MCP server in NextJS app crashes API Endpoints with error - No response is returned from route handler

[akshay-kumardhage-deel]

Problem

When a MCP server is built with latest versions mcp-handler@1.0.6 and @modelcontextprotocol/sdk@1.25.2 the Nextjs App router API endpoints throw below error

Error:
Error: No response is returned from route handler '[project]/app/api/health/route.ts'. Ensure you return a `Response` or a `NextResponse` in all branches of your handler. at ignore-listed frames

app/api/health/route.ts

import { NextResponse } from "next/server"
export const GET = () => {
    return NextResponse.json({
        status: "Ok"
    })
}

Same error persists for versions 1.25.X of @modelcontextprotocol/sdk along side of mcp-handler@1.0.6

The only properly working version found - mcp-handler@1.0.6 and @modelcontextprotocol/sdk@1.24.3

Expected

NextJS API endpoints must keep functioning correctly along side of MCP routes in same app, when using mcp-handler@1.0.6 and @modelcontextprotocol/sdk@1.25.x

Reproducible Steps

1. Create a NextJS App Router project
   e.g. npx create-next-app@latest my-app --yes
2. Install dependencies with latest versions - mcp-handler@1.0.6 and @modelcontextprotocol/sdk@1.25.2
3. Create a simple GET API endpoint
   e.g. app/api/health/route.ts as above
4. Create a simple MCP server
   e.g. follow mcp-handler example - https://www.npmjs.com/package/mcp-handler
5. Run the app and connect to the MCP Server from tools like Cursor or MCP Inspector (MUST connect to MCP first)
6. Ping the api/health endpoint. Observe that server starts to throw error -
   Error: No response is returned from route handler '[project]/app/api/health/route.ts'. Ensure you return a `Response` or a `NextResponse` in all branches of your handler

[balataca]

For those who need a workaround to use version @modelcontextprotocol/sdk@1.25.2 that doesn't have a security vulnerability:

1. Remove mcp-handler:

npm uninstall mcp-handler

2. Install @modelcontextprotocol/sdk@1.25.2

npm install @modelcontextprotocol/sdk

3. Create the mcp server and export the handlers you need:

import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { WebStandardStreamableHTTPServerTransport } from "@modelcontextprotocol/sdk/server/webStandardStreamableHttp.js";
import type { NextRequest } from "next/server";

const handler = async (req: NextRequest) => {
  // Create MCP server
  const server = new McpServer(
    {
      name: "mcp-server",
      version: "1.0.0",
    },
    {
      capabilities: {
        tools: {},
      },
    },
  );

  // Register tools, prompts, resources etc.
  // server.registerTool() [...]

  const transport = new WebStandardStreamableHTTPServerTransport();
  await server.connect(transport);

  return transport.handleRequest(req);
};

// You can also use DELETE and other methods with the handler
export { handler as GET, handler as POST };

It is working great for me after I removed the mcp-handler dependency. I moved the server creation logic to a factory createMcpServer, but you can also register directly on the handler.

[HemangRajvanshy]

The issue is tracked here: modelcontextprotocol/typescript-sdk#1369