Getting it into your agent
One page per mod, every tool's command on it. A separate URL per tool would split the same page into five that compete with each other.
npx agentmods add skills/biomejs/biome/testing-codegennpx skills add biomejs/biome --skill testing-codegengit clone --depth 1 https://github.com/biomejs/biomeWhat 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 | $0.00045 | $0.01074 |
| Opus 5 | $0.00023 | $0.00537 |
| Sonnet 5 | $0.00009 | $0.00215 |
| Haiku 4.5 | $0.00005 | $0.00107 |
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 2d 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 — 112 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 produce snapshots, then review every changed section:
cargo insta review
Use cargo insta accept or cargo insta reject only after inspecting the pending changes. A passing snapshot test proves output matches the checked-in snapshot, not that the snapshot describes correct behavior.
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:
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.
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.
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.
- 2d ago First seen · 112 lines · 45 tokens per session scan A f15a331e9c73
testing-codegen is a skill published in the GitHub repository biomejs/biome (25,686 stars, last pushed 2d ago), licensed Apache-2.0. It adds 45 tokens to every session and 1,074 once invoked, about $0.0002 per session on Opus 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.
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