Bot protection for Python + FastAPI

Arcjet bot detection lets you manage traffic by automated clients and bots.

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 you how to protect your application from bots and automated clients.

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 then 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>).
4
# 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 bot protection

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

import os

from arcjet import BotCategory, Mode, arcjet, detect_bot, is_spoofed_bot
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=[
        detect_bot(
            mode=Mode.LIVE,  # Blocks requests. Use Mode.DRY_RUN to log only
            # Block all bots except the following
            allow=[
                BotCategory.SEARCH_ENGINE,  # Google, Bing, etc
                # Uncomment to allow these other common bot categories
                # See the full list at https://arcjet.com/bot-list
                # BotCategory.MONITOR,  # Uptime monitoring services
                # BotCategory.PREVIEW,  # Link previews e.g. Slack, Discord
            ],
        ),
    ],
)


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

    # Bots not in the allow list will be blocked
    if decision.is_denied():
        return JSONResponse({"error": "Forbidden"}, status_code=403)

    # Requests from hosting IPs are likely from bots, so they can usually be
    # blocked. However, consider your use case — if this is an API endpoint
    # then hosting IPs might be legitimate.
    # https://docs.arcjet.com/blueprints/vpn-proxy-detection
    if decision.ip.is_hosting():
        return JSONResponse({"error": "Forbidden"}, status_code=403)

    # Paid Arcjet accounts include additional verification checks using IP data.
    # Verification isn't always possible, so we recommend checking the results
    # separately.
    # https://docs.arcjet.com/bot-protection/reference#bot-verification
    if any(is_spoofed_bot(r) for r in decision.results):
        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

Then make a curl request from your terminal. You get a 403 Forbidden response because curl is considered an automated client by default. See the reference guide to learn about configuring this.

curl -I http://localhost:8000

The requests also appear in the Arcjet dashboard.

5. Choose bots to allow or deny

Arcjet maintains a list of known bots so you can choose to either only allow specific bots or only deny specific bots. Pass exactly one of allow or deny on each rule. An empty allow list blocks every detected bot.

Bots can be identified individually or through managed categories. For more details, see Identifying bots.

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?

Bot protection reference Full details about bot protection.

Advanced bot signals Browser-based bot detection without CAPTCHAs.

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.