Receipt-signing middleware for MCP server frameworks

[tomjwxf]

Problem

MCP server implementations (Express, Hono, Cloudflare Workers) need a simple way to add receipt signing without adopting the full protect-mcp gateway. Developers want app.use(receiptMiddleware()) — not a separate proxy process.

Proposal

A lightweight middleware that plugs into existing MCP server frameworks:

import { receiptMiddleware } from 'protect-mcp/middleware';

// Express
app.use('/mcp', receiptMiddleware({
  keyPath: './keys/gateway.json',
  policy: './protect-mcp.json'
}));

// Hono
app.use('/mcp/*', receiptMiddleware({ ... }));

What the middleware does

1. Intercepts tools/call requests
2. Evaluates against policy (JSON, Cedar, or external PDP)
3. Signs the decision as an Ed25519 receipt
4. Attaches the receipt to the response headers
5. Logs to .protect-mcp-receipts.jsonl

What it doesn't do

• No process wrapping — runs in-process
• No stdio proxying — HTTP only
• No approval gates — those require the full gateway

Verification

npx @veritasacta/verify .protect-mcp-receipts.jsonl

Related

• IETF draft-farley-acta-signed-receipts-01
• Google ADK RFC #5164 — same pattern for Python