Use when integrating an external HTTP API (Stripe, GitHub, Twilio, a payment provider, etc.) into the project. Enforces the Client + Service split — Client lives in infrastructure and returns raw vendor-shaped Pydantic responses, Service lives in application and maps raw responses to application `Result` types. Covers where to put ports, vendor enums, raw response DTOs, how to wire DI, and how to let a use case access a raw response when it genuinely needs one.
Use when creating or editing a FastAPI endpoint under src/presentation/http/v1/endpoints/<audience>/. Enforces audience-first organization (public/user/admin/internal), the exact parameter-naming convention (data for bodies, query for GET filters), the five-parameter list shape, correct Require + Authorization wiring, and strict contract ↔ Result mapping.
Use when creating or extending a repository under src/database/psql/repositories/. Enforces the project's fixed repository vocabulary (create, select, select_many, update, delete, exists, count, upsert) — no invented methods. Shows exactly how to wrap CRUDRepository, use @on_integrity, build loads-aware selects, and return Result[T].
Use when creating or extending tests under tests/unit/, tests/integration/, or tests/e2e/. Enforces per-use-case unit coverage, per-endpoint e2e coverage, mutation-persistence verification (not just 200 status), contract-vs-Request field-mismatch detection, loads-opt-in flow tests, role/validation boundary tests, and the test infra gotchas (sync alembic fixture, session-scoped asyncio loop, httpx.AsyncClient + ASGITransport, CipherSettings with matching HS256 keys, argon2 hasher seeding).
Use when creating or editing a use case under src/application/v1/usecases/. Enforces the RequestBus + UseCase pattern (Request + UseCase in one file, @dataclass(slots=True), returns Result type) and the DBGateway transaction rules — never open two sessions in one use case, never touch self.database outside a context manager, never lazy-load relationships outside the session.
Use when the user asks to create a database migration, change a SQLAlchemy model, add a column, add an index, or otherwise modify the schema. Enforces the rule that migrations are generated via `make generate NAME="..."` (the project's Makefile target that runs Alembic autogenerate) — never hand-written — and never edit existing migration files in migrations/versions/.