principle-test-behavior-not-implementation

Call the code the way its users do and assert the observable result against a literal expected value, such as expect(slugify("Hello, World!")).toBe("hello-world"). Avoid asserting which internal calls were made or restating constants the code contains.

Ask whether the test would still pass if every function it imports returned undefined. If yes, it observes no behavior and cannot fail for a defect, so rewrite the assertion or delete the test.

Asserting only that a mock was called verifies no behavior. Instead assert the payload the mock received or the state after the call, so the test fails when the code produces a wrong result.

Pinning a constant like expect(LIMITS.maxTools).toBe(8) catches no defect and fails whenever someone legitimately edits the value. Test the mechanism that reads the constant with one concrete input instead of restating the value.

Delete a test when no behavioral assertion exists for it: when there is no concrete input whose literal output or observable effect the test can check. Keeping it only costs CI time and review attention.

Keep tests of a relation across a table's rows, such as a key present in two tables or a parent that exists, and compile-time checks in *.test-d.ts files. These verify real constraints even though they do not call a function's output.