protect-mcp — Policy Enforcement + Signed Receipts

Cryptographic governance for every Claude Code tool call. Each invocation is evaluated against a Cedar policy and produces an Ed25519-signed receipt that anyone can verify offline.

Overview

Claude Code runs powerful tools: Bash, Edit, Write, WebFetch. By default there is no audit trail, no policy enforcement, and no way to prove what was decided after the fact. protect-mcp closes all three gaps:

• Cedar policies (AWS's open authorization engine) evaluate every tool call before execution. Cedar deny is authoritative.
• Ed25519 receipts record each decision with its inputs, the policy that governed it, and the outcome. Receipts are hash-chained.
• Offline verification via npx @veritasacta/verify. No server, no account, no trust in the operator.

Problem

AI agents make decisions that affect money, safety, and rights. The Claude Code session log records what happened, but the log is:

• Mutable — anyone with access can edit it
• Unsigned — there is no way to prove integrity
• Operator-bound — verification requires trusting whoever holds the log

For compliance contexts (finance, healthcare, regulated research), this is not sufficient. You need tamper-evident evidence that can be verified by third parties without trusting you.

Solution

Add protect-mcp to your Claude Code project:

# 1. Install the plugin (adds hooks + skill to your project)
claude plugin install wshobson/agents/protect-mcp

# 2. Create ./protect.cedar (see below). The plugin installs the hooks.

# 3. Start the receipt-signing server (runs locally, no external calls)
npx protect-mcp@latest serve --enforce

# 4. Use Claude Code normally. Every tool call is now policy-evaluated
#    and produces a signed receipt in ./receipts/

Hook Configuration

Installing the plugin adds both hooks from hooks/hooks.json. Each hook runs a script bundled with the plugin:

{
  "hooks": {
    "PreToolUse": [
      {
        "matcher": ".*",
        "hooks": [
          { "type": "command", "command": "\"${CLAUDE_PLUGIN_ROOT}\"/hooks/evaluate.sh" }
        ]
      }
    ],
    "PostToolUse": [
      {
        "matcher": ".*",
        "hooks": [
          { "type": "command", "command": "\"${CLAUDE_PLUGIN_ROOT}\"/hooks/sign.sh" }
        ]
      }
    ]
  }
}

Claude Code passes the hook event to the command as JSON on stdin and does not set TOOL_NAME or TOOL_INPUT variables. evaluate.sh reads tool_name and tool_input from that payload and passes them to protect-mcp as flags; sign.sh reads tool_name only, because the 0.7.4 signer records nothing else. Set PROTECT_MCP_POLICY, PROTECT_MCP_RECEIPTS, and PROTECT_MCP_KEY to change the default paths. When the policy file is missing, the PreToolUse hook prints a warning to stderr and allows the call.

What each hook does

PreToolUse — Runs BEFORE the tool executes. Evaluates the tool call against your Cedar policy file. If Cedar returns deny, the hook exits with code 2 and Claude Code blocks the tool call entirely.

PostToolUse — Runs AFTER the tool completes. Signs a receipt containing the tool name, input hash, output hash, decision, policy digest, and timestamp. Writes the receipt to ./receipts/<timestamp>.json.

Cedar Policy File

Create ./protect.cedar at the project root:

// Allow read-only tools by default
permit (
    principal,
    action in [Action::"Read", Action::"Glob", Action::"Grep", Action::"WebFetch"],
    resource
);

// Require explicit allow for destructive tools
permit (
    principal,
    action == Action::"Bash",
    resource
) when {
    // Allow safe commands only
    context.command_pattern in ["git", "npm", "ls", "cat", "echo", "pwd", "test"]
};

// Never allow recursive deletion
forbid (
    principal,
    action == Action::"Bash",
    resource
) when {
    context.command_pattern == "rm -rf"
};

// Require confirmation for writes outside the project
forbid (
    principal,
    action in [Action::"Edit", Action::"Write"],
    resource
) when {
    context.path_starts_with != "."
};

Verification

Verify a single receipt:

npx @veritasacta/verify receipts/2026-04-15T10-30-00Z.json
# Exit 0 = valid
# Exit 1 = tampered
# Exit 2 = malformed

Verify the entire chain:

npx @veritasacta/verify receipts/*.json

Use the plugin's slash commands from within Claude Code:

/verify-receipt receipts/latest.json
/audit-chain ./receipts/ --last 20

Receipt Format

Each receipt is a JSON file with this structure:

{
  "receipt_id": "rec_8f92a3b1",
  "receipt_version": "1.0",
  "issuer_id": "claude-code-protect-mcp",
  "event_time": "2026-04-15T10:30:00.000Z",
  "tool_name": "Bash",
  "input_hash": "sha256:a3f8...",
  "decision": "allow",
  "policy_id": "autoresearch-safe",
  "policy_digest": "sha256:b7e2...",
  "parent_receipt_id": "rec_3d1ab7c2",
  "public_key": "4437ca56815c0516...",
  "signature": "4cde814b7889e987..."
}

• Ed25519 signatures (RFC 8032)
• JCS canonicalization (RFC 8785) before signing
• Hash-chained to the previous receipt via parent_receipt_id
• Offline verifiable — no network call, no vendor lookup

Why This Matters

Standards

• Ed25519 — RFC 8032 (digital signatures)
• JCS — RFC 8785 (deterministic JSON canonicalization)
• Cedar — AWS's open authorization policy language
• IETF draft — draft-farley-acta-signed-receipts

Related

• npm: protect-mcp
• Verify CLI: @veritasacta/verify
• Source: github.com/ScopeBlind/scopeblind-gateway
• Protocol: veritasacta.com
• Integrations: Microsoft Agent Governance Toolkit (PR #667), AWS cedar-policy/cedar-for-agents (PR #64)