Lesson 386 of 1596
Codex For Test Generation: From Coverage Gaps To Passing Suites
Codex can generate tests well when you give it the contract. It generates flaky theater when you ask for 'tests' with no spec.
Creators · Tools Literacy · ~5 min read
Tests are specifications
When you ask Codex to 'write tests for this file', it has to invent a specification first. The result is often technically passing tests that test the implementation, not the behavior — pure regression theater. Better: tell Codex what the function should do, and have it write tests against that contract.
The three test-generation modes
- 1Spec-first: you write the contract in prose, Codex writes the tests
- 2Coverage-driven: Codex sees a coverage report and fills the gaps
- 3Characterization: Codex reads existing behavior and writes tests that lock it in
Compare the options
| Mode | Best for | Trap |
|---|---|---|
| Spec-first | New code | If your spec is wrong, your tests are wrong |
| Coverage-driven | Existing code with gaps | Coverage percent != quality |
| Characterization | Pre-refactor lock-in | Tests pin in the bugs |
How to spot generated test theater
- Tests that mock everything and verify the mocks were called
- Tests with no assertions or trivial assertions
- Tests that pass when you delete the body of the function under test
- Tests that test implementation details you might want to change
- Tests with copy-paste boilerplate across cases
Applied exercise
- 1Pick a function with weak coverage
- 2Write the contract in 5 to 10 sentences
- 3Ask Codex for tests in spec-first mode
- 4Manually break the function in three ways. How many tests catch each break? That is your real coverage
Key terms in this lesson
The big idea: tests test contracts, not code. Tell Codex the contract and the tests get sharp. Skip the contract and you get theater.
End-of-lesson quiz
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