| name | tdd |
| description | Test-driven development with red-green-refactor loop. Use when user wants to build features or fix bugs using TDD, mentions "red-green-refactor", wants integration tests, or asks for test-first development. |
Test-Driven Development
Philosophy
Core principle: Tests should verify behavior through public interfaces, not implementation details. Code can change entirely; tests shouldn't.
Good tests are integration-style: they exercise real code paths through public APIs. They describe what the system does, not how it does it. A good test reads like a specification - "user can checkout with valid cart" tells you exactly what capability exists. These tests survive refactors because they don't care about internal structure.
Bad tests are coupled to implementation. They mock internal collaborators, test private methods, or verify through external means (like querying a database directly instead of using the interface). The warning sign: your test breaks when you refactor, but behavior hasn't changed. If you rename an internal function and tests fail, those tests were testing implementation, not behavior.
See tests.md for examples and mocking.md for mocking guidelines.
Anti-Pattern: Horizontal Slices
DO NOT write all tests first, then all implementation. This is "horizontal slicing" - treating RED as "write all tests" and GREEN as "write all code."
This produces crap tests:
- Tests written in bulk test imagined behavior, not actual behavior
- You end up testing the shape of things (data structures, function signatures) rather than user-facing behavior
- Tests become insensitive to real changes - they pass when behavior breaks, fail when behavior is fine
- You outrun your headlights, committing to test structure before understanding the implementation
Correct approach: Vertical slices via tracer bullets. One test → one implementation → repeat. Each test responds to what you learned from the previous cycle. Because you just wrote the code, you know exactly what behavior matters and how to verify it.
WRONG (horizontal):
RED: test1, test2, test3, test4, test5
GREEN: impl1, impl2, impl3, impl4, impl5
RIGHT (vertical):
RED→GREEN: test1→impl1
RED→GREEN: test2→impl2
RED→GREEN: test3→impl3
...
Workflow
1. Planning
When exploring the codebase, use the project's domain glossary so that test names and interface vocabulary match the project's language, and respect ADRs in the area you're touching.
Before writing any code:
Ask: "What should the public interface look like? Which behaviors are most important to test?"
You can't test everything. Confirm with the user exactly which behaviors matter most. Focus testing effort on critical paths and complex logic, not every possible edge case.
2. Tracer Bullet
Write ONE test that confirms ONE thing about the system:
RED: Write test for first behavior → test fails
GREEN: Write minimal code to pass → test passes
This is your tracer bullet - proves the path works end-to-end.
3. Incremental Loop
For each remaining behavior:
RED: Write next test → fails
GREEN: Minimal code to pass → passes
Rules:
- One test at a time
- Only enough code to pass current test
- Don't anticipate future tests
- Keep tests focused on observable behavior
4. Commit Per Slice
Land each slice as its own commit, the moment it goes GREEN. A slice is the smallest self-contained, shippable unit — usually one RED→GREEN cycle, sometimes a tight cluster of cycles that together form a single coherent capability (e.g., a pure utility plus its handful of example tests).
Why per-slice commits matter:
- Reviewability: small commits read like a story. A reviewer can follow the build-up cycle by cycle instead of unpicking a single mega-diff.
- Bisectability: each commit is green, so
git bisect can pinpoint regressions to a slice.
- Reversibility: a bad slice is one
git revert away — no manual untangling.
- Forced honesty: if a slice can't stand on its own as a commit, that's a signal it isn't really a vertical slice yet.
When you outline the plan in step 1, the slices in that plan are the commits. Confirm the slice list with the user up front; don't accumulate multiple slices in the working tree before committing.
Rules:
- Commit immediately after a slice goes GREEN. Don't defer commits to "the end."
- Each commit's diff should match exactly one item in the plan. If a single file's hunks belong to two different slices, untangle them — temporarily revert one part, commit, then re-apply.
- The commit message names the slice and the user-visible behaviour it adds, not the cycle number.
- Existing tests stay green at every commit; new tests added in this slice go in the same commit as the impl that makes them pass.
If you find yourself five slices deep with no commits, stop and split before continuing — a single git reset --soft plus per-slice git add is faster than letting the divergence grow.
5. Refactor
After all tests pass, look for refactor candidates:
Never refactor while RED. Get to GREEN first. Commit refactors separately from the slices that motivated them.
Checklist Per Cycle
[ ] Test describes behavior, not implementation
[ ] Test uses public interface only
[ ] Test would survive internal refactor
[ ] Code is minimal for this test
[ ] No speculative features added
[ ] Slice committed as soon as it went GREEN
