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TDD

Bucket: Engineering · Slash command: /zsl:tdd · Source: skills/engineering/tdd/SKILL.md

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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.

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
  ...

Flags

  • --no-ship — skip step 5 (Ship it). After the final refactor commit, stop and report back: branch name (git rev-parse --abbrev-ref HEAD), last commit sha (git rev-parse HEAD), and a one-paragraph summary of the changes. Do not push, do not open a PR, do not update the project board's "in review" Status. Used by /tdd-parallel so the orchestrator can integrate slice branches locally and ship a single consolidated PR. Step 1's "in progress" Status update still happens — the work is real, only the ship step is deferred. Also engages Progress heartbeat (see below) — the orchestrator depends on it for live status.

Progress heartbeat

Under --no-ship (running inside /tdd-parallel), emit one JSON line to .tdd-progress.jsonl in the current working directory at each phase below. The orchestrator tails the file to render live progress; without these lines it sees nothing until you return, and a silent slice looks indistinguishable from a hung one.

Schema: {"slice": <num>, "ts": "<ISO-8601 UTC>", "phase": "<phase>", "note": "<short>"}. slice is the input issue number, passed through verbatim.

Phases (in order; red / green repeat per cycle):

  • started — immediately after pre-flight, before planning.
  • planned — once the behavior list is decided. note: "N behaviors: ...".
  • red — each failing test written. note: "<n>/<N>: <behavior>".
  • green — each test now passing.
  • refactor — after each refactor step that touched files. note: what was extracted/renamed.
  • committed — after each commit. note: short sha.
  • done — final phase, just before returning to the parent.
  • escalating — instead of done, if you escalate. note: one-line question.
  • error — instead of done, if you halt. note: one-line cause.

Emit via bash -c "printf '%s\n' '<json>' >> .tdd-progress.jsonl" — one printf per line, no buffering, append-only. Without --no-ship, emission is optional and harmless.

Workflow

1. Planning

Pre-flight: refuse container issues. If you were given an issue identifier as input, fetch it and check whether it has open sub-issues. If it does, it's a tracking parent (likely a PRD), not a unit of work. Refuse and tell the user to run /to-issues to break it down first, then /tdd against one of the leaf children.

Pre-flight: signal "in progress" on the project board (if configured). If you were given an issue identifier and docs/agents/project-board.md exists, update the issue's project item Status to the option mapped to "work begins" (typically In progress). Use the same lookup-then-update procedure documented in triage/SKILL.md step 6: fetch the project item via gh api graphql filtered by the configured project node ID, then updateProjectV2ItemFieldValue with the mapped Status option ID. If the issue isn't on the configured project, log and continue. Best-effort — failure to update Status doesn't block the TDD work. Skip entirely if docs/agents/project-board.md doesn't exist or you weren't given an issue identifier.

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 (small interface, deep implementation)
  • [ ] Design interfaces for testability
  • [ ] 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.

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:

  • [ ] 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.

5. Ship it

Skip this entire step if --no-ship was passed. Stop after the final refactor commit and report back: branch name, last commit sha, one-paragraph summary. The orchestrator (/tdd-parallel) will integrate the branch and ship a consolidated PR.

Once tests are green and refactored, ship the slice. The repo's workflow is defined in docs/agents/ship-style.md (written by /setup-zsl-superpowers) — read it before doing anything.

  • Always commit via /commit — never craft commits yourself. The commit body must reference both the sub-task and the parent issue so git history is navigable. Use #<num> for GitHub/GitLab (auto-linked in the UI) or full URLs for other trackers:
<subject>

Sub-task: #<sub-task>
Parent: #<parent>
  • Close the sub-task on merge:
  • PR-style — put Closes #<sub-task> in the PR body, alongside the same sub-task/parent references.
  • Direct-push — put Closes #<sub-task> in the commit body itself (replaces the Sub-task: line).

GitHub auto-closes the parent once its last child closes (the sub-issue link was set at /to-issues time). - Confirm with the user before opening a PR or pushing to the default branch. - Signal "in review" on the project board (PR-style only, if configured). If docs/agents/project-board.md exists and the ship style is PR, update the issue's project item Status to the option mapped to "PR opened" (typically In review) once the PR is open. The project's existing Auto-close issue workflow will move Status to Done automatically when the PR merges and closes the issue. In direct-push mode, no skill-driven Status update is needed at ship time — the closing commit triggers the same Auto-close workflow directly.

If docs/agents/ship-style.md doesn't exist, run /setup-zsl-superpowers first.

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