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 the name of each tool that ran, signed with your key.
• 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. Create the signing key once (protect-mcp 0.7.4 sign does not create it).
#    An existing key is never replaced. See references/receipt-format.md to rotate.
if [ ! -e ./protect-mcp.key ]; then
  d=$(mktemp -d) && npx protect-mcp@0.7.4 init --dir "$d" && mv "$d/keys/gateway.json" ./protect-mcp.key
fi
echo "/protect-mcp.key" >> .gitignore

# 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. It signs a receipt that names the tool and appends it to ./receipts/receipts.jsonl. protect-mcp 0.7.4 does not record the tool input or output.

Cedar Policy File

Create ./protect.cedar at the project root:

// Read-only tools: one rule can name several tools in `when`. Add WebFetch
// with your own URL rule.
permit (principal, action == Action::"MCP::Tool::call", resource) when {
    resource == Tool::"Read" || resource == Tool::"Glob" || resource == Tool::"Grep"
};

// Safe commands only; git limited to read subcommands
permit (principal, action == Action::"MCP::Tool::call", resource == Tool::"Bash") when {
    context has input && context.input has command &&
    (context.input.command like "git status*" || context.input.command like "git diff*" ||
     context.input.command like "git log*" || context.input.command like "git show*" ||
     context.input.command like "npm*" || context.input.command like "ls*" ||
     context.input.command like "cat*" || context.input.command like "echo*" ||
     context.input.command like "pwd*" || context.input.command like "test*")
};

// No chaining (`&` also denies `2>&1`), `$` expansion, redirection (`>` or
// `<`, which covers `<(`), file output (`git diff --output`), or rm -rf
forbid (principal, action == Action::"MCP::Tool::call", resource == Tool::"Bash") when {
    context has input && context.input has command &&
    (context.input.command like "*;*" || context.input.command like "*&*" ||
     context.input.command like "*|*" || context.input.command like "*$*" ||
     context.input.command like "*`*" || context.input.command like "*>*" ||
     context.input.command like "*<*" || context.input.command like "*\n*" ||
     context.input.command like "*--output*" || context.input.command like "*rm -rf*")
};

// Writes only inside the project (paths are absolute), never via `..`
permit (principal, action == Action::"MCP::Tool::call", resource) when {
    (resource == Tool::"Write" || resource == Tool::"Edit") &&
    context has input && context.input has file_path &&
    context.input.file_path like "/path/to/project/*"
};
forbid (principal, action == Action::"MCP::Tool::call", resource) when {
    (resource == Tool::"Write" || resource == Tool::"Edit") &&
    context has input && context.input has file_path &&
    (context.input.file_path like "*/../*" || context.input.file_path like "*/..")
};

String matching is best-effort: like checks the raw string, not a resolved path, and an npm* permit runs arbitrary code, so it is only as safe as the project's scripts.

Verification

Verify every receipt against the public key in ./protect-mcp.key:

PUB=$(node -p 'JSON.parse(require("fs").readFileSync("./protect-mcp.key")).publicKey')
npx @veritasacta/verify@0.9.2 --replay-chain ./receipts/receipts.jsonl --key "$PUB"
# Exit 0 = every receipt verified
# Exit 1 = a receipt failed (tampered, wrong key, or malformed line)
# Exit 2 = the file could not be read

The plugin's slash commands do the same inside Claude Code. /verify-receipt takes one receipt in its own file, e.g., from tail -n 1 ./receipts/receipts.jsonl > receipt.json.

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

Receipt Format

Each receipt is one line of ./receipts/receipts.jsonl. See references/receipt-format.md for a sample.

• Ed25519 signatures (RFC 8032) over all fields but signature
• JCS canonicalization (RFC 8785) before signing
• No public key in the receipt, so pass it with --key
• No link to the previous receipt, so a deleted line goes undetected

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)