MCP Agent Governance with protect-mcp

Overview

Guidance for governing AI agent tool calls using Cedar policies and Ed25519 signed receipts. This skill teaches how to write access-control policies for MCP servers, run them in shadow mode for observation, and verify the cryptographic audit trail.

When to Use This Skill

• Use when you need to control which MCP tools an agent can call and under what conditions
• Use when you want a tamper-evident audit trail for agent tool executions
• Use when rolling out governance policies gradually (shadow mode first, then enforce)
• Use when authoring Cedar policies for MCP tool access control
• Use when verifying that a receipt or audit bundle has not been tampered with

Do Not Use This Skill

• When you need general application security auditing (use @security-auditor)
• When you need to scan code for vulnerabilities (use @security-audit)
• When you need compliance framework guidance without agent-specific governance

How It Works

protect-mcp intercepts MCP tool calls, evaluates them against Cedar policies (the same policy engine used by AWS Verified Permissions), and signs every decision as an Ed25519 receipt. The receipt is a cryptographic proof that a specific policy was evaluated against a specific tool call at a specific time.

Agent → protect-mcp → Cedar policy evaluation → MCP Server
                ↓
        Ed25519 signed receipt

Three modes of operation:

1. Shadow mode (default) — logs decisions without blocking. Use this to observe what your policies would do before enforcing them.
2. Enforce mode — blocks tool calls that violate policy. Use after shadow-mode validation.
3. Hooks mode — integrates with Claude Code hooks for pre/post tool-call governance.

Core Concepts

Cedar Policies

Cedar is a policy language designed for authorization. Policies are evaluated locally via WASM — no network calls required.

// Allow read-only file operations
permit(
  principal,
  action == Action::"call_tool",
  resource
) when {
  resource.tool_name in ["read_file", "list_directory", "search_files"]
};

// Deny destructive operations
forbid(
  principal,
  action == Action::"call_tool",
  resource
) when {
  resource.tool_name in ["execute_command", "delete_file", "write_file"]
  && resource has args
  && resource.args.contains("rm -rf")
};

Signed Receipts

Every policy decision produces a signed receipt:

{
  "payload": {
    "type": "protectmcp:decision",
    "tool_name": "read_file",
    "decision": "allow",
    "policy_digest": "sha256:9d0fd4c9e72c1d5d",
    "issued_at": "2026-04-05T14:32:04.102Z",
    "issuer_id": "sb:issuer:de073ae64e43"
  },
  "signature": {
    "alg": "EdDSA",
    "kid": "sb:issuer:de073ae64e43",
    "sig": "2a3b5022..."
  }
}

The receipt format follows IETF Internet-Draft draft-farley-acta-signed-receipts.

Step-by-Step Guide

1. Initialize Governance for a Project

# Install and initialize hooks (Claude Code integration)
npx protect-mcp init-hooks

# Or run as a standalone MCP gateway
npx protect-mcp serve

This creates a protect-mcp.config.json and a starter Cedar policy in your project root.

2. Write Your First Policy

Create policy.cedar in your project:

// Start permissive — allow everything in shadow mode
permit(
  principal,
  action == Action::"call_tool",
  resource
);

3. Run in Shadow Mode (Observe First)

# Shadow mode is the default — logs decisions without blocking
npx protect-mcp --policy policy.cedar -- node your-mcp-server.js

Review the shadow log to understand what your agent is doing before writing restrictive policies.

4. Tighten and Enforce

Once you understand the tool-call patterns, write specific policies:

// Allow file reads, deny writes outside src/
permit(
  principal,
  action == Action::"call_tool",
  resource
) when {
  resource.tool_name == "read_file"
};

permit(
  principal,
  action == Action::"call_tool",
  resource
) when {
  resource.tool_name == "write_file"
  && resource has args
  && resource.args.path like "src/*"
};

// Deny everything else
forbid(
  principal,
  action == Action::"call_tool",
  resource
);

Switch to enforce mode:

npx protect-mcp --policy policy.cedar --enforce -- node your-mcp-server.js

5. Verify Receipts

# Verify a single receipt

…