Ma1orek Replay Cloudflare MCP Server

Cloudflare MCP Server Skill

Build and deploy Model Context Protocol (MCP) servers on Cloudflare Workers with TypeScript.

Status: Production Ready ✅ Last Updated: 2026-01-21 Latest Versions: @modelcontextprotocol/sdk@1.25.3, @cloudflare/workers-oauth-provider@0.2.2, agents@0.3.6

Recent Updates (2025):

• September 2025: Code Mode (agents write code vs calling tools, auto-generated TypeScript API from schema)
• August 2025: MCP Elicitation (interactive workflows, user input during execution), Task Queues, Email Integration
• July 2025: MCPClientManager (connection management, OAuth flow, hibernation)
• April 2025: HTTP Streamable Transport (single endpoint, recommended over SSE), Python MCP support
• May 2025: Claude.ai remote MCP support, use-mcp React library, major partnerships

What is This Skill?

This skill teaches you to build remote MCP servers on Cloudflare - the ONLY platform with official remote MCP support.

Use when: Avoiding 24+ common MCP + Cloudflare errors (especially URL path mismatches - the #1 failure cause)

🚀 Quick Start (5 Minutes)

Start with Cloudflare's official template:

npm create cloudflare@latest -- my-mcp-server \
  --template=cloudflare/ai/demos/remote-mcp-authless
cd my-mcp-server && npm install && npm run dev

Choose template based on auth needs:

• remote-mcp-authless - No auth (recommended for most)
• remote-mcp-github-oauth - GitHub OAuth
• remote-mcp-google-oauth - Google OAuth
• remote-mcp-auth0 / remote-mcp-authkit - Enterprise SSO
• mcp-server-bearer-auth - Custom auth

All templates: https://github.com/cloudflare/ai/tree/main/demos

Production examples: https://github.com/cloudflare/mcp-server-cloudflare (15 servers with real integrations)

Deployment Workflow

# 1. Create from template
npm create cloudflare@latest -- my-mcp --template=cloudflare/ai/demos/remote-mcp-authless
cd my-mcp && npm install && npm run dev

# 2. Deploy
npx wrangler deploy
# Note the output URL: https://my-mcp.YOUR_ACCOUNT.workers.dev

# 3. Test (PREVENTS 80% OF ERRORS!)
curl https://my-mcp.YOUR_ACCOUNT.workers.dev/sse
# Expected: {"name":"My MCP Server","version":"1.0.0","transports":["/sse","/mcp"]}
# Got 404? See "HTTP Transport Fundamentals" below

# 4. Configure client (~/.config/claude/claude_desktop_config.json)
{
  "mcpServers": {
    "my-mcp": {
      "url": "https://my-mcp.YOUR_ACCOUNT.workers.dev/sse"  // Must match curl URL!
    }
  }
}

# 5. Restart Claude Desktop (config only loads at startup)

Post-Deployment Checklist:

• curl returns server info (not 404)
• Client URL matches curl URL exactly
• Claude Desktop restarted
• Tools visible in Claude Desktop
• Test tool call succeeds

⚠️ CRITICAL: HTTP Transport Fundamentals

The #1 reason MCP servers fail to connect is URL path configuration mistakes.

URL Path Configuration Deep-Dive

When you serve an MCP server at a specific path, the client URL must match exactly.

Example 1: Serving at /sse

// src/index.ts
export default {
  fetch(request: Request, env: Env, ctx: ExecutionContext) {
    const { pathname } = new URL(request.url);

    if (pathname.startsWith("/sse")) {
      return MyMCP.serveSSE("/sse").fetch(request, env, ctx);  // ← Base path is "/sse"
    }

    return new Response("Not Found", { status: 404 });
  }
};

Client configuration MUST include /sse:

{
  "mcpServers": {
    "my-mcp": {
      "url": "https://my-mcp.workers.dev/sse"  // ✅ Correct
    }
  }
}

❌ WRONG client configurations:

"url": "https://my-mcp.workers.dev"      // Missing /sse → 404
"url": "https://my-mcp.workers.dev/"     // Missing /sse → 404
"url": "http://localhost:8788"           // Wrong after deploy

