Filters for Python + FastAPI

Arcjet filters let you block requests using Wireshark-like display filter expressions over HTTP headers, IP addresses, and other request fields. This lets you enforce rules such as allow or deny by country, network, or user-agent.

What is Arcjet?

Arcjet is the runtime security platform that ships with your code. Enforce budgets, stop prompt injection, detect bots, and protect personal information with Arcjet's AI security building blocks.

Quick start

This guide shows how to protect your app with filters.

1. Install Arcjet

In your project root, install the SDK:

In your project root, install the Arcjet Python SDK along with FastAPI and Uvicorn:

uv add arcjet fastapi uvicorn

If you don’t use uv, pip install arcjet fastapi uvicorn (or your preferred package manager’s equivalent) works too.

2. Set your key

Create a free Arcjet account and follow the instructions to add a site and get a key.

Add your key to a .env file in your project root. Arcjet does not read ARCJET_KEY automatically – your application must pass it explicitly to arcjet(key=...).

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# Run Arcjet in development <https://docs.arcjet.com/environment#arcjet-env>.
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ARCJET_ENV=development
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# Arcjet key for your site (from <https://console.arcjet.com>).
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# More info: <https://docs.arcjet.com/environment#arcjet-key>.
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ARCJET_KEY=ajkey_yourkey

Install python-dotenv and call load_dotenv() before reading the environment, or use FastAPI’s pydantic-settings integration which loads .env automatically.

3. Add filters

Arcjet protection runs on HTTP requests. You can add Arcjet to your FastAPI app to protect routes.

Create a new file at main.py with the contents:

import os

from arcjet import Mode, arcjet, filter_request
from fastapi import FastAPI, Request
from fastapi.responses import JSONResponse

app = FastAPI()

aj = arcjet(
    key=os.environ["ARCJET_KEY"],  # Get your site key from https://console.arcjet.com
    rules=[
        filter_request(
            mode=Mode.LIVE,  # Blocks requests. Use Mode.DRY_RUN to log only
            # This will deny any traffic using a VPN, Tor, that matches the curl
            # user agent, or that has no user agent
            deny=[
                'ip.src.vpn or ip.src.tor or lower(http.request.headers["user-agent"]) matches "curl" or len(http.request.headers["user-agent"]) eq 0',
            ],
        ),
    ],
)


@app.get("/")
async def index(request: Request):
    decision = await aj.protect(request)

    if decision.is_denied():
        return JSONResponse({"error": "Forbidden"}, status_code=403)

    return {"message": "Hello world"}

4. Start server

Start the FastAPI dev server:

uv run uvicorn main:app --reload

5. Try it out

Make a curl request from your terminal to your app:

curl --head http://localhost:8000

You get a 403 Forbidden response.

The Curl request could result in a 200 OK if you have a .curlrc somewhere (probably ~/.curlrc) that sets a user-agent field to something other than Curl.

Try changing the filter expression to match "chrome" or "firefox" and visit your app with different browsers.

These requests also appear in the Arcjet dashboard. Make sure you configured ARCJET_KEY correctly if things don’t show up.

See the Filters reference to learn more about the expression language.

FAQs

Do I need to run any infrastructure e.g. Redis?

No, Arcjet handles all the infrastructure for you so you don't need to worry about deploying global Redis clusters, designing data structures to track rate limits, or keeping security detection rules up to date.

What is the performance overhead?

Arcjet SDK tries to do as much as possible asynchronously and locally to minimize latency for each request. Where decisions can be made locally or previous decisions are cached in-memory, latency is usually <1ms.

When a call to the Cloud API is required, such as when tracking a rate limit in a serverless environment, there is some additional latency before a decision is made. The Cloud API has been designed for high performance and low latency, and is deployed to multiple regions around the world. The SDK will automatically use the closest region which means the total overhead is typically no more than 20-30ms, often significantly less.

What happens if Arcjet is unavailable?

Where a decision has been cached locally e.g. blocking a client, Arcjet will continue to function even if the service is unavailable.

If a call to the Cloud API is needed and there is a network problem or Arcjet is unavailable, the default behavior is to fail open and allow the request. You have control over how to handle errors, including choosing to fail close if you prefer. See the reference docs for details.

How does Arcjet protect me against DDoS attacks?

Network layer attacks tend to be generic and high volume, so these are best handled by your hosting platform. Most cloud providers include network DDoS protection by default.

Arcjet sits closer to your application so it can understand the context. This is important because some types of traffic may not look like a DDoS attack, but can still have the same effect. For example, a customer making too many API requests and affecting other customers, or large numbers of signups from disposable email addresses.

Network-level DDoS protection tools find it difficult to protect against this type of traffic because they don't understand the structure of your application. Arcjet can help you to identify and block this traffic by integrating with your codebase and understanding the context of the request e.g. the customer ID or sensitivity of the API route.

Volumetric network attacks are best handled by your hosting provider. Application level attacks need to be handled by the application. That's where Arcjet helps.

What next?

Filter reference Full details about filters.

Testing Write tests for your rules.

Explore

Arcjet can be used with specific rules on individual routes or as general protection on your entire application. You can set up rate limiting for your API, minimize fraudulent registrations with the signup form protection and more.

Example apps See the example apps.

Learn how Arcjet works Arcjet's architecture.

SDK reference Python SDK reference guide.

Get help

Need help with anything? Email support@arcjet.com to get support from our engineering team.