fastapi ยท diff
git:20260518.3fc52ce to git:20260610.36508ae
430 added, 4 removed. Audit A to A.
---
name: fastapi
description: FastAPI best practices + Pydantic. Use when building or reviewing FastAPI APIs.
group: brana
keywords: [fastapi, python, pydantic, api, async, uvicorn]
allowed-tools: [Read, Glob, Grep, WebFetch, AskUserQuestion]
status: experimental
source: "https://skills.sh/fastapi/fastapi/fastapi"
acquired: "2026-05-17"
promoted: "2026-05-17"
---
+ # FastAPI
- <!-- PROCEDURE_FILE: procedures/fastapi.md -->
- This skill's full procedure is in a separate file for startup performance (ADR-034).
- Read and execute `system/procedures/fastapi.md` from the plugin root directory.
- If the path doesn't resolve, use Glob to find `**/procedures/fastapi.md`.
+ Official FastAPI skill to write code with best practices, keeping up to date with new versions and features.
+
+ ## Use the `fastapi` CLI
+
+ Run the development server on localhost with reload:
+
+ ```bash
+ fastapi dev
+ ```
+
+
+ Run the production server:
+
+ ```bash
+ fastapi run
+ ```
+
+ ### Add an entrypoint in `pyproject.toml`
+
+ FastAPI CLI will read the entrypoint in `pyproject.toml` to know where the FastAPI app is declared.
+
+ ```toml
+ [tool.fastapi]
+ entrypoint = "my_app.main:app"
+ ```
+
+ ### Use `fastapi` with a path
+
+ When adding the entrypoint to `pyproject.toml` is not possible, or the user explicitly asks not to, or it's running an independent small app, you can pass the app file path to the `fastapi` command:
+
+ ```bash
+ fastapi dev my_app/main.py
+ ```
+
+ Prefer to set the entrypoint in `pyproject.toml` when possible.
+
+ ## Use `Annotated`
+
+ Always prefer the `Annotated` style for parameter and dependency declarations.
+
+ It keeps the function signatures working in other contexts, respects the types, allows reusability.
+
+ ### In Parameter Declarations
+
+ Use `Annotated` for parameter declarations, including `Path`, `Query`, `Header`, etc.:
+
+ ```python
+ from typing import Annotated
+
+ from fastapi import FastAPI, Path, Query
+
+ app = FastAPI()
+
+
+ @app.get("/items/{item_id}")
+ async def read_item(
+ item_id: Annotated[int, Path(ge=1, description="The item ID")],
+ q: Annotated[str | None, Query(max_length=50)] = None,
+ ):
+ return {"message": "Hello World"}
+ ```
+
+ instead of:
+
+ ```python
+ # DO NOT DO THIS
+ @app.get("/items/{item_id}")
+ async def read_item(
+ item_id: int = Path(ge=1, description="The item ID"),
+ q: str | None = Query(default=None, max_length=50),
+ ):
+ return {"message": "Hello World"}
+ ```
+
+ ### For Dependencies
+
+ Use `Annotated` for dependencies with `Depends()`.
+
+ Unless asked not to, create a new type alias for the dependency to allow re-using it.
+
+ ```python
+ from typing import Annotated
+
+ from fastapi import Depends, FastAPI
+
+ app = FastAPI()
+
+
+ def get_current_user():
+ return {"username": "johndoe"}
+
+
+ CurrentUserDep = Annotated[dict, Depends(get_current_user)]
+
+
+ @app.get("/items/")
+ async def read_item(current_user: CurrentUserDep):
+ return {"message": "Hello World"}
+ ```
+
+ instead of:
+
+ ```python
+ # DO NOT DO THIS
+ @app.get("/items/")
+ async def read_item(current_user: dict = Depends(get_current_user)):
+ return {"message": "Hello World"}
+ ```
+
+ ## Do not use Ellipsis for *path operations* or Pydantic models
+
+ Do not use `...` as a default value for required parameters, it's not needed and not recommended.