Example 2: Serving at / (root)

export default {
  fetch(request: Request, env: Env, ctx: ExecutionContext) {
    return MyMCP.serveSSE("/").fetch(request, env, ctx);  // ← Base path is "/"
  }
};

Client configuration:

{
  "mcpServers": {
    "my-mcp": {
      "url": "https://my-mcp.workers.dev"  // ✅ Correct (no /sse)
    }
  }
}

How Base Path Affects Tool URLs

When you call serveSSE("/sse"), MCP tools are served at:

https://my-mcp.workers.dev/sse/tools/list
https://my-mcp.workers.dev/sse/tools/call
https://my-mcp.workers.dev/sse/resources/list

When you call serveSSE("/"), MCP tools are served at:

https://my-mcp.workers.dev/tools/list
https://my-mcp.workers.dev/tools/call
https://my-mcp.workers.dev/resources/list

The base path is prepended to all MCP endpoints automatically.

Request/Response Lifecycle

1. Client connects to: https://my-mcp.workers.dev/sse
                                ↓
2. Worker receives request: { url: "https://my-mcp.workers.dev/sse", ... }
                                ↓
3. Your fetch handler: const { pathname } = new URL(request.url)
                                ↓
4. pathname === "/sse" → Check passes
                                ↓
5. MyMCP.serveSSE("/sse").fetch() → MCP server handles request
                                ↓
6. Tool calls routed to: /sse/tools/call

If client connects to https://my-mcp.workers.dev (missing /sse):

pathname === "/" → Check fails → 404 Not Found

Testing Your URL Configuration

Step 1: Deploy your MCP server

npx wrangler deploy
# Output: Deployed to https://my-mcp.YOUR_ACCOUNT.workers.dev

Step 2: Test the base path with curl

# If serving at /sse, test this URL:
curl https://my-mcp.YOUR_ACCOUNT.workers.dev/sse

# Should return MCP server info (not 404)

Step 3: Update client config with the EXACT URL you tested

{
  "mcpServers": {
    "my-mcp": {
      "url": "https://my-mcp.YOUR_ACCOUNT.workers.dev/sse"  // Match curl URL
    }
  }
}

Step 4: Restart Claude Desktop

Post-Deployment Checklist

After deploying, verify:

• curl https://worker.dev/sse returns MCP server info (not 404)
• Client config URL matches deployed URL exactly
• No typos in URL (common: workes.dev instead of workers.dev)
• Using https:// (not http://) for deployed Workers
• If using OAuth, redirect URI also updated

Transport Selection

Two transports available:

1. SSE (Server-Sent Events) - Legacy, wide compatibility

   MyMCP.serveSSE("/sse").fetch(request, env, ctx)
   

2. Streamable HTTP - 2025 standard (recommended), single endpoint

   MyMCP.serve("/mcp").fetch(request, env, ctx)
   

Support both for maximum compatibility:

export default {
  fetch(request: Request, env: Env, ctx: ExecutionContext) {
    const { pathname } = new URL(request.url);

    if (pathname.startsWith("/sse")) {
      return MyMCP.serveSSE("/sse").fetch(request, env, ctx);
    }
    if (pathname.startsWith("/mcp")) {
      return MyMCP.serve("/mcp").fetch(request, env, ctx);
    }

    return new Response("Not Found", { status: 404 });
  }
};

CRITICAL: Use pathname.startsWith() to match paths correctly!

