Lesson 481 of 2116
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.
Lesson map
What this lesson covers
Learning path
The main moves in order
- 1Tests are specifications
- 2test generation
- 3coverage
- 4test contract
Concept cluster
Terms to connect while reading
Section 1
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.
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