Agent Guards

agent-guards core

The detection engines behind Package Guard, Agent Firewall, Payment Guard, Email Guard, Code Guard and Agent Web Tools, in one package, running on your machine.

Everything here is deterministic: regex rules, published lists, and parsers. No model is called in any detection path, so the same input gives the same verdict and every verdict names the rules version that produced it.

Run it

npx agent-guards

That starts an MCP stdio server with every tool from all six products. No configuration, no API key, no account.

npx agent-guards --offline          # local engines onlynpx agent-guards --only code-guard  # one productnpx agent-guards --list             # what you get, and which tools need the networknpx agent-guards --disable check_ip,check_password

The same package installs the guard CLI:

npx --yes --package agent-guards guard scan src/npx --yes --package agent-guards guard diffnpx --yes --package agent-guards guard stats --json

pre-commit

This repository publishes a staged-diff hook for the pre-commit framework. It needs Node 18 or newer. Add this to .pre-commit-config.yaml:

repos:  - repo: https://github.com/mlawsonking/MCP    rev: main  # pin this to a commit SHA in a shared repository    hooks:      - id: agent-guards-diff

Then run pre-commit install. The hook scans added lines only and uses the rules in that pinned checkout. It does not call a model or send file contents anywhere. A second manual hook, agent-guards-scan, scans the whole working tree when you run pre-commit run agent-guards-scan --hook-stage manual.

Local and cloud, and why the difference matters

Local tools run entirely on your machine. The text you scan never leaves it.

Cloud tools say which service they call in their own tool description. With --offline, they report that they could not check. They do not return a verdict.

That distinction is the whole design. A check that did not run is not a pass. This codebase shipped the opposite once: four endpoints answered "not sanctioned", "no known vulnerabilities", "not a honeypot" and "safe" when the lookup behind each had failed or was never attempted. A security tool that says "clear" when it means "I don't know" is worse than no tool.

What it catches, and what it does not

The injection scanner matches known instruction-override, jailbreak, prompt-leak, exfiltration and tool-poisoning phrasings, and the Unicode tricks used to hide them: zero-width characters, bidi overrides, the Unicode tag block, hidden CSS, instructions in HTML comments. It is a deterministic pattern scanner, not a classifier. Novel phrasing gets through. Anyone who reads the rules can write around them. It is one layer, not a wall.

The secret scanner matches 22 credential formats plus generic assignments and 3 PII patterns. A secret in a format it does not know, or split across lines, is not detected. What it does guarantee is that the value it found is gone from the redacted output, not just the label next to it.

The code scanner is 31 regex rules across JS/TS and Python. No parser, no data flow, no taint tracking, so exec(cmd) with a bare variable does not fire while exec("ls " + dir) does. That is a deliberate limit and there is a test pinning it.

Sanctions screening covers the EVM-format OFAC address lists. Honeypot checks only cover the chains honeypot.is actually simulates.

Using it as a library

const guards = require('agent-guards');
guards.injection.scan(untrustedText);      // { risk, score, verdict, findings, rules_version }guards.secrets.scan(text);                 // { found, verdict, findings, redacted, rules_version }guards.code.scanDiff(unifiedDiff, 'py');   // findings against new-file line numbersawait guards.safeFetch(url);               // SSRF-guarded: pins the resolved IP, rechecks every hop

safeFetch is the one to reach for whenever you fetch a URL you did not choose. It resolves the hostname once, refuses the request if anything it resolves to is private or loopback, and connects to that exact address, so there is no second lookup for DNS rebinding to poison. It re-checks every redirect hop and never pools sockets, because a pooled connection skips the pin.

Rule updates

The rules go stale. Addresses get sanctioned, packages get pulled from npm for shipping malware, and none of that reaches you if the rules only change when you reinstall. So about once a day the server and the CLI ask the rules feed whether there is a newer ruleset, and apply it if there is.

A pull sends two things: which surface asked (cli, plugin, mcp or facade) and the rules version you already have.

GET /api/rules/latest?surface=mcp&have=2026.07.31

No machine id, no user id, no file names, nothing you scanned. Bundles are signed with Ed25519 and verified against a public key compiled into this package, so it does not matter which mirror served one. A bundle that fails its signature, its schema, or its ReDoS check is discarded and you keep the rules you had. Rules never roll back to an older version on their own.

guard update --status     # what is in use, and when it last checkedguard update              # pull now

Every verdict carries rules_provenance, which is either bundled or feed@<version>, so you can always tell which ruleset produced a result.

To turn updates off: --offline, AGENT_GUARDS_NO_FEED=1, or {"feed": false} in ~/.agent-guards/config.json. AGENT_GUARDS_FEED_URL points it somewhere else. With updates off everything still works on the rules compiled into the package.

The bundle format, the verification steps, and how the malicious-package list is curated are in rules/README.md.

Tests

npm test

Offline suites only. Anything that asserts against a third-party endpoint is run by hand, because a suite that goes red at random teaches everyone to ignore red.

Licence

MIT.