2025 Knowledge Gaps

MCP Elicitation (August 2025)

MCP servers can now request user input during tool execution:

// Request user input during tool execution
const result = await this.elicit({
  prompt: "Enter your API key:",
  type: "password"
});

// Interactive workflows with Durable Objects state
await this.state.storage.put("api_key", result);

Use cases: Confirmations, forms, multi-step workflows State: Preserved during agent hibernation

Code Mode (September 2025)

Agents SDK converts MCP schema → TypeScript API:

// Old: Direct tool calls
await server.callTool("get_user", { id: "123" });

// New: Type-safe generated API
const user = await api.getUser("123");

Benefits: Auto-generated doc comments, type safety, code completion

MCPClientManager (July 2025)

New class for MCP client capabilities:

import { MCPClientManager } from "agents/mcp";

const manager = new MCPClientManager(env);
await manager.connect("https://external-mcp.com/sse");
// Auto-discovers tools, resources, prompts
// Handles reconnection, OAuth flow, hibernation

Task Queues & Email (August 2025)

// Task queues for background jobs
await this.queue.send({ task: "process_data", data });

// Email integration
async onEmail(message: Email) {
  // Process incoming email
  const response = await this.generateReply(message);
  await this.sendEmail(response);
}

HTTP Streamable Transport Details (April 2025)

Single endpoint replaces separate connection/messaging endpoints:

// Old: Separate endpoints
/connect  // Initialize connection
/message  // Send/receive messages

// New: Single streamable endpoint
/mcp      // All communication via HTTP streaming

Benefit: Simplified architecture, better performance

Security Considerations

PKCE Bypass Vulnerability (CRITICAL)

CVE: GHSA-qgp8-v765-qxx9 Severity: Critical Fixed in: @cloudflare/workers-oauth-provider@0.0.5

Problem: Earlier versions of the OAuth provider library had a critical vulnerability that completely bypassed PKCE protection, potentially allowing attackers to intercept authorization codes.

Action Required:

# Check current version
npm list @cloudflare/workers-oauth-provider

# Update if < 0.0.5
npm install @cloudflare/workers-oauth-provider@latest

Minimum Safe Version: @cloudflare/workers-oauth-provider@0.0.5 or later

Token Storage Best Practices

Always use encrypted storage for OAuth tokens:

// ✅ GOOD: workers-oauth-provider handles encryption automatically
export default new OAuthProvider({
  kv: (env) => env.OAUTH_KV,  // Tokens stored encrypted
  // ...
});

// ❌ BAD: Storing tokens in plain text
await env.KV.put("access_token", token);  // Security risk!

User-scoped KV keys prevent data leakage between users:

// ✅ GOOD: Namespace by user ID
await env.KV.put(`user:${userId}:todos`, data);

// ❌ BAD: Global namespace
await env.KV.put(`todos`, data);  // Data visible to all users!

Authentication Patterns

Choose auth based on use case:

1. No Auth - Internal tools, dev (Template: remote-mcp-authless)

2. Bearer Token - Custom auth (Template: mcp-server-bearer-auth)

   // Validate Authorization: Bearer <token>
   const token = request.headers.get("Authorization")?.replace("Bearer ", "");
   if (!await validateToken(token, env)) {
     return new Response("Unauthorized", { status: 401 });
   }
   

3. OAuth Proxy - GitHub/Google (Template: remote-mcp-github-oauth)

   import { OAuthProvider, GitHubHandler } from "@cloudflare/workers-oauth-provider";
   
   export default new OAuthProvider({
     authorizeEndpoint: "/authorize",
     tokenEndpoint: "/token",
     defaultHandler: new GitHubHandler({
       clientId: (env) => env.GITHUB_CLIENT_ID,
       clientSecret: (env) => env.GITHUB_CLIENT_SECRET,
       scopes: ["repo", "user:email"]
     }),
     kv: (env) => env.OAUTH_KV,
     apiHandlers: { "/sse": MyMCP.serveSSE("/sse") }
   });
   

   ⚠️ CRITICAL: All OAuth URLs (url, authorizationUrl, tokenUrl) must use same domain

4. Remote OAuth with DCR - Full OAuth provider (Template: remote-mcp-authkit)

Security levels: No Auth (⚠️) < Bearer (✅) < OAuth Proxy (✅✅) < Remote OAuth (✅✅✅)

Stateful MCP Servers (Durable Objects)

McpAgent extends Durable Objects for per-session state:

// Storage API
await this.state.storage.put("key", "value");
const value = await this.state.storage.get<string>("key");

// Required wrangler.jsonc
{
  "durable_objects": {
    "bindings": [{ "name": "MY_MCP", "class_name": "MyMCP" }]
  },
  "migrations": [{ "tag": "v1", "new_classes": ["MyMCP"] }]  // Required on first deploy!
}

Critical: Migrations required on first deployment

Cost: Durable Objects now included in free tier (2025)

Architecture: Internal vs External Transports

Important: McpAgent uses different transports for client-facing vs internal communication.

