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fastapi-ai-scaffold

Coding & development

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Updated 10-15 20:33

About this skill

FastAPI AI Scaffold

Generates a runnable, production-shaped FastAPI AI/ML service so you don't re-derive the same wiring every time. The skeleton encodes the patterns from the fastapi-ai-patterns skill as working code: nothing here is novel framework usage, it's the boring-but-correct baseline an inference service needs before it sees traffic.

Inspired by FastAPI for AI Engineers (AI Engineering Insider, 2026). All generated code is original.

When to use

  • "Start/bootstrap a new FastAPI service for serving a model / LLM / RAG / embeddings"
  • "Set up a FastAPI project with clean architecture (router/service/repository)"
  • The user wants the production basics pre-wired (lifespan model loading, auth, DB, probes, tests, Docker) rather than a single-file main.py

When NOT to use

  • The user just wants to understand a pattern or debug an existing app → use fastapi-ai-patterns
  • They're prepping for an interview → use fastapi-ai-interview-prep
  • They already have a project structure and want one endpoint added → don't regenerate; hand-write the endpoint following fastapi-ai-patterns

Workflow

1. Generate the project

bash skills/local/fastapi-ai-scaffold/scripts/new-fastapi-ai-service.sh ./my-service
# preview first, write nothing:
bash skills/local/fastapi-ai-scaffold/scripts/new-fastapi-ai-service.sh --dry-run ./my-service
# override the package/metadata slug:
bash skills/local/fastapi-ai-scaffold/scripts/new-fastapi-ai-service.sh --name churn-scorer ./services/churn

The script copies the assets/project/ skeleton, strips each file's .tmpl suffix, and substitutes the project slug. It prints a JSON summary (project, slug, path, files, next_steps[]) on stdout.

2. Install and run

cd ./my-service
uv venv && source .venv/bin/activate
uv pip install -e ".[dev]"
cp .env.example .env          # then set JWT_SECRET, DATABASE_URL, MODEL_PATH
uvicorn app.main:app --reload # http://127.0.0.1:8000/docs
pytest -q                     # the bundled tests should pass immediately

3. Make it yours

Replace the stubs with your real implementation — the structure stays:

  • app/ml/model.py — swap load_model/Model.predict for your real artifact; swap stream_llm_tokens for a real provider/vLLM call. Keep predict synchronous and CPU-bound; the service already offloads it off the event loop.
  • app/models/, app/schemas/, app/repositories/, app/services/ — add your domain. Keep HTTP concerns in routers and business logic in services.
  • Generate the first migration once models are real: alembic revision --autogenerate -m "init" then alembic upgrade head. Review the migration before applying — autogenerate misses renames/data moves.

What the generated project contains

Clean-architecture layout under app/ (router → service → repository), plus:

  • app/lifespan.py — model handle, shared httpx.AsyncClient, and DB engine created once at startup and stored on app.state (per-process singletons).
  • app/api/v1/health.py — cheap /health (liveness) vs dependency-checking /ready (readiness), so a DB blip removes a pod from rotation instead of restarting it.
  • app/api/v1/auth.py + app/core/security.py — bcrypt hashing, JWT with a pinned algorithm, a get_current_user dependency.
  • app/api/v1/inference.py — a /predict endpoint (CPU work offloaded via anyio.to_thread) and a /chat/stream LLM gateway using SSE.
  • app/ml/guardrails.py — input checks and the generate → validate → retry loop that turns LLM output into a typed Pydantic value.
  • tests/ — dependency_overrides + in-memory SQLite, exact-key-set leak assertions, generic-login-error assertion.
  • Ops — Dockerfile (multi-stage, non-root, HEALTHCHECK), gunicorn_conf.py (Uvicorn workers), pyproject.toml, .env.example, Alembic env.

Available scripts

  • scripts/new-fastapi-ai-service.sh <target-dir> — Generate the project.
    • Flags: --name SLUG, --force, --dry-run, --help.
    • Output: JSON summary on stdout; progress on stderr.

Bundled assets

  • assets/project/ — the skeleton. Every file ends in .tmpl; the script strips the suffix and substitutes PROJECT_SLUG_PLACEHOLDER. Edit these .tmpl files to change what new projects get.

Gotchas

  • .tmpl is mandatory on skeleton files. The script only strips .tmpl and substitutes the slug; a skeleton file without the suffix is copied verbatim (intentional, e.g. migrations/versions/.gitkeep). Keep real app files .tmpl so editors don't try to import their (uninstalled) dependencies.
  • The generated package is named app, not the slug. Only project metadata (pyproject name, README title, .env APP_NAME) uses the slug; imports are always app.*. Don't rename the package without updating every import.
  • The bundled tests use bare TestClient and skip lifespan on purpose — they inject app.state doubles + dependency_overrides. If you write a test that relies on real startup, use with TestClient(app) as client: instead.
  • The model and LLM in app/ml/model.py are deterministic stubs, not real inference. Tests assert shape/behavior, not model quality. Replace before deploy.
  • init_db (create_all) runs only in environment=development. Production schema must go through Alembic; don't rely on create_all in prod.