Intro
There's a pitch behind every "AI writes your tests too" workflow: more coverage, less manual toil, a safety net that used to take a sprint now takes minutes.
The pitch skips over what that safety net is actually made of. When the same model writes the implementation and the test suite, you haven't added a second, independent check. You've asked one reviewer to grade its own homework and handed you the green checkmark as if someone else had signed off.
The blind spot loop
A model reasons about a function once, forms an implicit set of assumptions (input shapes, timezone handling, what counts as "empty"), and writes the implementation against those assumptions. Ask the same model to write tests for that function, and it doesn't re-derive correct behavior from scratch. It writes tests against the same assumptions it just used to write the code. If it assumed dates always arrive as ISO strings in UTC, the implementation assumes that, and the tests assume it too. The suite goes green. The assumption is still wrong.