Source: GitHub Issue #172

Transport Architecture

Client --- (SSE or HTTP) --> Worker --- (WebSocket) --> Durable Object

Client → Worker (External):

• SSE transport: /sse endpoint
• HTTP Streamable: /mcp endpoint
• Client chooses transport

Worker → Durable Object (Internal):

• Always WebSocket
• Required by PartyServer (McpAgent's internal dependency)
• Automatic upgrade, invisible to client

What This Means

1. SSE clients are fully supported - External interface can be SSE
2. WebSocket is mandatory for DO - Internal Worker-DO communication always uses WebSocket
3. This is not a limitation - It's an implementation detail of McpAgent's architecture

Example

export default {
  fetch(request: Request, env: Env, ctx: ExecutionContext) {
    const { pathname } = new URL(request.url);

    // Client uses SSE
    if (pathname.startsWith("/sse")) {
      // ✅ Client → Worker: SSE
      // ✅ Worker → DO: WebSocket (automatic)
      return MyMCP.serveSSE("/sse").fetch(request, env, ctx);
    }

    return new Response("Not Found", { status: 404 });
  }
};

Key Takeaway: You can serve SSE to clients without worrying about the internal WebSocket requirement.

Common Patterns

Tool Return Format (CRITICAL)

Source: Stytch Blog - Building MCP Server with OAuth

All MCP tools must return this exact format:

this.server.tool(
  "my_tool",
  { /* schema */ },
  async (params) => {
    // ✅ CORRECT: Return object with content array
    return {
      content: [
        { type: "text", text: "Your result here" }
      ]
    };

    // ❌ WRONG: Raw string
    return "Your result here";

    // ❌ WRONG: Plain object
    return { result: "Your result here" };
  }
);

Common mistake: Returning raw strings or plain objects instead of proper MCP content format. This causes client parsing errors.

Conditional Tool Registration

Source: Cloudflare Blog - Building AI Agents

Dynamically add tools based on authenticated user:

export class MyMCP extends McpAgent<Env> {
  async init() {
    this.server = new McpServer({ name: "My MCP" });

    // Base tools for all users
    this.server.tool("public_tool", { /* schema */ }, async (params) => {
      // Available to everyone
    });

    // Conditional tools based on user
    const userId = this.props?.userId;
    if (await this.isAdmin(userId)) {
      this.server.tool("admin_tool", { /* schema */ }, async (params) => {
        // Only available to admins
      });
    }

    // Premium features
    if (await this.isPremiumUser(userId)) {
      this.server.tool("premium_feature", { /* schema */ }, async (params) => {
        // Only for premium users
      });
    }
  }

  private async isAdmin(userId?: string): Promise<boolean> {
    if (!userId) return false;
    const userRole = await this.state.storage.get<string>(`user:${userId}:role`);
    return userRole === "admin";
  }
}

Use cases:

• Feature flags per user
• Premium vs free tier tools
• Role-based access control (RBAC)
• A/B testing new tools

Caching with DO Storage

async getCached<T>(key: string, ttlMs: number, fetchFn: () => Promise<T>): Promise<T> {
  const cached = await this.state.storage.get<{ data: T, timestamp: number }>(key);
  if (cached && Date.now() - cached.timestamp < ttlMs) {
    return cached.data;
  }
  const data = await fetchFn();
  await this.state.storage.put(key, { data, timestamp: Date.now() });
  return data;
}

Rate Limiting

async rateLimit(key: string, maxRequests: number, windowMs: number): Promise<boolean> {
  const requests = await this.state.storage.get<number[]>(`ratelimit:${key}`) || [];
  const recentRequests = requests.filter(ts => Date.now() - ts < windowMs);
  if (recentRequests.length >= maxRequests) return false;
  recentRequests.push(Date.now());
  await this.state.storage.put(`ratelimit:${key}`, recentRequests);
  return true;
}

