insta snapshot workflows, expectation comments, orphan checks, or required code generators. Do not use for subsystem implementation design.\ncompatibility: Designed for coding agents working on the Biome codebase (github.com/biomejs/biome).Choose the narrowest test that exercises the changed behavior, then broaden only when shared infrastructure or integration risk justifies it.
\n| Change | \nStart with | \n
|---|---|
| Lint rule | \njust test-lintrule <ruleName> | \n
| One crate | \ncargo test -p <crate> or the crate's focused test target | \n
| Parser or formatter investigation | \njust qt <package> | \n
| CLI migration | \nfocused biome_cli migration tests | \n
| Documentation code | \njust test-doc | \n
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.
\nRun the focused test to produce snapshots, then review every changed section:
\ncargo insta review\nUse 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.
Do not delete suspected orphan snapshots manually. Deletion is safe only after running the complete workspace snapshot suite without package, target, or test filters:
\ncargo insta test --workspace --unreferenced delete\nFor 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.
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.
The test utilities recognize these marker texts in source comments:
\nshould generate diagnostics\nshould not generate diagnostics\nCurrent enforcement:
\nvalid but not invalid must contain one of the expectation markers;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;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.
\nUse 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.
Use internal specs for behavior introduced or fixed by the change. External Prettier snapshots record comparison results but do not replace focused internal coverage.
\nThe formatter harness performs its idempotency reformat during one test invocation for eligible files. Inspect both formatted output and any IR shown for a mismatch.
\n| Changed source | \nCommand | \n
|---|---|
.ungram grammar | \njust gen-grammar <lang> | \n
| Formatter source | \njust gen-formatter <lang> | \n
| Lint rule or assist | \njust gen-rules and just gen-configuration | \n
| Bindings needed locally | \njust gen-bindings | \n
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.
insta.CONTRIBUTING.md#testingcrates/biome_analyze/CONTRIBUTING.mdcrates/biome_test_utils/src/lib.rscrates/biome_formatter_test/src/spec.rsjustfile