Prompt injection detection for Mastra

Arcjet prompt injection detection evaluates each incoming prompt for injection patterns inside your application before it reaches the AI provider. Detected attacks are blocked before the AI call is made, protecting both your application behavior and your AI budget.

What is Arcjet?

Arcjet is the AI agent runtime security platform. Discover the agents running in your organization, enforce policy across every action, prompt, and tool call, and keep the evidence to prove what happened. Detect prompt injection, authorize agent tool calls, redact PII, and block bots and abuse.

Quick start

This example screens inbound user text for prompt injection before the model runs.

We assume you already have a Mastra project set up. For helper options and denial behavior, see the Mastra agent guard.

Install the dependencies:

# Export your Arcjet API key from https://console.arcjet.com
export ARCJET_KEY="ajkey_..."

npm install @arcjet/guard @mastra/core

Create the example:

agent.ts

import { launchArcjet, detectPromptInjection } from "@arcjet/guard";
import { guardProcessor } from "@arcjet/guard/mastra/v1";
import { Agent } from "@mastra/core/agent";

const arcjet = launchArcjet({ key: process.env.ARCJET_KEY! });

const inbound = guardProcessor(arcjet, {
  action: "message.received",
  rules: ({ text }) => [detectPromptInjection()(text)],
});

export const agent = new Agent({
  id: "support-agent",
  name: "support-agent",
  instructions: "Help the user.",
  model: "openai/gpt-4o-mini",
  inputProcessors: [inbound],
});

Then start or invoke the agent with a test prompt.

Requests appear in your Arcjet dashboard in real time.

What next?

Prompt injection detection intro Learn how prompt injection detection works and when to use it.

AI abuse protection Combine with bot detection for complete AI app protection.

AI data loss prevention Prevent PII from entering model context alongside prompt injection protection.

Testing Write tests for your rules.

Get help

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

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