24 Known Errors (With Solutions)

1. McpAgent Class Not Exported

Error: TypeError: Cannot read properties of undefined (reading 'serve')

Cause: Forgot to export McpAgent class

Solution:

export class MyMCP extends McpAgent { ... }  // ✅ Must export
export default { fetch() { ... } }

2. Base Path Configuration Mismatch (Most Common!)

Error: 404 Not Found or Connection failed

Cause: serveSSE("/sse") but client configured with https://worker.dev (missing /sse)

Solution: Match base paths exactly

// Server serves at /sse
MyMCP.serveSSE("/sse").fetch(...)

// Client MUST include /sse
{ "url": "https://worker.dev/sse" }  // ✅ Correct
{ "url": "https://worker.dev" }      // ❌ Wrong - 404

Debug steps:

1. Check what path your server uses: serveSSE("/sse") vs serveSSE("/")
2. Test with curl: curl https://worker.dev/sse
3. Update client config to match curl URL

3. Transport Type Confusion

Error: Connection failed: Unexpected response format

Cause: Client expects SSE but connects to HTTP endpoint (or vice versa)

Solution: Match transport types

// SSE transport
MyMCP.serveSSE("/sse")  // Client URL: https://worker.dev/sse

// HTTP transport
MyMCP.serve("/mcp")     // Client URL: https://worker.dev/mcp

Best practice: Support both transports (see Transport Selection Guide)

4. pathname.startsWith() Logic Error

Error: Both /sse and /mcp routes fail or conflict

Cause: Incorrect path matching logic

Solution: Use startsWith() correctly

// ✅ CORRECT
if (pathname.startsWith("/sse")) {
  return MyMCP.serveSSE("/sse").fetch(...);
}
if (pathname.startsWith("/mcp")) {
  return MyMCP.serve("/mcp").fetch(...);
}

// ❌ WRONG: Exact match breaks sub-paths
if (pathname === "/sse") {  // Breaks /sse/tools/list
  return MyMCP.serveSSE("/sse").fetch(...);
}

5. Local vs Deployed URL Mismatch

Error: Works in dev, fails after deployment

Cause: Client still configured with localhost URL

Solution: Update client config after deployment

// Development
{ "url": "http://localhost:8788/sse" }

// ⚠️ MUST UPDATE after npx wrangler deploy
{ "url": "https://my-mcp.YOUR_ACCOUNT.workers.dev/sse" }

Post-deployment checklist:

• Run npx wrangler deploy and note output URL
• Update client config with deployed URL
• Test with curl
• Restart Claude Desktop

6. OAuth Redirect URI Mismatch

Error: OAuth error: redirect_uri does not match

Cause: OAuth redirect URI doesn't match deployed URL

Solution: Update ALL OAuth URLs after deployment

{
  "url": "https://my-mcp.YOUR_ACCOUNT.workers.dev/sse",
  "auth": {
    "type": "oauth",
    "authorizationUrl": "https://my-mcp.YOUR_ACCOUNT.workers.dev/authorize",  // Must match deployed domain
    "tokenUrl": "https://my-mcp.YOUR_ACCOUNT.workers.dev/token"
  }
}

CRITICAL: All URLs must use the same protocol and domain!

7. Missing OPTIONS Handler (CORS Preflight)

Error: Access to fetch at '...' blocked by CORS policy or Method Not Allowed

Cause: Browser clients send OPTIONS requests for CORS preflight, but server doesn't handle them

Solution: Add OPTIONS handler

export default {
  fetch(request: Request, env: Env, ctx: ExecutionContext) {
    // Handle CORS preflight
    if (request.method === "OPTIONS") {
      return new Response(null, {
        status: 204,
        headers: {
          "Access-Control-Allow-Origin": "*",
          "Access-Control-Allow-Methods": "GET, POST, OPTIONS",
          "Access-Control-Allow-Headers": "Content-Type, Authorization",
          "Access-Control-Max-Age": "86400"
        }
      });
    }

