backend-fastapi-python
Use this skill for any Python backend work in this project: building FastAPI endpoints, writing service functions, defining Pydantic/SQLModel schemas, running Alembic migrations, or debugging 422 errors. Essential for authentication and authorization patterns — setting up get_current_user, is_superuser checks, admin-only guards, role-based access, and dependency injection chains like Depends(). Also covers middleware, background tasks, async SQLAlchemy sessions, ORM relationship loading, and request/response design. Activate whenever the question involves Python API code, FastAPI patterns, or backend architecture in this codebase. Not for frontend, Docker, CI/CD, or infrastructure.
git clone --depth 1 https://github.com/avibebuilder/claude-prime /tmp/backend-fastapi-python && cp -r /tmp/backend-fastapi-python/.claude/starter-skills/backend-fastapi-python ~/.claude/skills/backend-fastapi-pythonSKILL.md
# Backend FastAPI Python
Project-specific conventions for FastAPI with SQLModel, pydantic-settings, and async SQLAlchemy.
## Architecture Decisions
1. **Services are stateless functions** — Not classes. First param is `db: AsyncSession`.
2. **Generic response wrapper** — Always use `ApiResponse[T]` for consistency.
3. **Dependencies chain** — `get_current_user` -> `require_auth` -> `require_admin`.
4. **Module-scoped config** — Each module can have its own `{module}_config.py`.
5. **Error codes for frontend** — `AppException(status, message, error_code)`.
## Gotchas
- SQLModel `Relationship()` fields are NOT included in API responses by default. You must explicitly add them to `model_config` or use a separate response schema with those fields.
- `AsyncSession.refresh()` does not load relationships. After commit, re-query with `.options(selectinload(...))` if you need related objects.
- Pydantic V2 uses `model_validator` not `validator`. The `@validator` decorator is V1 and will break silently or raise deprecation warnings.
- `Depends()` in FastAPI creates a NEW instance per request — don't store state in dependency return values expecting it to persist.
- Background tasks (`BackgroundTasks`) run AFTER the response is sent. If they fail, the client already got a 200. Use proper task queues (Celery, ARQ) for anything that must not silently fail.
- Alembic `--autogenerate` misses: table renames (generates drop+create), index changes on existing columns, and `Enum` type modifications in PostgreSQL. Always review generated migrations.
- `async def` endpoints block the event loop if you call sync I/O inside them. Use `run_in_executor` for sync libraries or define the endpoint as `def` (FastAPI runs sync endpoints in a threadpool).
- `HTTPException` from FastAPI and `HTTPException` from Starlette are different classes. Importing the wrong one causes middleware to miss exception handlers.
- SQLAlchemy's `lazy="selectin"` on relationships causes N+1 queries in async sessions. Use explicit `selectinload()` in queries instead.
- `Optional[str] = None` in query params makes the field optional. `str = None` also works but loses type information — prefer the explicit `Optional` form.
- When using `response_model`, FastAPI filters OUT any fields not in the model. If your response is missing data, check that the response model includes all fields, not just the ORM model.
## References
| When you need... | Read |
|------------------|------|
| Directory layout | [file-structure.md](./references/file-structure.md) |
| Settings and env vars | [configuration.md](./references/configuration.md) |
| Database sessions and connections | [database.md](./references/database.md) |
| ORM models | [models.md](./references/models.md) |
| Request/response schemas | [schemas.md](./references/schemas.md) |
| Router and endpoint patterns | [routing.md](./references/routing.md) |
| Service layer patterns | [services.md](./references/services.md) |
| Dependency injection | [dependencies.md](./references/dependencies.md) |
| Middleware setup | [middleware.md](./references/middleware.md) |
| Error handling | [error-handling.md](./references/error-handling.md) |
| Auth flow example | [auth.md](./references/auth.md) |Browser automation CLI for AI agents. Use when the user needs to interact with websites, including navigating pages, filling forms, clicking buttons, taking screenshots, extracting data, testing web apps, or automating any browser task. Triggers include requests to "open a website", "fill out a form", "click a button", "take a screenshot", "scrape data from a page", "test this web app", "login to a site", "automate browser actions", or any task requiring programmatic web interaction.
