Validate a request body with a Pydantic model

I wrote the route but I'm still hand-checking every field myself, and I know FastAPI is supposed to do this for me.

BeginnerRead 4 minDoing: half an hour, with your project open6 steps

▶ Start

What you'll have at the end

An endpoint that rejects a malformed request with a 422 before your function runs

Before you start

You need

A working route that already accepts a POST request, currently reading the body as a plain dictionary or an untyped parameter with no checks of its own.

Not covered

Confirming the email or username isn't already taken. That's a database lookup your route runs after this validation passes, not a rule the model itself can enforce.

Leans on

See the shape enforce itself

Define the shape of a signup request
Type the route's body parameter with it
Send it something wrong and read the answer

Teach it the rules a type can't express

Add real limits with field constraints
Block a value no length rule can catch
Catch the two passwords disagreeing

Where to go next

Every API error looks different right now

for the errors you raise yourself on purpose, like a 404 for a record that doesn't exist, which need the same consistent shape as this recipe's automatic 422 but are a different check entirely.

Checked 18 Aug 2026

Part of the FastAPI cookbook