    // ... rest of your fetch handler
  }
};

When needed: Browser-based MCP clients (like MCP Inspector in browser)

8. Request Body Validation Missing

Error: TypeError: Cannot read properties of undefined or Unexpected token in JSON parsing

Cause: Client sends malformed JSON, server doesn't validate before parsing

Solution: Wrap request handling in try/catch

export default {
  async fetch(request: Request, env: Env, ctx: ExecutionContext) {
    try {
      // Your MCP server logic
      return await MyMCP.serveSSE("/sse").fetch(request, env, ctx);
    } catch (error) {
      console.error("Request handling error:", error);
      return new Response(
        JSON.stringify({
          error: "Invalid request",
          details: error.message
        }),
        {
          status: 400,
          headers: { "Content-Type": "application/json" }
        }
      );
    }
  }
};

9. Environment Variable Validation Missing

Error: TypeError: env.API_KEY is undefined or silent failures (tools return empty data)

Cause: Required environment variables not configured or missing at runtime

Solution: Add startup validation

export class MyMCP extends McpAgent<Env> {
  async init() {
    // Validate required environment variables
    if (!this.env.API_KEY) {
      throw new Error("API_KEY environment variable not configured");
    }
    if (!this.env.DATABASE_URL) {
      throw new Error("DATABASE_URL environment variable not configured");
    }

    // Continue with tool registration
    this.server.tool(...);
  }
}

Configuration checklist:

• Development: Add to .dev.vars (local only, gitignored)
• Production: Add to wrangler.jsonc vars (public) or use wrangler secret (sensitive)

Best practices:

# .dev.vars (local development, gitignored)
API_KEY=dev-key-123
DATABASE_URL=http://localhost:3000

# wrangler.jsonc (public config)
{
  "vars": {
    "ENVIRONMENT": "production",
    "LOG_LEVEL": "info"
  }
}

# wrangler secret (production secrets)
npx wrangler secret put API_KEY
npx wrangler secret put DATABASE_URL

10. McpAgent vs McpServer Confusion

Error: TypeError: server.registerTool is not a function or this.server is undefined

Cause: Trying to use standalone SDK patterns with McpAgent class

Solution: Use McpAgent's this.server.tool() pattern

// ❌ WRONG: Mixing standalone SDK with McpAgent
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";

const server = new McpServer({ name: "My Server" });
server.registerTool(...);  // Not compatible with McpAgent!

export class MyMCP extends McpAgent { /* no server property */ }

// ✅ CORRECT: McpAgent pattern
export class MyMCP extends McpAgent<Env> {
  server = new McpServer({
    name: "My MCP Server",
    version: "1.0.0"
  });

  async init() {
    this.server.tool("tool_name", ...);  // Use this.server
  }
}

Key difference: McpAgent provides this.server property, standalone SDK doesn't.

11. WebSocket Hibernation State Loss

Error: Tool calls fail after reconnect with "state not found"

Cause: In-memory state cleared on hibernation

Solution: Use this.state.storage instead of instance properties

// ❌ DON'T: Lost on hibernation
this.userId = "123";

// ✅ DO: Persists through hibernation
await this.state.storage.put("userId", "123");

12. Durable Objects Binding Missing

Error: TypeError: Cannot read properties of undefined (reading 'idFromName')

Cause: Forgot DO binding in wrangler.jsonc

Solution: Add binding (see Stateful MCP Servers section)

{
  "durable_objects": {
    "bindings": [
      {
        "name": "MY_MCP",
        "class_name": "MyMCP",
        "script_name": "my-mcp-server"
      }
    ]
  }
}

13. Migration Not Defined

Error: Error: Durable Object class MyMCP has no migration defined

Cause: First DO deployment requires migration

Solution:

{
  "migrations": [
    { "tag": "v1", "new_classes": ["MyMCP"] }
  ]
}

14. serializeAttachment() Not Used

Error: WebSocket metadata lost on hibernation wake

Cause: Not using serializeAttachment() to preserve connection metadata

Solution: See WebSocket Hibernation section

15. OAuth Consent Screen Disabled

Security risk: Users don't see what permissions they're granting

Cause: allowConsentScreen: false in production

Solution: Always enable in production

export default new OAuthProvider({
  allowConsentScreen: true,  // ✅ Always true in production
  // ...
});

