Deploy a remote MCP server on Vercel Functions

Vercel Functions suit a remote MCP server that does a bounded amount of work per request. You can publish one Node function without creating a Next.js application. Start with this echo server, then add your real tools once the production URL passes a browser test.

The general deployment guide compares hosting models. This guide focuses on a complete plain-function project and Vercel's limits, reviewed on 24 October 2026.

Run the complete sample locally

The complete deployment sample (opens in a new tab) includes app.mjs, node.mjs, pinned dependencies, all platform adapters, and a smoke runner. Download just that directory from the repository archive into a fresh folder:

mkdir echo-mcp
cd echo-mcp
curl -fL https://github.com/integry/mcptest/archive/refs/heads/master.tar.gz \
  | tar -xz --strip-components=6 mcptest-master/src/content/articles/samples/remote-deploy
npm install
npm run smoke
npm start

Use Node.js 22.23.4. npm start serves http://localhost:3000/mcp; stop it before rerunning smoke, which uses ports 3000–3003. Leave PUBLIC_URL unset locally. The sample pins @modelcontextprotocol/server 2.3.1, @hono/node-server 2.1.5 and zod 4.6.7. The smoke client is SDK 2.3.1.

The app registers one read-only echo tool with a required text string and optional waitMs from 0 to 1000. It emits progress when the caller requests it, then returns the text. createMcpHandler serves 2026-07-28 stateless requests and uses legacy: 'stateless' for 2025 clients. It issues no session ID; GET /mcp returns 405. The same URL handles JSON responses and POST responses that upgrade to SSE when progress is emitted. There is no separate legacy /sse endpoint.

This is an unauthenticated echo demonstration. Before attaching private data, add real bearer authorization and protected resource metadata. The Hono guide provides a complete authenticated application if that is your next step.

The Vercel entry point

The downloaded project already contains the complete function entry:

// api/mcp.mjs
import app from '../app.mjs';
export default app;

app.mjs exports an object with a fetch(request) method. Vercel's Node runtime docs (opens in a new tab) support this web-standard export in the root api directory. Keep the project root at the sample directory so both the function and its shared app are included.

Its complete vercel.json mounts the friendly URL and sets an explicit duration:

{
  "$schema": "https://openapi.vercel.sh/vercel.json",
  "functions": { "api/mcp.mjs": { "maxDuration": 60 } },
  "rewrites": [{ "source": "/mcp", "destination": "/api/mcp" }]
}

The public endpoint is /mcp. The rewrite routes that path to the function; no catch-all page rewrite is needed. Do not cache POST responses or put a page redirect on this route.

Publish the function

1. Import a repository containing the downloaded sample into Vercel, selecting this directory as the root. Use framework Other, with no build command or output directory override. The server files are ordinary JavaScript.
2. Choose a production hostname, such as your project's .vercel.app domain, and set the production environment variable PUBLIC_URL=https://YOUR-PROJECT.vercel.app/mcp.
3. Deploy, then test that exact production URL. Changing environment settings requires a new deployment.
4. If you add a custom hostname in Project Settings → Domains, follow Vercel's DNS instructions, wait for TLS, change PUBLIC_URL to https://YOUR-DOMAIN/mcp, and redeploy.

Vercel Deployment Protection (opens in a new tab) can put a Vercel login in front of previews. A preview URL reaching an authentication page is not the same as the MCP app's OAuth. Use a reachable production domain or an intentionally configured automation bypass. Do not expose a private project just to make a test green.

Duration, streaming and cold starts

Vercel's duration table (opens in a new tab) currently gives Fluid Compute a 300-second default: Hobby's maximum is 300 seconds, Pro and Enterprise have an 800-second generally available maximum, and an extended 1800-second beta exists for eligible configurations. This sample uses 60 seconds so it does not depend on the extended beta. Plans without Fluid Compute have different limits; check the project's settings.

A streaming response still consumes the invocation's duration. SSE progress can make a tool's work visible, but cannot extend maxDuration. Vercel supports streaming function responses (opens in a new tab); return the response body directly as this wrapper does. Converting the body to text first would buffer it. Set downstream API deadlines shorter than the function deadline and move longer work to a separately managed job.

Fluid Compute can reuse an instance for concurrent invocations. Instances can also start cold or disappear on redeploy. Module-scope SDK setup can be reused, but per-user tokens, tool results and sessions must not live in mutable globals. Our factory creates request-scoped server work and uses no in-memory session map. A stateful 2025 transport would need a different design; “one function file” does not mean “one durable process.”

CORS and the public URL

Set PUBLIC_URL to your exact production endpoint, including /mcp. The app validates the incoming Host against that hostname and rejects other Origins. It allows browser requests from https://mcptest.io, answers OPTIONS before the MCP handler, allows the MCP request headers, and exposes WWW-Authenticate, MCP-Protocol-Version, Mcp-Session-Id and Retry-After.

To support another browser app, add its exact origin to the origins array in app.mjs. The SDK Origin check compares hostnames; the CORS response uses the exact origin allowlist. CORS is not user authentication. Non-browser clients can omit Origin, so protect private tools independently. The CORS reference covers the browser checks in detail.

A new custom domain means a new canonical resource URL if you later add OAuth. Update PUBLIC_URL, protected resource metadata and the provider's resource/audience configuration together. Test redirects too: an HTML landing page or a login page in front of /mcp is not a successful MCP response.

Test it on mcptest.io

Open the deployed HTTPS /mcp endpoint in the mcptest.io playground. Check discovery, list tools, and call echo with {"text":"hello","waitMs":500}. Confirm the result is hello, the browser preflight succeeds, the response headers are readable, and there is no session ID. Repeat once after the platform has been idle and once after a new deployment.

The fixture's npm run smoke was run locally: pinned 2026 discovery, legacy initialization, tool listing/calling, SSE progress, no-session behavior, allowed-origin preflight, rejected Host/Origin, and GET 405 all passed. Local success does not verify the cloud front door. Deployment CLIs and credentials were unavailable during authoring; cloud deployment, DNS/TLS, platform timeouts and live streaming must be checked in your account. Add the pre-deployment release gate once the public URL is stable.