An API can pass every unit test in its suite and still break every client that calls it, because unit tests check that functions do what the code says they do, not that the API does what the contract says it does. That gap is where most production API incidents actually come from: a field renamed, a status code changed from 200 to 204, a required parameter quietly made optional. Testing an API well means testing at several different levels, because each one catches a different class of failure.

Unit tests verify that individual functions behave correctly in isolation. They run fast, catch regressions close to where they are introduced, and document expected behavior at the code level. They do not verify that the API endpoints exposed to consumers behave according to the documented contract, that changes in one service do not break the consumers that depend on it, or that the system behaves correctly under the conditions that production traffic creates.