16. JWT Signing Key Missing

Error: Error: JWT_SIGNING_KEY environment variable not set

Cause: OAuth Provider requires signing key for tokens

Solution:

# Generate secure key
openssl rand -base64 32

# Add to wrangler secret
npx wrangler secret put JWT_SIGNING_KEY

17. Tool Schema Validation Error

Error: ZodError: Invalid input type

Cause: Client sends string, schema expects number (or vice versa)

Solution: Use Zod transforms

// Accept string, convert to number
param: z.string().transform(val => parseInt(val, 10))

// Or: Accept both types
param: z.union([z.string(), z.number()]).transform(val =>
  typeof val === "string" ? parseInt(val, 10) : val
)

18. Multiple Transport Endpoints Conflicting

Error: /sse returns 404 after adding /mcp

Cause: Incorrect path matching (missing startsWith())

Solution: Use startsWith() or exact matches correctly (see Error #4)

19. Local Testing with Miniflare Limitations

Error: OAuth flow fails in local dev, or Durable Objects behave differently

Cause: Miniflare doesn't support all DO features

Solution: Use npx wrangler dev --remote for full DO support

# Local simulation (faster but limited)
npm run dev

# Remote DOs (slower but accurate)
npx wrangler dev --remote

20. Client Configuration Format Error

Error: Claude Desktop doesn't recognize server

Cause: Wrong JSON format in claude_desktop_config.json

Solution: See "Connect Claude Desktop" section for correct format

Common mistakes:

// ❌ WRONG: Missing "mcpServers" wrapper
{
  "my-mcp": {
    "url": "https://worker.dev/sse"
  }
}

// ❌ WRONG: Trailing comma
{
  "mcpServers": {
    "my-mcp": {
      "url": "https://worker.dev/sse",  // ← Remove comma
    }
  }
}

// ✅ CORRECT
{
  "mcpServers": {
    "my-mcp": {
      "url": "https://worker.dev/sse"
    }
  }
}

21. Health Check Endpoint Missing

Issue: Can't tell if Worker is running or if URL is correct

Impact: Debugging connection issues takes longer

Solution: Add health check endpoint (see Transport Selection Guide)

Test:

curl https://my-mcp.workers.dev/health
# Should return: {"status":"ok","transports":{...}}

22. CORS Headers Missing

Error: Access to fetch at '...' blocked by CORS policy

Cause: MCP server doesn't return CORS headers for cross-origin requests

Solution: Add CORS headers to all responses

// Manual CORS (if not using OAuthProvider)
const corsHeaders = {
  "Access-Control-Allow-Origin": "*",  // Or specific origin
  "Access-Control-Allow-Methods": "GET, POST, OPTIONS",
  "Access-Control-Allow-Headers": "Content-Type, Authorization"
};

// Add to responses
return new Response(body, {
  headers: {
    ...corsHeaders,
    "Content-Type": "application/json"
  }
});

Note: OAuthProvider handles CORS automatically!

23. IoContext Timeout During MCP Initialization

Error: IoContext timed out due to inactivity, waitUntil tasks were cancelled

Source: GitHub Issue #640

Cause: When implementing MCP servers using McpAgent with custom Bearer authentication, the IoContext times out during the MCP protocol initialization handshake (before any tools are called).

