Typed codebases and the tests that mean something
Coverage percentage is not a quality signal. Here is what we require instead before a service goes on-call.
We have never been persuaded by coverage percentages. A codebase at 95% coverage can still be untested in every way that matters.
What we require before a service joins an on-call rotation: types at the boundaries, contract tests against real consumers, one end-to-end path exercised against production-shaped data, and a documented failure mode for every external dependency.
That set is smaller than most test suites we inherit and catches considerably more. It also survives refactors, because it asserts on behaviour rather than implementation.
The honest reason to write tests is so the next engineer can change things without fear. Anything that does not serve that is ceremony.
An engineer replies within one working day — not a sales sequence.
