ruff "unknown rule selector" config error in CI

ruff rejects a rule code in your select/ignore/extend-select that it does not recognize. It fails to parse the configuration and exits before linting, so no files are checked.

What this error means

ruff fails at startup with a config error like "Failed to parse ... unknown rule selector: XYZ", naming the offending code in pyproject.toml or ruff.toml.

ruff

ruff failed
  Cause: Failed to parse [tool.ruff.lint.select]
  Cause: unknown rule selector: `PT1XX`

Common causes

A misspelled or non-existent rule code

The code in select/ignore does not correspond to any ruff rule, so configuration parsing fails.

A rule code removed or renamed across versions

A ruff upgrade renamed or dropped the code, leaving an unknown selector in the committed config.

How to fix it

Correct the selector to a real code

List valid rules with ruff rule --all or ruff linter to find the right code.
Replace the unknown selector with the correct prefix or code.
Re-run ruff to confirm the config parses.

Terminal

ruff linter        # list rule prefixes
ruff check .       # confirm config parses

Match config to the installed ruff version

Pin ruff and update selectors together when a code is renamed, so config and binary agree.

How to prevent it

Validate config after upgrading ruff.
Pin the ruff version so selectors stay valid.
Use rule prefixes (E, F, PT) deliberately and check the changelog.