+
+ Do this, without Ellipsis (`...`):
+
+ ```python
+ from typing import Annotated
+
+ from fastapi import FastAPI, Query
+ from pydantic import BaseModel, Field
+
+
+ class Item(BaseModel):
+ name: str
+ description: str | None = None
+ price: float = Field(gt=0)
+
+
+ app = FastAPI()
+
+
+ @app.post("/items/")
+ async def create_item(item: Item, project_id: Annotated[int, Query()]): ...
+ ```
+
+ instead of this:
+
+ ```python
+ # DO NOT DO THIS
+ class Item(BaseModel):
+ name: str = ...
+ description: str | None = None
+ price: float = Field(..., gt=0)
+
+
+ app = FastAPI()
+
+
+ @app.post("/items/")
+ async def create_item(item: Item, project_id: Annotated[int, Query(...)]): ...
+ ```
+
+ ## Return Type or Response Model
+
+ When possible, include a return type. It will be used to validate, filter, document, and serialize the response.
+
+ ```python
+ from fastapi import FastAPI
+ from pydantic import BaseModel
+
+ app = FastAPI()
+
+
+ class Item(BaseModel):
+ name: str
+ description: str | None = None
+
+
+ @app.get("/items/me")
+ async def get_item() -> Item:
+ return Item(name="Plumbus", description="All-purpose home device")
+ ```
+
+ **Important**: Return types or response models are what filter data ensuring no sensitive information is exposed. And they are used to serialize data with Pydantic (in Rust), this is the main idea that can increase response performance.
+
+ The return type doesn't have to be a Pydantic model, it could be a different type, like a list of integers, or a dict, etc.
+
+ ### When to use `response_model` instead
+
+ If the return type is not the same as the type that you want to use to validate, filter, or serialize, use the `response_model` parameter on the decorator instead.
+
+ ```python
+ from typing import Any
+
+ from fastapi import FastAPI
+ from pydantic import BaseModel
+
+ app = FastAPI()
+
+
+ class Item(BaseModel):
+ name: str
+ description: str | None = None
+
+
+ @app.get("/items/me", response_model=Item)
+ async def get_item() -> Any:
+ return {"name": "Foo", "description": "A very nice Item"}
+ ```
+
+ This can be particularly useful when filtering data to expose only the public fields and avoid exposing sensitive information.
+
+ ```python
+ from typing import Any
+
+ from fastapi import FastAPI
+ from pydantic import BaseModel
+
+ app = FastAPI()
+
+
+ class InternalItem(BaseModel):
+ name: str
+ description: str | None = None
+ secret_key: str
+
+
+ class Item(BaseModel):
+ name: str
+ description: str | None = None
+
+
+ @app.get("/items/me", response_model=Item)
+ async def get_item() -> Any:
+ item = InternalItem(
+ name="Foo", description="A very nice Item", secret_key="supersecret"
+ )
+ return item
+ ```
+
+ ## Performance
+
+ Do not use `ORJSONResponse` or `UJSONResponse`, they are deprecated.
+
+ Instead, declare a return type or response model. Pydantic will handle the data serialization on the Rust side.
+
+ ## Including Routers
+
+ When declaring routers, prefer to add router level parameters like prefix, tags, etc. to the router itself, instead of in `include_router()`.
+
+ Do this:
+
+ ```python
+ from fastapi import APIRouter, FastAPI
+
+ app = FastAPI()
+
+ router = APIRouter(prefix="/items", tags=["items"])
+
+
+ @router.get("/")
+ async def list_items():
+ return []
+
+
+ # In main.py
+ app.include_router(router)
+ ```
+
+ instead of this:
+
+ ```python
+ # DO NOT DO THIS
+ from fastapi import APIRouter, FastAPI
+
+ app = FastAPI()
+
+ router = APIRouter()
+
+
+ @router.get("/")
+ async def list_items():
+ return []
+
+
+ # In main.py
+ app.include_router(router, prefix="/items", tags=["items"])
+ ```
+
+ There could be exceptions, but try to follow this convention.
+
+ Apply shared dependencies at the router level via `dependencies=[Depends(...)]`.
+
+ ## Dependency Injection
+
+ See the dependency injection reference (upstream `references/dependencies.md`, not installed locally) for detailed patterns including `yield` with `scope`, and class dependencies.
+
+ Use dependencies when the logic can't be declared in Pydantic validation, depends on external resources, needs cleanup (with `yield`), or is shared across endpoints.
