Inspector Sending Incorrect Data

Oct 13, 2025 by wbrum.org 44 views

Introduction

The Model Context Protocol (MCP) SDK is a powerful tool for building serverless applications. However, like any complex software, it can be prone to errors and bugs. In this article, we will explore a specific issue with the SSE (Server-Sent Events) example in the MCP SDK, where the messages endpoint returns a 400 error due to body parsing failure. We will walk through the steps to reproduce the behavior, the expected behavior, and the additional context to help identify the source of the error.

Describe the Bug

The messages endpoint returns a 400 error because body parsing fails. This error occurs when the MCP SDK is unable to parse the request body, resulting in a 400 Bad Request response.

To Reproduce

To reproduce the behavior, follow these steps:

import express from "express";
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { SSEServerTransport } from "@modelcontextprotocol/sdk/server/sse.js";

const server = new McpServer({
  name: "example-server",
  version: "1.0.0"
});

// ... set up server resources, tools, and prompts ...

const app = express();

let transport: SSEServerTransport | undefined = undefined;

app.get("/sse", async (req, res) => {
  transport ??= new SSEServerTransport("/messages", res);
  await server.connect(transport);
});

app.post("/messages", async (req, res) => {
  await transport.handlePostMessage(req, res);
});

app.listen(3001);

Expected Behavior

The expected behavior is to be able to connect to the server via npx @modelcontextprotocol/inspector and set the SSE endpoint to http://localhost:3001. This should allow the inspector to send data to the messages endpoint, which is then handled by the MCP SDK.

Additional Context

I was able to work around the issue by passing the body to handlePostMessage: await transport.handlePostMessage(req, res, req.body);. This suggests that the issue may be related to the way the inspector sends data to the messages endpoint, or it may be a bug in the MCP SDK.

Debugging the Issue

To debug the issue, we need to identify the source of the error. There are several possible causes:

Inspector Sending Incorrect Data: The inspector may be sending incorrect data to the messages endpoint, which is causing the body parsing failure.
MCP SDK Bug: The MCP SDK may have a bug that is causing the body parsing failure.
Server Configuration: The server configuration may be incorrect, causing the body parsing failure.

To investigate further, we can try the following:

Inspect the Request Body: Use a tool like Postman or cURL to inspect the request body sent by the inspector. This can help us determine if the issue is related to the data being sent.
Check the MCP SDK Documentation: Review the MCP SDK documentation to see if there are any known issues or workarounds related to body parsing.
Verify Server Configuration: Verify that the server configuration is correct and that the body parsing middleware is properly configured.

Conclusion

In conclusion, the SSE example in the MCP SDK is broken due to a body parsing failure. We have walked through the steps to reproduce the behavior, the expected behavior, and the additional context to help identify the source of the error. By debugging the issue and investigating the possible causes, we can determine the root cause of the problem and implement a solution to fix the issue.

Future Work

To further improve the MCP SDK, we can:

Improve Inspector Data Sending: Improve the inspector to send correct data to the messages endpoint.
Fix MCP SDK Bug: Fix the bug in the MCP SDK that is causing the body parsing failure.
Enhance Server Configuration: Enhance the server configuration to properly handle body parsing.

Q&A: Debugging the Issue with Model Context Protocol SDK

Q: What is the issue with the SSE example in the Model Context Protocol SDK?

A: The issue is that the messages endpoint returns a 400 error due to body parsing failure. This error occurs when the MCP SDK is unable to parse the request body, resulting in a 400 Bad Request response.

Q: How do I reproduce the behavior?

A: To reproduce the behavior, follow these steps:

import express from "express";
import { McpServer } from "@modelcontextprotocol/sdk/server/mcp.js";
import { SSEServerTransport } from "@modelcontextprotocol/sdk/server/sse.js";

const server = new McpServer({
  name: "example-server",
  version: "1.0.0"
});

// ... set up server resources, tools, and prompts ...

const app = express();

let transport: SSEServerTransport | undefined = undefined;

app.get("/sse", async (req, res) => {
  transport ??= new SSEServerTransport("/messages", res);
  await server.connect(transport);
});

app.post("/messages", async (req, res) => {
  await transport.handlePostMessage(req, res);
});

app.listen(3001);

Q: What is the expected behavior?

A: The expected behavior is to be able to connect to the server via npx @modelcontextprotocol/inspector and set the SSE endpoint to http://localhost:3001. This should allow the inspector to send data to the messages endpoint, which is then handled by the MCP SDK.

Q: How do I work around the issue?

A: I was able to work around the issue by passing the body to handlePostMessage: await transport.handlePostMessage(req, res, req.body);. This suggests that the issue may be related to the way the inspector sends data to the messages endpoint, or it may be a bug in the MCP SDK.

Q: What are the possible causes of the issue?

A: There are several possible causes:

Inspector Sending Incorrect Data: The inspector may be sending incorrect data to the messages endpoint, which is causing the body parsing failure.
MCP SDK Bug: The MCP SDK may have a bug that is causing the body parsing failure.
Server Configuration: The server configuration may be incorrect, causing the body parsing failure.

Q: How do I debug the issue?

A: To debug the issue, we need to identify the source of the error. We can try the following:

Inspect the Request Body: Use a tool like Postman or cURL to inspect the request body sent by the inspector. This can help us determine if the issue is related to the data being sent.
Check the MCP SDK Documentation: Review the MCP SDK documentation to see if there are any known issues or workarounds related to body parsing.
Verify Server Configuration: Verify that the server configuration is correct and that the body parsing middleware is properly configured.

Q: What is the conclusion of the issue?

A: In conclusion, the SSE example in the MCP SDK is broken due to a body parsing failure. We have walked through the steps to reproduce the behavior, the expected behavior, and the additional context to help identify the source of the error. By debugging the issue and investigating the possible causes, we can determine the root cause of the problem and implement a solution to fix the issue.

Q: What is the future work for the MCP SDK?

A: To further improve the MCP SDK, we can:

Improve Inspector Data Sending: Improve the inspector to send correct data to the messages endpoint.
Fix MCP SDK Bug: Fix the bug in the MCP SDK that is causing the body parsing failure.
Enhance Server Configuration: Enhance the server configuration to properly handle body parsing.

By addressing these issues, we can make the MCP SDK more reliable and easier to use, which will benefit developers building serverless applications.