building-mcp-server-on-cloudflare

SKILL.md

Building MCP Servers on Cloudflare

Your knowledge of the MCP SDK and Cloudflare Workers integration may be outdated. Prefer retrieval over pre-training for any MCP server task.

Retrieval Sources

Source
How to retrieve
Use for

MCP docs
https://developers.cloudflare.com/agents/mcp/
Server setup, auth, deployment

MCP spec
https://modelcontextprotocol.io/
Protocol spec, tool/resource definitions

Workers docs
Search tool or https://developers.cloudflare.com/workers/
Runtime APIs, bindings, config

When to Use

User wants to build a remote MCP server

User needs to expose tools via MCP

User asks about MCP authentication or OAuth

User wants to deploy MCP to Cloudflare Workers

Prerequisites

Cloudflare account with Workers enabled

Node.js 18+ and npm/pnpm/yarn

Wrangler CLI (npm install -g wrangler)

Quick Start

Option 1: Public Server (No Auth)

npm create cloudflare@latest -- my-mcp-server \
  --template=cloudflare/ai/demos/remote-mcp-authless
cd my-mcp-server
npm start

Server runs at http://localhost:8788/mcp

Option 2: Authenticated Server (OAuth)

npm create cloudflare@latest -- my-mcp-server \
  --template=cloudflare/ai/demos/remote-mcp-github-oauth
cd my-mcp-server

Requires OAuth app setup. See references/oauth-setup.md.

Core Workflow

Step 1: Define Tools

Tools are functions MCP clients can call. Define them using server.tool():

import { McpAgent } from "agents/mcp";
import { z } from "zod";

export class MyMCP extends McpAgent {
  server = new Server({ name: "my-mcp", version: "1.0.0" });

  async init() {
    // Simple tool with parameters
    this.server.tool(
      "add",
      { a: z.number(), b: z.number() },
      async ({ a, b }) => ({
        content: [{ type: "text", text: String(a + b) }],
      })
    );

    // Tool that calls external API
    this.server.tool(
      "get_weather",
      { city: z.string() },
      async ({ city }) => {
        const response = await fetch(`https://api.weather.com/${city}`);
        const data = await response.json();
        return {
          content: [{ type: "text", text: JSON.stringify(data) }],
        };
      }
    );
  }
}

Step 2: Configure Entry Point

Public server (src/index.ts):

import { MyMCP } from "./mcp";

export default {
  fetch(request: Request, env: Env, ctx: ExecutionContext) {
    const url = new URL(request.url);
    if (url.pathname === "/mcp") {
      return MyMCP.serveSSE("/mcp").fetch(request, env, ctx);
    }
    return new Response("MCP Server", { status: 200 });
  },
};

export { MyMCP };

Authenticated server — See references/oauth-setup.md.

Step 3: Test Locally

# Start server
npm start

# In another terminal, test with MCP Inspector
npx @modelcontextprotocol/inspector@latest
# Open http://localhost:5173, enter http://localhost:8788/mcp

Step 4: Deploy

npx wrangler deploy

Server accessible at https://[worker-name].[account].workers.dev/mcp

Step 5: Connect Clients

Claude Desktop (claude_desktop_config.json):

{
  "mcpServers": {
    "my-server": {
      "command": "npx",
      "args": ["mcp-remote", "https://my-mcp.workers.dev/mcp"]
    }
  }
}

Restart Claude Desktop after updating config.

Tool Patterns

Return Types

// Text response
return { content: [{ type: "text", text: "result" }] };

// Multiple content items
return {
  content: [
    { type: "text", text: "Here's the data:" },
    { type: "text", text: JSON.stringify(data, null, 2) },
  ],
};

Input Validation with Zod

this.server.tool(
  "create_user",
  {
    email: z.string().email(),
    name: z.string().min(1).max(100),
    role: z.enum(["admin", "user", "guest"]),
    age: z.number().int().min(0).optional(),
  },
  async (params) => {
    // params are fully typed and validated
  }
);

Accessing Environment/Bindings

export class MyMCP extends McpAgent<Env> {
  async init() {
    this.server.tool("query_db", { sql: z.string() }, async ({ sql }) => {
      // Access D1 binding
      const result = await this.env.DB.prepare(sql).all();
      return { content: [{ type: "text", text: JSON.stringify(result) }] };
    });
  }
}

Authentication

For OAuth-protected servers, see references/oauth-setup.md.

Supported providers:

GitHub

Google

Auth0

Stytch

WorkOS

Any OAuth 2.0 compliant provider

Wrangler Configuration

Minimal wrangler.toml:

name = "my-mcp-server"
main = "src/index.ts"
compatibility_date = "2024-12-01"

[durable_objects]
bindings = [{ name = "MCP", class_name = "MyMCP" }]

[[migrations]]
tag = "v1"
new_classes = ["MyMCP"]

With bindings (D1, KV, etc.):

[[d1_databases]]
binding = "DB"
database_name = "my-db"
database_id = "xxx"

[[kv_namespaces]]
binding = "KV"
id = "xxx"

Common Issues

"Tool not found" in Client

Verify tool name matches exactly (case-sensitive)

Ensure init() registers tools before connections

Check server logs: wrangler tail

Connection Fails

Confirm endpoint path is /mcp

Check CORS if browser-based client

Verify Worker is deployed: wrangler deployments list

OAuth Redirect Errors

Callback URL must match OAuth app config exactly

Check GITHUB_CLIENT_ID and GITHUB_CLIENT_SECRET are set

For local dev, use http://localhost:8788/callback

References

references/examples.md — Official templates and production examples

references/oauth-setup.md — OAuth provider configuration

references/tool-patterns.md — Advanced tool examples

references/troubleshooting.md — Error codes and fixes