protect-mcp

Local policy enforcement and signed decision receipts for AI agent tool calls.

Put the gateway between an MCP client and its tool server, or use its hooks in a coding agent. Review a Cedar policy, enable enforcement, and keep a signed record of the decisions. Local evaluation and verification need no ScopeBlind account and send no decision telemetry.

• Enforce at the tool boundary. Cedar denies on missing policies, engine errors or evaluation errors in fail-closed mode. Shadow mode is the default for the MCP wrapper: use --enforce when ready to block.
• Check what was signed. Ed25519 receipts include the decision and policy digest. A separate verifier checks the bytes against the public key you trust.
• Start with existing tools. Use MCP stdio, the HTTP gateway or coding-agent hooks. Optional hosted collaboration has its own permissions and data flows.

A gateway covers calls routed through it. It is not an OS sandbox, and a signed receipt does not establish that an action executed or that no action was omitted. The security policy describes the boundary and reporting process.

Try it without an agent

Use Node.js 22 and npm in a new directory. Install the pinned gateway and verifier:

mkdir protect-mcp-demo
cd protect-mcp-demo
npm init -y
npm install --save-exact [email protected] @veritasacta/[email protected]

This example evaluates two proposed calls and signs their decisions. It does not execute either tool or run git push. Everything after installation is local.

# Create a starter policy and a private signing key.
npx --no-install protect-mcp init --starter

# A normal read is allowed; a force-push to main is denied (exit 2).
npx --no-install protect-mcp evaluate --policy ./protect.cedar --tool Read --input '{"path":"README.md"}'
npx --no-install protect-mcp evaluate --policy ./protect.cedar --tool Bash --input '{"command":"git push --force origin main"}' || test "$?" -eq 2

# Evaluate again at signing time and record both decisions, in order.
npx --no-install protect-mcp sign --cedar builtin:starter --tool Read --input '{"path":"README.md"}' --key ./protect-mcp.key --receipts ./receipts
npx --no-install protect-mcp sign --cedar builtin:starter --tool Bash --input '{"command":"git push --force origin main"}' --key ./protect-mcp.key --receipts ./receipts

# Take only the public key from the key you generated, not from the receipt.
PROTECT_MCP_PUBLIC_KEY=$(node -p 'JSON.parse(require("fs").readFileSync("protect-mcp.key", "utf8")).publicKey')
npx --no-install verify --replay-chain ./receipts/receipts.jsonl --key "$PROTECT_MCP_PUBLIC_KEY" --json

# Change the signed deny to allow. The original receipt log stays intact.
node -e 'const fs=require("fs");const r=JSON.parse(fs.readFileSync("receipts/receipts.jsonl","utf8").trim().split("\n")[1]);r.payload.decision="allow";fs.writeFileSync("tampered.json",JSON.stringify(r))'
npx --no-install verify tampered.json --key "$PROTECT_MCP_PUBLIC_KEY" --json || test "$?" -eq 1

Expected: allow, then deny; both original receipt signatures and their link verify; the modified receipt reports invalid_signature. Keep protect-mcp.key private. To verify someone else's record, obtain their expected public key through an independent trusted channel.

Verifier exit codes are 0: valid, 1: invalid, 2: not checked/error. An invalid result is not necessarily tampering: the wrong expected key can also fail verification. Chain replay checks the supplied links; it does not establish that you received the entire history.

CI runs this exact example against the packed candidate and the pinned published verifier. See validation and releases.

Connect your agent

Choose the integration that matches your host:

| Use case | Start here | |---|---| | Wrap an existing MCP server | MCP setup and shadow-to-enforce workflow | | Claude Code hooks | Hook setup | | Codex, Cursor, Gemini or Hermes hooks | Host payload formats | | Call the policy evaluator as MCP tools | The gate as an MCP server | | Host an HTTP gateway | HTTP deployment notes |

For an existing MCP server, initialize its configuration in a separate directory from the example above, then run the wrapper with your server command:

npx --yes [email protected] init
npx --yes [email protected] --policy ./protect-mcp.json -- node your-mcp-server.js

This starts in shadow mode. Review the observed calls and policy before adding --enforce. wrap prints client configuration and exits; it is not the launch command. Full setup instructions.

Starter shell rules inspect command text. They are useful guardrails, but can miss alternate command spellings and can match harmless quoted text. Review the starter policy's coverage and limits before relying on it.

Documentation

• Local gateway guide: policies, hooks, dashboard, credentials and receipt compatibility.
• Receipt and disclosure guide: local records, selective disclosure and optional hosted digest anchoring.
• Command reference.
• Hosted coordination: separate human approvals, invoice sandbox and repository review workflows.
• Release validation and provenance.
• Changes by version.

Contribute

Bug reports, minimal reproductions, documentation fixes and compatibility tests are welcome. See CONTRIBUTING.md for setup and checks, and GOVERNANCE.md for maintainership. Report vulnerabilities privately through SECURITY.md, not a public issue.

The shared conformance corpus records compatibility results across receipt producers. A passing corpus is bounded interoperability evidence, not a security certification.

MIT licensed; see LICENSE and the existing patent notice. Maintained by Tom Farley at ScopeBlind. Receipt format: Acta individual Internet-Draft.