HOL Guard alternative to Invariant Labs mcp-scan: Runtime Firewall vs. Static Scanner

When you're deciding between Invariant Labs mcp-scan and HOL Guard, you're choosing between a static scanner and a runtime firewall. That difference is the whole ballgame. Here's why teams are switching to HOL Guard — and why a scan alone won't keep your agents safe.

Why buyers actually switch

Teams don't switch because of a feature checklist. They switch because they've had a close call — or they've realized that scanning isn't the same as protection. Here are the concrete reasons we hear most often:

• Scans miss zero-day attacks. mcp-scan inspects known artifacts and configs. If a malicious MCP server ships a novel payload, a static scan may not flag it. HOL Guard intercepts every tool call at runtime, so so calls that violate your policy are blocked before they execute. No tool is a safety guarantee.

• No enforcement. mcp-scan produces a report. You still have to manually act on it. HOL Guard enforces policies automatically — if a command is disallowed, it never runs. That's the difference between a smoke alarm and a fire suppression system.

• Runtime attacks are invisible to scanners. Prompt injection can manipulate an agent into calling a tool that wasn't in the original config. mcp-scan can't see that because it never watches live calls. HOL Guard sees every call and can block it.

• Cost of a breach is higher than any tool. A single exfiltrated secret can cost thousands in remediation and lost trust. HOL Guard is free, so the ROI is immediate — you're saving potential breach costs without spending a cent.

• Setup time. mcp-scan requires you to integrate scanning into your CI/CD pipeline. HOL Guard installs in minutes with pipx install hol-guard && hol-guard init and starts protecting immediately.

Runtime interception vs. static analysis

The core difference is when protection happens. mcp-scan is a static analysis tool: it reads MCP server definitions, configuration files, and code artifacts, then reports potential vulnerabilities. That's useful for a pre-deployment audit, but it's a point-in-time check. Once your agent is live, mcp-scan is no longer watching.

HOL Guard is a runtime firewall. It sits between your agent and the tools it calls — shell commands, secrets access, MCP calls, package installs. Every call is checked against your policy before it executes. If a call violates a rule, it's blocked instantly. This is the difference between checking a door is locked and having a guard at the door.

How interception works

HOL Guard hooks into the agent's tool-call layer. When your agent tries to run rm -rf / or access ~/.ssh/id_rsa, HOL Guard evaluates the call against your rules. If it's allowed, it proceeds; if not, it's blocked and you get an alert. This happens in milliseconds, with no perceptible latency. mcp-scan cannot do this because it's not in the execution path.

Secrets protection: blocking vs. flagging

Secrets are the crown jewels. mcp-scan can scan your MCP configs for hardcoded secrets and flag them — useful for cleanup. But it can't stop an agent from reading a secret at runtime. HOL Guard intercepts access to sensitive files and environment variables. You can define patterns like ~/.aws/credentials or API_KEY and block any tool call that tries to read them.

Consider a scenario: an agent is compromised via prompt injection and tries to read /etc/passwd. mcp-scan never sees this because it's not watching. HOL Guard blocks the read and alerts you. That's the difference between a report and a shield.

MCP call filtering: definitions vs. live calls

mcp-scan does scan MCP server definitions, which is valuable for finding misconfigurations before deployment. But MCP calls happen at runtime, and that's where attacks occur. HOL Guard intercepts each MCP call, validates it against your policy, and blocks malicious ones. For example, if an MCP server tries to call a tool that accesses the network, HOL Guard can block it if your policy disallows network access.

This is not a knock on mcp-scan — it does what it does well. But if you're relying on it for runtime safety, you have a gap. HOL Guard fills that gap.

Package install protection

Supply-chain attacks are on the rise. An agent might be tricked into installing a malicious package. mcp-scan doesn't monitor package installs. HOL Guard can block any pip install, npm install, or other package manager command that isn't explicitly allowed. You can even restrict installs to a specific registry or version range. This is a layer of defense that static scanners simply don't provide.

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Who this is actually for

HOL Guard is for teams that run AI agents in production and need active protection. Here are three concrete scenarios:

• DevOps teams automating infrastructure. If your agent can run shell commands, you need a firewall. HOL Guard blocks destructive commands and limits access to production secrets. mcp-scan can't help here.

• Security teams auditing agent behavior. HOL Guard gives you a real-time log of every blocked call, which is invaluable for incident response. mcp-scan gives you a static report that goes stale.

• Startups building agent-based products. You need to ship fast without worrying about your agent being exploited. HOL Guard is free and easy to integrate, so you can add protection without slowing down.

Migration walkthrough

Switching from mcp-scan to HOL Guard is straightforward. Here's how:

1. Install HOL Guard: Run pipx install hol-guard on the machine where your agent runs.

2. Initialize: Run hol-guard init to create a default policy file.

3. Configure policies: Edit the policy to allow the tools and commands your agent legitimately needs. Start with a deny-by-default stance and add exceptions.

4. Integrate with your agent: HOL Guard provides a simple API or CLI wrapper. Point your agent's tool calls through HOL Guard. This usually takes minutes.

5. Test: Run your agent through a few scenarios to ensure legitimate calls pass and malicious ones are blocked.

6. Deploy: Roll out to production. You can keep mcp-scan in your CI for pre-deployment audits, but HOL Guard becomes your runtime shield.

Setup time varies by environment. And because HOL Guard is free, there's no cost barrier.

Conclusion

If you're serious about AI agent security, you need runtime protection. mcp-scan is a useful tool for static analysis, but it's not a substitute for a firewall. HOL Guard gives you real-time interception, policy enforcement, and secrets protection — all for free. Start protecting your agents today. HOL Guard is Apache-2.0, maintained by HOL, and installs with pipx install hol-guard && hol-guard init. No scan or firewall is a safety guarantee, and there is no published cloud pricing. This comparison was published with Alt Hunt.