guard

Guard non-HTTP JavaScript

Use @arcjet/guard when there is no HTTP request. MCP tools, queue workers, and agent tool calls are Guard. HTTP routes are @arcjet/skills#protect.

For a specific vendor SDK, load the matching skill from @arcjet/guard (@arcjet/guard#integrate-arcjet-guard-agents, -eve, -mastra, -langgraph, -langchain, -openai-agents, -genkit, -google-adk, -strands-agents, -tanstack-ai, -cloudflare-think, -claude-agent-sdk, -claude-managed-agents).

Claude Code / Copilot HTTP hooks are not this package. Publish a coding-agent policy via @arcjet/skills#mcp (Execute on: Tool call, Prompt, or Model switch) and install hooks from https://docs.arcjet.com/coding-agents (omit ?surface=).

Client

import { launchArcjet } from "@arcjet/guard";

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

One client at module scope. Get the key with @arcjet/skills#cli first. Declare rules at module scope so .deniedResult(decision) works.

One guard() per operation

Hardcode the label. Do not interpolate in a generic dispatcher.

Hardcode labels as slugs: ASCII letters of either case, digits, -, ., and _ (tools.get-weather). Start and end with a letter or digit; max 256 bytes. Labels match case-sensitively. Prefer dash/dot in new labels. Check a label you build yourself with validateGuardLabel.

const decision = await arcjet.guard({
  label: "tools.get-weather",
  rules: [/* ... */],
  metadata: { user: { id: userId } },
});

if (decision.conclusion === "DENY") {
  const rateLimited = toolCallLimit.deniedResult(decision);
  if (rateLimited) {
    return { error: `rate limited, retry after ${rateLimited.resetAtUnixSeconds}` };
  }
  return { error: decision.reason };
}
if (decision.hasFailedOpen()) {
  // ALLOW only because a rule could not run — deny here if the site is sensitive
}

Core guard() fails open — check hasFailedOpen(). warnings never change the conclusion. Rate-limit rules need key and bucket. Nested metadata is Console-only; no secrets or PII. decision.reason is a flat string on DENY and undefined on ALLOW.