netlify-mcp-servers

---
name: netlify-mcp-servers
description: Build, deploy, and secure Model Context Protocol (MCP) servers on Netlify. Use whenever the task involves creating an MCP server, exposing an app or API to AI agents as MCP tools, letting Claude / Cursor / Claude Code call a custom remote server, or adding MCP tools to an existing Netlify site. Covers the MCP SDK + Streamable HTTP transport on a Netlify Function, authentication (single shared secret vs per-user API keys with Netlify Identity), read/write safety, file uploads, and connecting clients. Use even when the user just says "MCP", "tool server for an agent", or "let an AI use my API".
---

# Netlify MCP Servers

An MCP server exposes **tools** (and optionally resources/prompts) that an AI client — Claude Desktop, Claude Code, Cursor — can call. On Netlify, a remote MCP server is just **one Netlify Function** that speaks the MCP protocol over HTTP. This skill gets you a working, secure server and connects a client to it.

**"Netlify MCP" means two different things — make sure you're building the right one.** Netlify publishes its *own* hosted MCP server that lets an AI client operate the **Netlify platform** on your behalf — create projects, trigger deploys, manage env vars and infrastructure through your Netlify account. You don't write that one; you point your client at Netlify's hosted MCP server per Netlify's MCP-server docs (and see the **netlify-agent-runner** skill for running agents against your site). This skill is the *other* thing: building **your own** MCP server — an endpoint that exposes *your* app's tools and data to an agent — hosted on a Netlify Function. If the ask is "let my agent manage my Netlify sites/deploys/env vars," that's the hosted Netlify MCP server, not a function you write.

The same setup works two ways:

- **Standalone server** — a repo whose only job is the MCP endpoint (e.g. wrapping a third-party API).
- **Added to an existing app** — one more function alongside your site. Have its tools call the **same service/data layer your UI and REST routes already use**, so logic isn't duplicated.

## Before you build

Decide one thing up front, because it shapes the auth code:

- **Who calls this server?** Just you (a personal/single-user server) → use a **single shared secret**. Multiple people, each acting as themselves → use **per-user API keys** backed by Netlify Identity. See [authentication](references/authentication.md).

If you're not sure, start with the single shared secret — it's a few lines and you can layer per-user keys on later. I'll default to that unless you say otherwise.

## Stack

Use the official MCP SDK with its Web-standard Streamable HTTP transport, running statelessly inside a Netlify Function.

```bash
npm install @modelcontextprotocol/sdk zod
```

A Netlify Function already speaks the web platform — it receives a `Request` and returns a `Response`. The SDK ships a transport built on exactly those primitives, `WebStandardStreamableHTTPServerTransport` (the same core the SDK runs on internally, and what Cloudflare Workers / Deno / Bun use): you hand it the `Request` and return the `Response` it produces — no adapter, no version pin. Older guides reach for the Node-flavored `StreamableHTTPServerTransport` plus a `fetch-to-node` bridge to synthesize the Node `req`/`res` objects it expects; on Netlify you need neither, and skipping them is both simpler and what's verified to work here.

One gotcha, independent of all this: the transport returns **HTTP 406** to any POST whose `Accept` header lacks *both* `application/json` and `text/event-stream`. That's an MCP-spec requirement the *client* must satisfy — a 406 means fix the client's `Accept` header, not the server. Letting the SDK own the protocol also means you don't hand-maintain JSON-RPC framing or the protocol-version handshake.

## The server function

With the Web-standard transport this is a few lines — most of what older guides show was the Node bridge, which you don't need. Put it in `netlify/functions/mcp.ts`:

```typescript
import type { Config, Context } from "@netlify/functions";
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { WebStandardStreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/webStandardStreamableHttp.js";
import { z } from "zod";
import { checkBearer } from "../lib/mcp/bearer"; // see Authentication

function buildServer() {
  const server = new McpServer({ name: "my-mcp", version: "0.1.0" });

  server.tool(
    "get_item",
    "Fetch a single item by id. Read-only.",
    { id: z.string().describe("The item's unique id") },
    async ({ id }) => ({
      content: [{ type: "text", text: JSON.stringify(await getItem(id)) }],
    }),
  );

  return server;
}

export default async (req: Request, _context: Context) => {
  if (!checkBearer(req)) return new Response("Unauthorized", { status: 401 });

  // Stateless JSON server: it only does request/response over POST. Reject other
  // methods — a GET makes the transport open an SSE stream that never closes, which
  // a serverless function can't serve (you'll get a 502).
  if (req.method !== "POST") return new Response("Method not allowed", { status: 405 });

  // Fresh server + transport per request, no session to persist. enableJsonResponse
  // returns one application/json body instead of an SSE stream — the right fit here.
  const server = buildServer();
  const transport = new WebStandardStreamableHTTPServerTransport({
    sessionIdGenerator: undefined,
    enableJsonResponse: true,
  });

  // Hand over the Web Request, return the Web Response. The transport owns JSON-RPC
  // framing, body parsing (a malformed body comes back as a clean 400), and the handshake.
  await server.connect(transport);
  return transport.handleRequest(req);
};

export const config: Config = { path: "/mcp" };
```

That's a complete, deployable server. Everything else is tools, auth, and safety.

