biome check exits zero despite lint warnings — should CI treat warnings as failures?

Answer

Yes – configure CI to treat warnings as failures so that local biome check and CI behave the same.

Likely explanation

By default biome check returns exit code 0 for warning‑severity diagnostics; only error‑severity diagnostics produce a non‑zero code. This causes CI to pass even when the linter reports warnings, creating a drift between local runs and CI.

Confirmed facts (from Biome documentation)

• The --error-on-warnings flag (or the alias biome ci) makes Biome treat any warning as an error for exit‑code purposes, yielding a non‑zero status when warnings exist.
• biome ci is defined as biome check --ci, which internally enables the same --error-on-warnings behavior, so both commands produce identical exit‑code results.
• Promoting a rule to the error level in biome.json makes the failure deterministic and independent of any CLI flag, ensuring local and CI runs match without needing to remember a flag.
• Nursery rules may change or be removed in minor Biome releases; treating them as errors via configuration or --error-on-warnings can lead to unexpected build breaks after an upgrade.

Steps to align local and CI behavior

1. Decide whether you want a global flag or per‑rule configuration.
2. If you prefer a simple CI‑only switch, add --error-on-warnings to your CI command (or use biome ci).
3. If you want the same behavior locally and in CI without remembering a flag, promote the specific rules you want to block on to error in biome.json.
4. For nursery rules, either keep them at warn level or explicitly exclude them from the error‑on‑warnings policy to avoid surprise failures after upgrades.
5. Verify locally: run biome check (or biome check --error-on-warnings) and echo $? to confirm the exit code matches expectations.

Missing diagnostic detail

If you are using custom formatter or CI output parsing that relies on the warning severity being unchanged, let me know whether you need the diagnostic output to retain warning levels; that would affect whether you choose a flag versus rule promotion.