Fastapi Knowledge Patch

FastAPI Knowledge Patch

Use this skill when changing a FastAPI application or its Pydantic, SQLModel, Starlette, or Uvicorn integration. Inspect the project's pinned versions before applying version-dependent guidance, and trust its code and tests when they demonstrate different behavior.

Reference index

Breaking changes and deprecations

Meet the runtime and dependency floors

Check FastAPI, Python, Pydantic, Starlette, HTTPX, and optional serializer pins together. Python 3.14 applications must use Pydantic V2, and FastAPI 0.129+ requires Python 3.10. Upgrade pydantic and pydantic-core together because Pydantic 2.12 rejects a mismatched core at startup. Starlette compatibility has changed repeatedly, so confirm the range supported by the pinned FastAPI release rather than upgrading it independently. Read upgrading-and-compatibility.md before changing dependency constraints.

Finish Pydantic V1 migration

FastAPI 0.119 allowed V1 and V2 models together only as a migration bridge. Move every pydantic.v1 import to V2 before FastAPI 0.128 and before Python 3.14. Do not plan new mixed-model designs around the removed bridge.

Replace deprecated integrations

• Replace fastapi.middleware.wsgi.WSGIMiddleware with a2wsgi.WSGIMiddleware.
• Depend on fastapi, not the deprecated fastapi-slim wrapper.
• Replace ORJSONResponse and UJSONResponse with an ordinary typed return or response_model; FastAPI's standard JSON path now serializes with Pydantic.
• Replace Starlette registration decorators and startup/shutdown callbacks with lifespan plus declarative routes, middleware, and exception handlers.
• Stop calling removed Uvicorn Config.setup_event_loop() and migrate away from the deprecated uvicorn.workers module.

Send an explicit JSON media type

FastAPI 0.132 rejects a JSON request without a valid JSON Content-Type by default. Fix clients to send application/json. Use the compatibility switch only while migrating clients:

from fastapi import FastAPI

app = FastAPI(strict_content_type=False)

The strict default also blocks a narrow class of credential-free, headerless browser requests from bypassing CORS preflight; it does not replace authentication for privileged endpoints.

Update schema consumers

• Expect bytes schemas to use contentMediaType: application/octet-stream, not format: binary, from FastAPI 0.129.1.
• Expect validation-error schemas to include input and ctx.
• Expect Pydantic 2.12 changes for decimal patterns, function titles, typed-dictionary additionalProperties, and configurable primitive-union type arrays.
• Preserve literal schema attributes named $ref and accept OpenAPI type arrays.

Dependency lifecycles

FastAPI 0.118 keeps default yield dependencies alive until the response has finished, including streaming responses. FastAPI 0.121 makes cleanup timing explicit:

Choose the shortest scope that still covers resource use:

from typing import Annotated
from fastapi import Depends, FastAPI

app = FastAPI()

def session():
    resource = open_resource()
    try:
        yield resource
    finally:
        resource.close()

@app.get("/items")
def items(
    resource: Annotated[Resource, Depends(session, scope="function")],
):
    return read_items(resource)

A request-scoped dependency cannot use a function-scoped child because it may need that child during teardown. Scoped dependency trees also do not use the ordinary unscoped cache path. Re-raise exceptions caught after yield; merely swallowing them can hide useful server logging.

Read dependency-injection.md before changing scopes, wrappers, partials, callable classes, or forward annotations.

Streaming response selection

Choose the response path deliberately:

from collections.abc import AsyncIterable
from fastapi import FastAPI
from fastapi.sse import EventSourceResponse, ServerSentEvent

app = FastAPI()

@app.get("/events", response_class=EventSourceResponse)
async def events() -> AsyncIterable[ServerSentEvent]:
    yield ServerSentEvent(
        data={"status": "ready"}, event="status", id="1", retry=5000
    )
    yield ServerSentEvent(raw_data="[DONE]", event="done")

data is JSON encoded; mutually exclusive raw_data carries preformatted text. SSE defaults include a 15-second keepalive, Cache-Control: no-cache, and X-Accel-Buffering: no. Read responses-and-streaming.md for status codes, model filtering, cancellation, router inclusion, and multiline events.

Requests and security

Built-in authentication dependencies return 401 Unauthorized, rather than 403 Forbidden, when credentials are missing. Preserve an old 403 contract by overriding make_not_authenticated_error() and returning an exception instance. Credentials from Authorization have surrounding whitespace removed, and nested security scopes now propagate into runtime checks and OpenAPI.

Parameter parsing now handles union-valued forms, empty controls, extra list values, aliases on Query, Header, and Cookie models, optional sequences, and tagged discriminated-union bodies. Read requests-and-security.md before keeping compatibility workarounds.

Pydantic and OpenAPI essentials

For Pydantic 2.12:

• Define @model_validator(mode="after") as an instance method.
• Use MISSING to distinguish omission from None.
• Use Field(exclude_if=...) and exclude_computed_fields=True for selective serialization.
• Use model_validate(data, extra="forbid") for a one-call extra-field policy.
• Use union_format="primitive_type_array" where a consumer prefers eligible primitive unions as a type array.
• Rebuild dynamic fields from FieldInfo.asdict() instead of mutating reused FieldInfo objects.
• Use ValidateAs to validate a custom class through a supported intermediate type.

FastAPI accepts OpenAPI type arrays and external_docs, emits top-level security schemes correctly, and supports PEP 695 aliases in endpoint types. Read openapi-and-pydantic.md before changing validation, serialization, dynamic models, discriminators, or schema generation.

Static frontends and CLI

app.frontend() and router.frontend() serve already-built static output after API route matching. They are not server-side rendering. Normal routes win, and frontend responses remain inside middleware and inherit dependencies. fallback="auto" prefers 404.html; otherwise it uses index.html only for missing browser navigations, not missing assets.

Persist application discovery for fastapi dev, editors, and deployment:

[tool.fastapi]
entrypoint = "backend.main:app"

Read frontend-cli-and-docs.md before changing fallback selection, directory checks, frontend dependencies, CLI extras, deployment, Vibe, or ReDoc customization.

Ecosystem checks

• Read sqlmodel.md before changing SQLModel package pins, Pydantic field forms, DML execution, relationships, or cascades.
• Read starlette.md before adopting Starlette 1.0 or changing lifespan, templates, CORS, cookies, multipart limits, files, or response transmission.
• Read uvicorn.md before changing workers, reload, custom protocols, proxy trust, Unix sockets, root paths, WebSockets, environment configuration, or logging.