Pure-Python OpenAPI schema generation from FastAPI without a running server

FastAPI's OpenAPI schema can be generated purely from Python imports without starting a server or connecting to a database. The key insight is that get_openapi(routes=app.routes) only introspects Pydantic models and route signatures — it has no I/O dependencies.

from fsrv.main import get_app  # or however your app factory works
from fastapi.openapi.utils import get_openapi

app = get_app()
schema = get_openapi(
    title=app.title, version=app.version,
    routes=app.routes,
    separate_input_output_schemas=False,
)

Side effects of get_app() that are harmless in a build-tool context:

• Config loading (reads TOML/env — already available in dev)
• Sentry SDK init (no-op consequence for codegen)
• SQLAlchemy engine creation (lazy — no actual DB connection until a query runs, objects GC'd immediately)
• Middleware registration (stored on app object, never invoked)

This is useful for CI/CD pipelines and SDK generation tools (like oazapfts) where you want to generate the schema without requiring a running backend. The schema is byte-for-byte identical to what the server would serve at /openapi.json.

One gotcha: if your app factory conditionally registers routes based on service_mode config, make sure the config used at generation time matches your production service mode.