Scalar has started a vendor-neutral series with the sentence most FastAPI teams need to hear. In OpenAPI with FastAPI, part one of three, the post notes that FastAPI generates an OpenAPI document automatically from type hints, Pydantic models and decorators, served at /openapi.json. “The interesting work is not turning it on. It is making the document accurate enough that other tools can rely on it.” Everything in part one is plain FastAPI and Pydantic, and “it applies whatever you later point at the document: a documentation UI, an SDK generator, a linter, or a contract test.”
There are no figures beyond a version pin, which is itself good practice: behaviour is described “as of September 2026, when the latest release was 0.141.1.” The most practical warning concerns versions. Since FastAPI 0.99.0 the output is OpenAPI 3.1.0 with JSON Schema 2020-12, so nullable fields come out as anyOf with a null type, which “catches people who feed the document to a tool that only reads 3.0.” The most common gap is errors: FastAPI knows the success response and adds a 422 for validation, but “it does not know about the 404 you raise with HTTPException inside the function, because that happens at runtime.” The fix is to declare every response, with a model when the body has a shape. The smaller advice adds up: tags with descriptions because SDK generators use them as namespaces, docstrings as operation descriptions, and explicit summaries for functions named like handle_v2_lookup.
The catalog holds Scalar as one of the better-described tool vendors. The Scalar provider page lists API pages for its products, including Scalar API References, which renders documents like the one the post describes, the Scalar Registry that stores them, and Scalar SDKs that generate from them. On the Agent Readiness score, the MCP server, agent skills, a well-known catalog, error semantics, examples and the full identity layer are lit.
The Kin Score is 70.8, exemplar band, carried by contract quality at 78.1 and developer ergonomics at 73.2, with contract governance at 45.5. The Agent Readiness score is 57.6, agent-ready. The post’s central advice, document every response including the errors raised at runtime, is the error semantics dimension written as a FastAPI tutorial, and Scalar’s own record lights it. Idempotency is unlit, as are dry-run mode and the agent card. Scalar has written the guide that turns a generated document into one other tools can trust. Its own contract already follows most of it.