Write tests for this [function/module/component]:
[paste your code]
Stack: [e.g. Python + pytest, TypeScript + vitest, Go stdlib]
Cover:
- The happy path (expected inputs → expected outputs)
- Edge cases (empty input, nulls, boundaries, huge values)
- Failure modes (what should throw/error and what shouldn’t)
No mocks unless absolutely necessary. If you mock something, explain why a real call won’t work here. Give me tests I can run right now without setup.