Codex Test Generator

---
name: codex-test-gen
description: |
  Generate unit and integration tests for existing code in Codex. Trigger
  when the user asks for test coverage on a file, module, or function.
  Phrases: "generate tests", "write tests for", "add coverage", "unit tests",
  "test this function".
allowed-tools:
  - Read
  - Write
  - Edit
  - Grep
  - Glob
  - Bash(npm test *)
  - Bash(pytest *)
---

# Codex Test Gen

Generate meaningful tests for a target file or function. Favor behavior
coverage over line coverage. Never produce tests that only assert that
mocks were called.

## Prerequisites

- Test runner configured (Jest, Vitest, pytest, go test, cargo test)
- Target file exists and is imported somewhere

## Steps

1. **Read the target file end to end.** Map exported surfaces, inputs,
   outputs, and side effects. Note which branches exist.

2. **Find existing tests** in the repo for style conventions. Match
   import paths, describe/it structure, assertion library, and fixture
   patterns already in use.

3. **Plan cases before writing code.** Happy path, edge cases (empty,
   null, boundary), error cases, and one integration case that exercises
   real collaborators.

4. **Write tests** in the repo's idiomatic location (`__tests__/`,
   `*.test.ts` sibling, `tests/` folder). Keep one behavior per test.
   Use table-driven tests for similar shape with different inputs:
   ```ts
   it.each([
     [1, 2, 3],
     [0, 0, 0],
     [-1, 1, 0],
   ])("add(%i, %i) = %i", (a, b, expected) => {
     expect(add(a, b)).toBe(expected);
   });
   ```

5. **Run the suite.** If a test fails on a real bug, report it and stop;
   do not silently relax the assertion to make it pass.

6. **Report coverage delta** for the touched file only. Whole-repo
   coverage targets are noise.

## Anti-patterns

- Mock-heavy tests that only verify implementation details
- Snapshot tests on UI with no assertions on behavior
- Single giant test that exercises half the module

## Output

- Test file next to or alongside the target
- Green run on the new tests
- Summary of cases covered and any bugs surfaced