/tdd

name: tdd
description: Test-driven development with red-green-refactor loop, plus a value gate and audit workflow for tests. Use when user wants to build features or fix bugs using TDD, mentions "red-green-refactor", wants integration tests, asks for test-first development, or asks to review, audit, prune, or clean up low-value, duplicated, or implementation-coupled tests.

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](tests.md) for examples, [mocking.md](mocking.md) for mocking guidelines, and [test-audit.md](test-audit.md) for the value gate every new test must pass and the workflow for auditing existing tests.

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:

• [ ] Confirm with user what interface changes are needed
• [ ] Confirm with user which behaviors to test (prioritize)
• [ ] Identify opportunities for [deep modules](deep-modules.md) (small interface, deep implementation)
• [ ] Design interfaces for [testability](interface-design.md)
• [ ] List the behaviors to test (not implementation steps)
• [ ] Get user approval on the plan

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.

Before writing each test, pass it through the [authoring gate](test-audit.md#authoring-gate): name the behavior it protects, the regression that would break it, why existing coverage doesn't already catch that, and whether it needs a production seam nothing else uses. For a bug fix, the first RED test must fail on the unfixed code for the reason the bug describes.

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. Refactor

After all tests pass, look for [refactor candidates](refactoring.md):

• [ ] Extract duplication
• [ ] Deepen modules (move complexity behind simple interfaces)
• [ ] Apply SOLID principles where natural
• [ ] Consider what new code reveals about existing code
• [ ] Run tests after each refactor step

**Never refactor while RED.** Get to GREEN first.

Auditing Existing Tests

When asked to review or prune tests rather than write them, follow the [audit workflow](test-audit.md#audit-workflow): read the test, its production owner, and its history first; discover read-only and record evidence per candidate; edit one coherent batch; and keep any test that meets the [retention bar](test-audit.md#retention-bar).

Checklist Per Cycle

[ ] Test passes the authoring gate (behavior, regression, not already covered, no test-only seam)
[ ] Test describes behavior, not implementation
[ ] Test uses public interface only
[ ] Test would survive internal refactor
[ ] Bug fixes only: the regression test failed on the pre-fix code for the intended reason
[ ] Code is minimal for this test
[ ] No speculative features added