Canon TDD

Test-driven development the way Kent Beck describes as "Canon TDD", per https://adam-young.co.uk/blog/canon-tdd/. CLAUDE.md mandates a TDD approach for this package; this skill is the detailed how.

Red-Green-Refactor is the engine. The Test List is the steering wheel.

Agent behaviour contract

The point of this skill: do these by default, without being reminded.

1. Start with a test list, and show it. Before touching production code, enumerate the behaviours and edge cases as a checklist and confirm it with the user (or state it explicitly). This is the steering wheel — it defines "done".
2. No production code before a failing test. Write exactly one test, run it, and watch it fail for the right reason. Never write the implementation first and back-fill tests.
3. One test at a time. Do not convert the whole test list into code up front — the first passing test often forces a design change that affects the rest.
4. Refactor only on green, and keep refactoring out of the make-it-pass step.
5. Repeat until the test list is empty, adding newly-discovered cases to the list as you go.

The five steps

1. Write a test list — every behavioural variant and edge case you can think of, as a plain checklist. Not code yet. It's a living list: add to it whenever implementation reveals a new case.
2. Write a test — pick the next item and write one real, automated test: setup, invocation, and assertions. Define what the system does and how it's invoked — not how it works inside. Run it; confirm it fails — and state the failure: quote the failing assertion/message and check it is the expected reason. (For brand-new API the first red is normally a compile error — the symbol doesn't exist yet; that is expected. Stub the minimal declaration and re-run so the red becomes the failing assertion.) A compile error in existing code, a missing import, or an unrelated crash is a red for the wrong reason — it proves nothing about the behaviour, so fix the test and re-run until it is red for the right reason.
3. Make it pass — change the system to satisfy that test with the simplest thing that works. Nothing more; resist solving items still on the list.
4. Optionally refactor — improve names, structure, and duplication while every test stays green. Skip if there's nothing to improve.
5. Repeat — next item, until the list is empty.

Separate interface from implementation

A test fixes a decision about the interface (the call shape, inputs, outputs) and should be silent about the internals. Tests that assert on private mechanics break under refactoring and defeat step 4.

Anti-patterns (don't)

• Tests without assertions — a test that invokes but asserts nothing proves nothing.
• Pre-converting the whole list to code — write one test, learn, then the next. Batching tests locks in design decisions before you've learned anything.
• Mixing refactor into make-it-pass — keep "make it work" and "make it clean" as separate steps on either side of green.
• Premature abstraction from duplication — a little duplication across two green tests is fine; abstract when the shape is clear, not at the first repeat.

In this codebase (TMDb)

Apply the loop with the project's tooling and conventions:

• Framework: Swift Testing — @Suite, @Test, #expect, #require. Never force-unwrap in tests; unwrap optionals with try #require(...).
• Run tests via the delegated skills, not make directly, so output stays out of context: /test (unit) and /integration-test (live API). Re-run after each red→green and after refactoring.
• Both layers are required for new behaviour. Unit tests with JSON fixtures AND integration tests against the live API — put both kinds of items on the test list from the start.
• Models / new endpoints: when the test list involves decoding, the fixture must exercise every decode branch (all N optional appended properties, plus a paired "without appended data" test that asserts they're nil). Use the TMDb MCP server (mcp__tmdb__*) to fetch real API responses for fixtures rather than inventing JSON — see .claude/docs/tmdb-api.md → "Workflow for New Endpoints".
• Bug fixes: the first item on the list is a test that reproduces the bug (red), then the fix (green) — never fix first.
• New public API: the test list should include the doc/DocC and README updates as completion items, even though they aren't tests (per the document-swift skill's consistency checklist).

When NOT to use it

Pure refactors with existing coverage, formatting, config/CI edits, and docs-only changes don't start from a new failing test — keep the existing tests green instead. TDD is for new behaviour and bug fixes.