How do you receive and validate a JSON request body in FastAPI?

Declare a Pydantic BaseModel as a function parameter. FastAPI automatically reads the JSON body, validates it against the model, and provides it as a typed Python object. If validation fails it returns a 422 with details.

from fastapi import FastAPI
from pydantic import BaseModel
from typing import Annotated
from pydantic import Field

app = FastAPI()

class Item(BaseModel):
    name: str
    description: str | None = None  # optional field
    price: float
    tax: float | None = None

@app.post("/items")
def create_item(item: Item):  # body parsed + validated automatically
    total = item.price + (item.tax or 0)
    return {**item.model_dump(), "total_price": total}

# Mixing path param + query param + body in the same endpoint
@app.put("/items/{item_id}")
def update_item(
    item_id: int,          # path
    q: str | None = None,  # query
    item: Item | None = None,  # body (optional)
):
    result = {"item_id": item_id}
    if q:
        result["q"] = q
    if item:
        result.update(item.model_dump())
    return result

FastAPI resolves the source of each parameter automatically:

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