen 2026-08-30.
Other skills, from other repositories
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prettier-docs
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insta-snapshots
Review and update Oxc Insta snapshots non-interactively after expected output changes or snapshot test failures.
video-qa-frameproof
A post-production checker examines finished MP4 videos, optional subtitles and audio, and a render manifest that links video sections to source code. It reports visual, subtitle, audio, encoding, and platform-format problems in machine-readable and human-readable forms.
oxc-docs
Comprehensive reference for the JavaScript Oxidation Compiler (Oxc) — a collection of high-performance JavaScript tools written in Rust. Covers parser design (lexer, AST, parser, errors, semantic analysis), architecture (parser, linter, test infrastructure, AST tools), ECMAScript specification and grammar, performance…
compiler-commit
Use when you want to verify compiler changes and commit with the correct convention. Runs tests, lint, and format, then commits with the [compiler] or [rust-compiler] prefix.