Biome VCS Integration

Problem

Biome does not automatically skip .gitignore'd files. This causes CI failures and false positives when local-only files (e.g., .claude/settings.local.json) don't match Biome's formatting rules.

The fix is non-obvious: you need to enable Biome's VCS integration explicitly in biome.json.

Diagnosis

Check for these symptoms before applying the fix:

1. Biome reports errors on files that are listed in .gitignore.
2. CI fails on files that only exist locally (untracked).
3. Running biome check or biome format processes files you expected to be ignored.

Verify with:

# Check if the file causing errors is gitignored
git check-ignore -v <problematic-file>

If git check-ignore confirms the file is ignored but Biome still processes it, VCS integration is the fix.

Fix

Step 1 — Enable VCS integration in biome.json

Add or merge the vcs section into the project's biome.json (or biome.jsonc):

{
  "vcs": {
    "enabled": true,
    "clientKind": "git",
    "useIgnoreFile": true
  }
}

Important: The clientKind field is required. Omitting it causes a Biome config validation error.

Step 2 — Remove redundant manual exclusions (if any)

With VCS integration enabled, manual exclusions for .gitignore'd paths are redundant. Check the files section for patterns that duplicate .gitignore entries and remove them.

Biome v1.x — uses the ignore key:

{
  "files": {
    "ignore": [".claude/settings.local.json"]
  }
}

Remove entries that .gitignore already covers.

Biome v2.x — uses includes with ! negation patterns (the old ignore key no longer exists):

{
  "files": {
    "includes": ["!.claude/settings.local.json"]
  }
}

Remove ! negation entries that .gitignore already covers.

Step 3 — Verify

# Confirm Biome now skips gitignored files
biome check .

The previously-erroring gitignored files should no longer appear in output.

Biome v2.x Migration Notes

If migrating from Biome v1.x to v2.x while applying this fix:

• files.ignore is replaced by files.includes with ! prefix for exclusions.
• The vcs config section works the same in both versions.
• Run biome migrate for automated config migration.

Common Pitfalls

Pitfall	Fix
Missing clientKind	Always include "clientKind": "git" — it's required
Nested .gitignore files	Biome respects nested .gitignore files when VCS integration is enabled; ensure useIgnoreFile is true and each subdirectory's .gitignore is committed
biome.jsonc vs biome.json	VCS config works in both; use whichever the project already has

Reference

• Biome VCS integration docs