## Browser-based clients and CORS

Netlify Functions do **not** add CORS headers for you, and the server above returns 405 to every non-POST method — including the `OPTIONS` preflight a browser sends. That's fine for the normal case: native MCP clients (Claude Code, Cursor, Claude Desktop, the `mcp-remote` bridge) are **not** browsers and don't enforce the same-origin policy, so they need no CORS at all — which is why those clients work while a browser call doesn't.

It only matters when your MCP client runs **in a browser** — a web app calling the server cross-origin. Then the browser blocks the request unless the response carries `Access-Control-Allow-Origin`, and it first sends an `OPTIONS` preflight that must come back `2xx` with `Access-Control-Allow-Methods` (including `POST`) and `Access-Control-Allow-Headers` (including `Authorization` and `Content-Type`). A "blocked by CORS policy: No Access-Control-Allow-Origin header" error in the browser console is this — not a broken server or a platform bug. Answer the preflight in the function itself, **before** the 405 check, and echo the CORS headers on the POST response too:

```typescript
const CORS = {
  "Access-Control-Allow-Origin": Netlify.env.get("MCP_ALLOWED_ORIGIN") ?? "*",
  "Access-Control-Allow-Methods": "POST, OPTIONS",
  "Access-Control-Allow-Headers": "Authorization, Content-Type, Mcp-Session-Id",
};

// In the handler, before the 405 check:
if (req.method === "OPTIONS") return new Response(null, { status: 204, headers: CORS });
// ...then reject other non-POST methods with 405, and add CORS to the transport's Response.
```

The function must set these headers itself — don't treat a browser CORS error as something to escalate to Netlify or route around by loosening auth.

## Defining tools

Each tool is a `name`, a one-line `description`, a `zod` input schema, and a handler that returns `{ content: [...] }`. The description and parameter `.describe()` text are the only thing the model sees — write them like API docs for an agent: say what the tool does, when to use it, and call out anything irreversible.

As the count grows, give each tool its own module and register them in `buildServer()`. Servers with many tools often keep a registry (an array of `{ name, description, inputSchema, handler }`) and wire `tools/list` + `tools/call` once — the transport setup above is identical either way.

## Authentication

The MCP client must prove it's allowed to call your server. Every request carries `Authorization: Bearer <token>`; reject anything else with a 401.

**Single shared secret** (personal / single-user). One env var, compared in constant time. Put this in `netlify/lib/mcp/bearer.ts`:

```typescript
import { timingSafeEqual } from "node:crypto";

export function checkBearer(req: Request): boolean {
  const expected = Netlify.env.get("MCP_BEARER_TOKEN");
  if (!expected) return false;
  const match = req.headers.get("authorization")?.match(/^Bearer\s+(.+)$/i);
  if (!match) return false;
  const a = Buffer.from(match[1]);
  const b = Buffer.from(expected);
  // Length check first because timingSafeEqual throws (RangeError) on unequal-length
  // buffers. The token is fixed-length, so the early return leaks nothing useful.
  return a.length === b.length && timingSafeEqual(a, b);
}
```

Generate the token with `openssl rand -hex 32` and store it as a secret env var.

**Per-user API keys** (multi-user). Netlify Identity gates a web UI where each user mints their own keys; you store only a **hash** of each key (never the plaintext) tied to that user, resolve the key to a user on every request, and flow that user into your tool handlers so tools act as the right person. Full pattern — schema, generation, hashing, revocation, resolving the user — in [authentication](references/authentication.md).

**Start simple with scoping.** The simplest model is all-or-nothing: a valid key can call every tool as the user it belongs to — usually the right starting point. Add per-key scopes when a concrete need appears (e.g. a read-only key), and grow into per-tool scopes or role tiers if the app genuinely calls for them. If a fuller RBAC design is requested, lead with the simple baseline and layer scopes on top of it, rather than treating the full hierarchy as required up front.

## Safety and permissions

Tools are a public API handed to an autonomous agent. Be deliberate:

- **Expose the least that does the job.** Separate reads from writes, and think hard before exposing destructive tools. A common, sound choice is to **omit delete tools entirely** and keep destructive actions in a human-operated UI.
- **Guard irreversible or public actions** by putting explicit instructions in the tool's description — e.g. "show the user the exact text and get confirmation before posting." This is a soft, model-level guard, so back it with a real kill switch: a token you can revoke instantly.
- **Keep the client's credential separate from your backend's.** The client authenticates to your server (bearer/API key); your server authenticates to the database or third-party API with its *own* secret. Never pass your backend god-key out to the client.
- **Use least-privilege backend credentials** — app passwords or scoped tokens, not account-level ones, so a leak is contained and revocable.
- **Validate inputs** (your `zod` schemas do this) and **log every tool call** so you can see what the agent did — `console.info` shows up in Netlify function logs.