Answer questions about code, architecture, and technical decisions — no implementation. Trigger on questions asking 'why', 'what does this do', 'what is the purpose of', 'explain', 'what's the difference', 'compare', or 'what are the tradeoffs' — even when referencing specific files, code snippets, or inline code. The key signal is the user wants to UNDERSTAND something, not change it. Do NOT trigger for requests to build, fix, plan, review, research, or add/modify code.
Implement, build, create, or add any feature, endpoint, page, component, or functionality. Use this skill whenever the user asks you to write new code or make code changes — whether it's adding an API endpoint, building a UI page, creating an export feature, wiring up a webhook, implementing a search/filter, or any other hands-on coding task. This is the default skill for all 'build this', 'add this', 'create this', 'wire up', 'implement' requests. Covers the full cycle: clarify requirements, plan if needed, write code, verify, and review. Do NOT use for pure research, debugging, documentation, or explanation — only when the user wants working code delivered.
Use when the user wants to save knowledge as a file so others don't have to rediscover it — \"turn this into a doc\", \"write this up\", \"document how X works\", \"we figured this out and want to capture it\", \"nobody should have to figure this out again\". Covers any request to create or update durable written artifacts: onboarding guides, runbooks, ADRs, API docs, architecture notes, postmortems, changelogs, setup guides. The trigger: user wants knowledge captured in a file for future reference, not just a conversation. Do NOT use when still making decisions (→ give-plan), just asking for explanation without a file (→ ask), or writing code (→ cook).
Investigate unexpected behavior and mysterious bugs. Use when the cause of a problem is unknown and the user needs to understand WHY something is happening — symptoms like: sudden unexplained changes in metrics or behavior, works locally but not in staging/production, inconsistent or intermittent failures, correct code producing wrong results, operations succeeding but having no effect, environment-specific failures, duplicate executions, stale data, or any \"why did this change?\" or \"why is this happening?\" situation. Covers infrastructure anomalies (cache hit rates dropping, latency spikes, queue behavior shifts) as well as code bugs. The key signal is confusion about root cause, not a request to implement a known fix. Do NOT use for feature requests, known fixes, planning, or documentation tasks.
Brainstorms and debates approaches, then drives toward an actionable decision. Use whenever someone needs a thinking partner for a decision they're facing: 'discuss', 'debate', 'brainstorm', 'weigh options', 'tradeoffs', 'should I do X or Y', 'help me decide', 'I'm torn between', 'sanity check my thinking', or 'what do you think about'. The user must be asking for help reasoning through a choice — not asking to build, fix, evaluate, plan, or modify something (even if the topic involves this skill itself). Picks the right decision lens, surfaces tradeoffs and blind spots, pushes back when reasoning is genuinely weak, and never implements.
Fetch up-to-date documentation for any library, framework, API, or service into context. Use when the user wants to look up API references, check function signatures or required fields, find feature-specific docs, or verify how an external tool actually works. Triggers for queries about third-party libraries like Stripe, SQLAlchemy, Tailwind, FastAPI, shadcn, Drizzle, Hono, Better Auth — any time the answer lives in official docs rather than in the project codebase. Use this instead of guessing from trained knowledge, which is stale.
Fix bugs and broken behavior when there is enough evidence to act on a repair path. Use for errors, crashes, incorrect results, API failures (500, 404, 403), CORS problems, database exceptions, broken rendering, duplicated or wrong data, off-by-one mistakes, timezone/date bugs, broken forms, config-caused runtime failures, and regressions. Trigger when the user wants the bug repaired and the conversation already contains a clear failing area, a reproducible failing test, a concrete error path, or a prior diagnosis to implement. Do NOT use for new features, pure explanation, architecture discussion, broad research, or bug reports where the main need is figuring out why the behavior happens — use diagnose for that.