+
+ Apply shared dependencies at the router level via `dependencies=[Depends(...)]`.
+
+ ## Async vs Sync *path operations*
+
+ Use `async` *path operations* only when fully certain that the logic called inside is compatible with async and await (it's called with `await`) or that doesn't block.
+
+ ```python
+ from fastapi import FastAPI
+
+ app = FastAPI()
+
+
+ # Use async def when calling async code
+ @app.get("/async-items/")
+ async def read_async_items():
+ data = await some_async_library.fetch_items()
+ return data
+
+
+ # Use plain def when calling blocking/sync code or when in doubt
+ @app.get("/items/")
+ def read_items():
+ data = some_blocking_library.fetch_items()
+ return data
+ ```
+
+ In case of doubt, or by default, use regular `def` functions, those will be run in a threadpool so they don't block the event loop.
+
+ The same rules apply to dependencies.
+
+ Make sure blocking code is not run inside of `async` functions. The logic will work, but will damage the performance heavily.
+
+ When needing to mix blocking and async code, see Asyncer in the other tools reference (upstream `references/other-tools.md`, not installed locally).
+
+ ## Streaming (JSON Lines, SSE, bytes)
+
+ See the streaming reference (upstream `references/streaming.md`, not installed locally) for JSON Lines, Server-Sent Events (`EventSourceResponse`, `ServerSentEvent`), and byte streaming (`StreamingResponse`) patterns.
+
+ ## Tooling
+
+ See the other tools reference (upstream `references/other-tools.md`, not installed locally) for details on uv, Ruff, ty for package management, linting, type checking, formatting, etc.
+
+ ## Other Libraries
+
+ See the other tools reference (upstream `references/other-tools.md`, not installed locally) for details on other libraries:
+
+ * Asyncer for handling async and await, concurrency, mixing async and blocking code, prefer it over AnyIO or asyncio.
+ * SQLModel for working with SQL databases, prefer it over SQLAlchemy.
+ * HTTPX for interacting with HTTP (other APIs), prefer it over Requests.
+
+ ## Do not use Pydantic RootModels
+
+ Do not use Pydantic `RootModel`, instead use regular type annotations with `Annotated` and Pydantic validation utilities.
+
+ For example, for a list with validations you could do:
+
+ ```python
+ from typing import Annotated
+
+ from fastapi import Body, FastAPI
+ from pydantic import Field
+
+ app = FastAPI()
+
+
+ @app.post("/items/")
+ async def create_items(items: Annotated[list[int], Field(min_length=1), Body()]):
+ return items
+ ```
+
+ instead of:
+
+ ```python
+ # DO NOT DO THIS
+ from typing import Annotated
+
+ from fastapi import FastAPI
+ from pydantic import Field, RootModel
+
+ app = FastAPI()
+
+
+ class ItemList(RootModel[Annotated[list[int], Field(min_length=1)]]):
+ pass
+
+
+ @app.post("/items/")
+ async def create_items(items: ItemList):
+ return items
+
+ ```
+
+ FastAPI supports these type annotations and will create a Pydantic `TypeAdapter` for them, so that types can work as normally and there's no need for the custom logic and types in RootModels.
+
+ ## Use one HTTP operation per function
+
+ Don't mix HTTP operations in a single function, having one function per HTTP operation helps separate concerns and organize the code.
+
+ Do this:
+
+ ```python
+ from fastapi import FastAPI
+ from pydantic import BaseModel
+
+ app = FastAPI()
+
+
+ class Item(BaseModel):
+ name: str
+
+
+ @app.get("/items/")
+ async def list_items():
+ return []
+
+
+ @app.post("/items/")
+ async def create_item(item: Item):
+ return item
+ ```
+
+ instead of this:
+
+ ```python
+ # DO NOT DO THIS
+ from fastapi import FastAPI, Request
+ from pydantic import BaseModel
+
+ app = FastAPI()
+
+
+ class Item(BaseModel):
+ name: str
+
+
+ @app.api_route("/items/", methods=["GET", "POST"])
+ async def handle_items(request: Request):
+ if request.method == "GET":
+ return []
+ ```