## Rate limiting

An MCP server is a public endpoint an autonomous agent can hit in a tight loop — cap it. Netlify Functions have **built-in declarative rate limiting**, so don't hand-roll a counter (a per-instance in-memory counter wouldn't hold across function instances anyway — see the next section). Add a `rateLimit` block to the function's `config` export:

```typescript
export const config: Config = {
  path: "/mcp",
  rateLimit: {
    windowSize: 60,               // time window in seconds; capped at 180
    windowLimit: 100,             // max requests per window
    aggregateBy: ["ip", "domain"], // group by ip, domain, or both
  },
};
```

Over the limit the platform returns HTTP `429` by default (or set `action: "rewrite"` with a `to` path to send excess traffic to a dedicated page). Function rate limits live **only** in the function's `config` export — they **cannot** be defined in `netlify.toml`.

## File uploads

When a tool needs the agent to supply a file (an image to post, a doc to attach), don't push the bytes through the tool call as base64 — it bloats the model's context and runs into payload limits. Instead hand the agent a short-lived, single-use **presigned URL** to `PUT` the raw bytes to, store them in **Netlify Blobs**, and reference the file by a stable key from your other tools. Sign the URL with an **HMAC-SHA256** over the upload id, content-type, size, and expiry, keyed by a **secret env var**, and **verify it in constant time** — the signature *is* the authorization, so the `PUT` carries no bearer token. On the upload endpoint, enforce the declared content-type and size and reject replays. Full three-step flow (`prepare_upload` → `PUT` → `finalize_upload`) with code: [file uploads](references/file-uploads.md).

## State doesn't survive between requests

Every request builds a fresh server and transport, and any invocation may land on a **different** — or cold-started — function instance. Module-level memory is not shared between instances and not durable across cold starts. So state you need to persist between calls **cannot** live in a module-scoped `Set`/`Map`/variable: single-use / replay tracking for the presigned uploads above, idempotency keys, "already processed this id" guards, per-user counters you track by hand. An in-memory guard *looks* correct locally and on one warm instance, then silently lets a replayed upload through (or double-processes a call) the moment another instance serves the request. Keep that state in a **durable store** — Netlify Blobs or your database — keyed by the upload/request id, and check-and-mark it there. (This is also why the server itself runs stateless, with `sessionIdGenerator: undefined`.)

## Connecting a client

Native remote-MCP support is now the norm; reach for the `mcp-remote` bridge only as a fallback.

- **Claude Code** — `claude mcp add --transport http my-mcp https://<site>.netlify.app/mcp --header "Authorization: Bearer <token>"`
- **Cursor** — add the server to `mcp.json` with the URL and an `Authorization` header.
- **Claude Desktop / claude.ai** — add a **Custom Connector** (Settings → Connectors). Connectors are OAuth-oriented; for a static-bearer server the `mcp-remote` bridge is the reliable path.
- **Fallback (older / stdio-only clients)** — `npx mcp-remote https://<site>.netlify.app/mcp --header "Authorization: Bearer <token>"`

Full client matrix and the OAuth / Custom Connector deep-dive: [connecting clients](references/connecting-clients.md).

## Local dev and deploy

- **Run it:** `netlify dev` serves the function at `http://localhost:8888/mcp`.
- **Test it:** the MCP Inspector — `npx @modelcontextprotocol/inspector` — connect via Streamable HTTP to your URL with an `Authorization: Bearer` header and list/call tools. Or point `claude mcp add --transport http` at the localhost URL.
- **Identity caveat:** Netlify Identity does **not** work under `netlify dev`, so per-user-key auth must be tested on a deploy preview. See the **netlify-identity** skill.
- **Deploy:** push to Git, or `netlify deploy --build --prod`.
- **Secrets:** set tokens/keys as env vars (`netlify env:set MCP_BEARER_TOKEN <value> --secret`) — never in code.

## Cross-cutting rules

- Never hardcode secrets. Store tokens, API keys, and signing secrets as Netlify environment variables (mark them secret). Beyond the leak risk, a bearer token or signing secret written into source (or any file the build publishes) trips **Netlify's secrets scanning and fails the deploy** even after an otherwise-green build — the fix is to move it to a secret env var and read it at runtime with `Netlify.env.get(...)`, and rotate the token if it was committed, *not* to disable the scanner. See **netlify-deploy** for the scan controls.
- Inside functions, read env vars with `Netlify.env.get("VAR")`, not `process.env`.
- Add `.netlify` to `.gitignore`.

## Related skills and references

- [authentication](references/authentication.md) — single-secret vs per-user API keys (Identity) in depth.
- [connecting clients](references/connecting-clients.md) — full client matrix, OAuth, and Custom Connectors.
- [file uploads](references/file-uploads.md) — letting an agent upload images/files via presigned URLs to Netlify Blobs.
- **netlify-functions** — function syntax, routing, limits. **netlify-identity** — Identity setup. **netlify-database** / **netlify-blobs** — where to store keys and files. **netlify-deploy** — deploys. **netlify-config** — env vars.

From netlify/context-and-tools at b5894c0, shown as plain text under its license: MIT (permissive). The license text is in agent-plugin/LICENSE.

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• authentication.md4.3 KB · b5744bc1708d
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