image-relay-mcp

A tiny MCP server with one job: fetch an image from an allowed host (Hugging Face Space outputs) server-side, and hand it back as inline image content — so Claude gets the bytes directly through the MCP connector, without going through its code-execution sandbox's domain-restricted network.

Tested locally with @modelcontextprotocol/sdk v1.29 and Node 18+.

Why this works

Claude's code-execution sandbox (the one used to build files) only has outbound access to a small package-manager allowlist. Custom MCP connectors are a separate path — they run on your own server, so they have normal internet access. This tool exploits that: it does the fetching on your infrastructure, then returns the result as an MCP image content block, which Claude can read directly from the tool result.

Files

• server.js — the whole thing. One tool: fetch_image({ url }).
• test-client.mjs — local smoke test, talks to localhost:3000/mcp.
• package.json — two dependencies: @modelcontextprotocol/sdk, express.

Security notes (read before deploying)

• Host allowlist: ALLOWED_HOST_SUFFIXES in server.js restricts which domains this will fetch from. It ships set to .hf.space and huggingface.co. Don't widen this to arbitrary URLs — an open fetch-any-URL relay is an SSRF vector.
• Optional auth: set the RELAY_AUTH_TOKEN env var on your host to require a Authorization: Bearer <token> header. Recommended once this is public, since anyone with the URL could otherwise call it.
• Size cap: hard-capped at 15MB per image (MAX_BYTES) so nothing huge gets base64-encoded into a response.
• Stateless by design (sessionIdGenerator: undefined) — no server-side session state to leak or clean up.

Run locally

npm install
npm start
# listening on :3000, POST /mcp
node test-client.mjs   # optional smoke test

Deploy (pick one — all have a free tier)

Any plain Node 18+ host works, since this only needs express + fetch.

Render (easiest, free web service):

1. Push this folder to a GitHub repo.
2. render.com → New → Web Service → connect the repo.
3. Build command: npm install. Start command: npm start.
4. Once live, your MCP endpoint is https://<your-app>.onrender.com/mcp.

Railway / Fly.io: same idea — Node buildpack, expose $PORT, start command npm start.

Your own VPS: npm install --production && RELAY_AUTH_TOKEN=xxx npm start behind a reverse proxy (Caddy/nginx) for TLS.

Connect it to Claude

1. claude.ai → Settings → Connectors → Add custom connector
2. URL: https://<your-deployed-host>/mcp
3. If you set RELAY_AUTH_TOKEN, add it as a header/auth value when the connector setup prompts for one.
4. In a chat: "use my image-relay connector to fetch <hf.space url>" — or just paste the url once it's a named tool Claude can see.

Note: custom connectors require a paid Claude plan (Pro or above).