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

Exit code 0 from biome check with warnings present

Our pipeline runs biome check and passes even when the linter reports dozens of warning-severity diagnostics. My understanding is that the CLI only returns a non-zero exit code for error-severity diagnostics, so warnings never block a merge unless we change something. I want one consistent gating policy between local runs and CI instead of the current drift.

Constraints

Rules in biome.json are currently a mix of error and warn, mostly inherited from the recommended defaults. Some nursery rules are enabled, and I'm wary of promoting those to error since they can change or be removed in minor releases. We're on Biome 2.x, so config keys like assist apply rather than the older organizeImports layout.

Questions

• Is --error-on-warnings the intended mechanism for making warnings fail CI, and does it behave identically under biome ci versus biome check?
• Is promoting individual rules to error in config considered better practice than a global CLI flag, given we want local and CI behavior to match?
• Should nursery rules be excluded from any blocking policy to avoid surprise failures after upgrades?