Symptoms:

• Timeout occurs before any tools are called
• ~2 minute gap between initial request and agent initialization
• Internal methods work (setInitializeRequest, getInitializeRequest, updateProps)
• Both GET and POST to /mcp are canceled
• Error: "IoContext timed out due to inactivity, waitUntil tasks were cancelled"

Affected Code Pattern:

// Custom Bearer auth without OAuthProvider wrapper
export default {
  fetch: async (req, env, ctx) => {
    const authHeader = req.headers.get("Authorization");
    if (!authHeader?.startsWith("Bearer ")) {
      return new Response("Unauthorized", { status: 401 });
    }

    if (url.pathname === "/sse") {
      return MyMCP.serveSSE("/sse")(req, env, ctx);  // ← Timeout here
    }
    return new Response("Not found", { status: 404 });
  }
};

Root Cause Hypothesis:

• May require OAuthProvider wrapper even for custom Bearer auth
• Possible missing timeout configuration for Durable Object IoContext
• May need CloudflareMCPServer instead of standard McpServer

Workaround: Use official templates with OAuthProvider pattern instead of custom Bearer auth:

// Use OAuthProvider wrapper (recommended)
import { OAuthProvider } from "@cloudflare/workers-oauth-provider";

export default new OAuthProvider({
  authorizeEndpoint: "/authorize",
  tokenEndpoint: "/token",
  // ... OAuth config
  apiHandlers: { "/sse": MyMCP.serveSSE("/sse") }
});

Status: Investigation ongoing (issue open as of 2026-01-21)

24. OAuth Remote Connection Failures

Error: Connection to remote MCP server fails when using OAuth (works locally but fails when deployed)

Source: GitHub Issue #444

Cause: When deploying MCP client from Cloudflare Agents repository to Workers, client fails to connect to MCP servers secured with OAuth.

Symptoms:

• Works perfectly in local development
• Fails after deployment to Workers
• OAuth handshake never completes
• Client can't establish connection

Troubleshooting Steps:

1. Verify OAuth tokens are handled correctly during remote connection attempts

   // Check token is being passed to remote server
   console.log("Connecting with token:", token ? "present" : "missing");
   

2. Check network permissions to access OAuth provider

   // Ensure Worker can reach OAuth endpoints
   const response = await fetch("https://oauth-provider.com/token");
   

3. Verify CORS configuration on OAuth provider

   // OAuth provider must allow Worker origin
   headers: {
     "Access-Control-Allow-Origin": "https://your-worker.workers.dev",
     "Access-Control-Allow-Methods": "POST, OPTIONS",
     "Access-Control-Allow-Headers": "Content-Type, Authorization"
   }
   

4. Check redirect URIs match deployed URLs

   {
     "url": "https://mcp.workers.dev/sse",
     "auth": {
       "authorizationUrl": "https://mcp.workers.dev/authorize",  // Must match deployed domain
       "tokenUrl": "https://mcp.workers.dev/token"
     }
   }
   

Deployment Checklist:

• All OAuth URLs use deployed domain (not localhost)
• CORS headers configured on OAuth provider
• Network requests to OAuth provider allowed in Worker
• Redirect URIs registered with OAuth provider
• Environment variables set in production (wrangler secret)

Related: Issue #640 (both involve OAuth/auth in remote deployments)

Testing & Deployment

# Local dev
npm run dev                    # Miniflare (fast)
npx wrangler dev --remote      # Remote DOs (accurate)

# Test with MCP Inspector
npx @modelcontextprotocol/inspector@latest
# Open http://localhost:5173, enter http://localhost:8788/sse

# Deploy
npx wrangler login  # First time only
npx wrangler deploy
# ⚠️ CRITICAL: Update client config with deployed URL!

# Monitor logs
npx wrangler tail

Official Documentation

• Cloudflare Agents: https://developers.cloudflare.com/agents/
• MCP Specification: https://modelcontextprotocol.io/
• Official Templates: https://github.com/cloudflare/ai/tree/main/demos
• Production Servers: https://github.com/cloudflare/mcp-server-cloudflare
• workers-oauth-provider: https://github.com/cloudflare/workers-oauth-provider

Package Versions: @modelcontextprotocol/sdk@1.25.3, @cloudflare/workers-oauth-provider@0.2.2, agents@0.3.6 Last Verified: 2026-01-21 Errors Prevented: 24 documented issues (100% prevention rate) Skill Version: 3.1.0 | Changes: Added IoContext timeout (#23), OAuth remote failures (#24), Security section (PKCE vulnerability), Architecture clarification (internal WebSocket), Tool return format pattern, Conditional tool registration

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