Trailing slash behavior in FastAPI

Introduction

Trailing slash behavior controls how FastAPI handles URLs that end with or without a slash. It helps avoid confusion and broken links.

When you want to decide if '/items' and '/items/' should be treated the same or differently.

When building APIs that need consistent URL patterns for clients.

When you want to avoid 404 errors caused by missing or extra slashes in URLs.

Syntax

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/path/")
async def with_slash():
    return {"message": "URL ends with slash"}

@app.get("/path")
async def without_slash():
    return {"message": "URL without slash"}

FastAPI treats routes with and without trailing slashes as different by default.

You can define both versions if you want to support both URLs.

Examples

This route matches only URLs ending with a slash, like '/hello/'.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/hello/")
async def hello_slash():
    return {"msg": "Hello with slash"}

This route matches only URLs without a trailing slash, like '/hello'.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/hello")
async def hello_no_slash():
    return {"msg": "Hello without slash"}

Defining both routes lets you handle both '/hello' and '/hello/' separately.

FastAPI

from fastapi import FastAPI

app = FastAPI()

@app.get("/hello")
async def hello_no_slash():
    return {"msg": "Hello without slash"}

@app.get("/hello/")
async def hello_slash():
    return {"msg": "Hello with slash"}

Sample Program

This example shows two routes: one with and one without a trailing slash. It tests both URLs and prints their JSON responses.

FastAPI

from fastapi import FastAPI
from fastapi.testclient import TestClient

app = FastAPI()

@app.get("/greet")
async def greet_no_slash():
    return {"message": "Hello without slash"}

@app.get("/greet/")
async def greet_with_slash():
    return {"message": "Hello with slash"}

client = TestClient(app)

response1 = client.get("/greet")
response2 = client.get("/greet/")

print(response1.json())
print(response2.json())

Important Notes

FastAPI does not automatically redirect between slash and no-slash URLs.

Be consistent with your URL style to avoid confusion for API users.

You can use middleware or custom logic if you want to enforce one style.

Summary

Trailing slash matters in FastAPI URLs and affects route matching.

Define routes carefully to support or avoid both slash and no-slash URLs.

Test your API URLs to ensure they behave as expected.