Tdd

Test-driven development

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TDD determines the order of development, not how much of the system a test boots. Use integration-tests for a real adapter boundary and the repository's full-system test workflow when the assembled application must run; either boundary can support a TDD cycle. Ordinary test creation does not require a full TDD workflow.

See tests.md for examples and mocking.md for when and how to mock system boundaries.

Anti-pattern: horizontal slices

DO NOT write all the tests first and then all the implementation. That is "horizontal slicing" — treating RED as "write all the tests" and GREEN as "write all the code".

It produces bad tests:

• Tests written in bulk test imagined behavior, not actual behavior.
• You end up testing the shape of things (data structures, function signatures) instead of user-facing behavior.
• The tests become insensitive to real changes — they pass when behavior breaks and fail when everything is fine.
• You lock yourself into a test structure before you understand the implementation.

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

The right approach: vertical slices via tracer bullets. One test → one implementation → repeat. Each test answers what you just learned from the previous cycle.

Workflow

1. Plan

If you are following a plan or task brief from the calling workflow, stick to the behaviors there. Use domain vocabulary or binding decisions when the repository exposes them, but do not depend on a particular documentation path.

Discover the test contract before writing: inspect neighboring tests, build manifests, runner configuration, scripts and CI. Record the framework, test style, public seam, focused command, broader gate and fixture conventions. Do not assume Gradle, Kotest, Ktor, Jest, Vitest, pytest, containers or any other stack choice until repository evidence establishes it.

Identify the public interface and list the observable behaviors to prove, using the existing brief and repository contract. Prioritize critical paths and complex logic such as authorization, validation and state transitions. Prefer a small interface that can be tested without exposing internal collaborators.

An agreed brief or a concrete request to add a regression test and fix the bug is sufficient to begin. Ask only about material behavior or interface choices that remain unresolved; starting a TDD cycle does not require another approval.

2. Tracer bullet

Write ONE test that confirms ONE thing end-to-end through the public interface:

RED:   Write a test for the first behavior → fails
GREEN: Minimal code to make it pass → passes

For a new entry point, choose the thinnest production-shaped test supported by the detected stack. It should prove that the entry point, composition and observable response or outcome hang together without locking the test to internal collaborators.

3. Incremental loop

For each remaining behavior:

RED:   Next test → fails
GREEN: Minimal code → passes

Rules:

• One test at a time
• Only enough code to pass the current test
• Do not anticipate future tests
• Keep the tests on observable behavior

Run the discovered focused command while you work:

<focused-test-command>
echo "exit: $?"

A GREEN claim needs an actual passing run against the relevant code state. Report the command, result and exit code. Reuse that evidence while the code and relevant environment remain unchanged; rerun affected checks after changes. An unrun check remains UNVERIFIED.

4. Refactor

When all tests pass, look for refactoring candidates:

• Extract duplication
• Deepen modules — move complexity behind simple interfaces
• Apply SOLID where it falls naturally
• Consider what the new code reveals about existing code
• Run the tests after each refactoring step

Never refactor while RED. Get to GREEN first.

When the implementation is done and green, return the command that was run and the result to the calling workflow so it can close the verification phase.

Checklist per cycle

[ ] Test describes behavior, not implementation
[ ] Test uses only the public interface
[ ] Test would have survived internal refactoring
[ ] Code is minimal for this test
[ ] No speculative features added
[ ] GREEN supported by a passing run against the current relevant code state

Bug fixing is TDD

A bug fix starts with a reproduction test: write a test that fails because the bug exists (RED), then fix until it turns green (GREEN). That way you have both proven the bug and prevented regression. Never write the fix first.

For a bug that is hard to reproduce (flaky, timing- or environment-dependent): start with diagnosing-bugs to build a tight, red-capable repro loop first — the reproduction test here is then the minimized loop.