Trusted proxies

import { FastMCP, AuthorizationError } from 'fastmcp-ts/server'
import type { RequestVerifier } from 'fastmcp-ts/server'
import { timingSafeEqual } from 'node:crypto'

const PROXY_SECRET = process.env.PROXY_SHARED_SECRET!

function safeEqual(a: string, b: string): boolean {
  const ab = Buffer.from(a)
  const bb = Buffer.from(b)
  return ab.length === bb.length && timingSafeEqual(ab, bb)
}

const proxyAuth: RequestVerifier = {
  async verifyRequest(request) {
    // 1. Verify provenance FIRST. Headers are forgeable by any direct caller.
    //    The verifier sees the full wire headers; ctx.http does not.
    const secret = request.headers.get('x-proxy-secret')
    if (!secret || !safeEqual(secret, PROXY_SECRET)) {
      throw new Error('Request did not come through the proxy')
    }
    // 2. Read the identity the proxy established.
    const userId = request.headers.get('x-auth-request-user')
    if (!userId) throw new Error('Proxy did not forward an identity')
    // 3. Return it as an AccessToken. `token` MUST be a stable, non-empty,
    //    per-identity value: it keys response-cache partitioning.
    return {
      token: userId,
      clientId: userId,
      scopes: (request.headers.get('x-auth-request-groups') ?? '').split(',').filter(Boolean),
      claims: { sub: userId },
    }
  },
}

const mcp = new FastMCP({
  name: 'internal-tools',
  auth: proxyAuth,
  // Keep the proxy secret out of ctx.http and out of anything forwarded:
  http: { redactHeaders: ['x-proxy-secret'] },
})