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Test-Driven Development
A few end-to-end happy-path tests of the public contract beat a pile of unit tests. This skill is opt-in: only when the user asked for TDD or a contract test.
When to use
- The user asked for TDD, test-first, or red-green-refactor
- A public contract (HTTP, store API, migration of live rows) is new or changed and nothing covers the happy path
When not to use
- Ordinary implementation, refactors, helpers, comment or docs edits
- "Every new function/method"
- Edges an advisory invented that are not the contract
- Existing tests already cover the public behavior
Default is no new test. Smoke the changed path. See when-to-write-tests.
What to write
One (or a few) tests that walk the real path: input at the public boundary → through the system → observable result.
Good: seed the old inbound tables, apply the migration, list events, assert facts and metadata survived.
Bad: a test per helper, encoder, or bind-list rewrite.
Prefer real collaborators over mocks. Prefer the route/store/UI path over isolated functions. Supporting notes: tests.md, mocking.md.
Where tests live
Next to the source: foo.test.ts beside foo.ts. Do not create a separate top-level test/ or tests/ directory in a package for these.
Exception: an end-to-end test that spawns the real server or CLI, or spans packages, lives in the repo-root tests/ (for example tests/smoke.test.ts).
Cycle (only when this skill applies)
- Name the happy path in one sentence.
- Write that one failing E2E test.
- Write the thinnest code that makes it pass.
- Stop. Do not add unit tests for the internals you just wrote.
Do not delete working code to restart TDD. Do not require a failing test before every production change.
Conflicts
User instructions and when-to-write-tests beat this skill. If both fire, write no unit tests and at most a few E2E happy-path tests.
