Writing tests worth keeping

The rationale and the same rules in human-facing form live in the handbook: Backend coding conventions › Testing (docs/published/handbook/engineering/conventions/backend-coding.md). This skill is the operational gate — run it before writing tests. It carries the decision procedure plus a catalog of the bug shapes we actually ship.

The gate: one question

Before writing any test, answer in one sentence:

What realistic regression does this test catch that no existing test already catches?

If you can't answer it concretely — name the bug, the code path, the input that would break — do not write the test. "Increases coverage", "good practice", and "the function exists" are not answers.

A good answer sounds like: "if someone makes parse_filters drop the team_id clause, this fails" or "empty-cohort input used to 500; this locks in the 400." That is a test worth keeping.