handlePostMessage fails when reqBody is not passed

[Rajesh-Narayanappa87]

Describe the bug
When creating mcp using SSE server, await transport.handlePostMessage(req, res); expects reqbody. Not providing is failing run

Machine config:
Tried with node 20 and 22.
Mac M3
@modelcontextprotocol/sdk - 1.7.0

To Reproduce
Steps to reproduce the behavior:

1. Run node index.js to run mcp server
2. Run node client.js to run mcp client

index.js

  // Create an MCP server
  const server = new McpServer({
    name: "Demo",
    version: "1.0.0",
    });
  
  // Add an addition tool
  server.tool("add", { a: z.number(), b: z.number() }, async ({ a, b}) => ({
    content: [{ type: "text", text: String(a + b) }],
  }));
  
  
  const axiosInstance = axios.create({
      httpsAgent: new https.Agent({
        rejectUnauthorized: false,
      }),
    });
  
  server.tool("pullTestResults", { a: z.number() }, async ({ a }) => {
    let result = await axiosInstance.get(`https://xyz/test-case/api-result?count=${a}`);
    return {
      content: [{ type: "text", text: JSON.stringify(result.data) }],
    };
  });
  
  
  const app = express();
  app.use(express.json());
  
  const transports = {};


  app.get("/sse", async (req, res) => {
    const transport = new SSEServerTransport('/messages', res);
    transports[transport.sessionId] = transport;
    res.on("close", () => {
      delete transports[transport.sessionId];
    });
    await server.connect(transport);
  });
  
  app.post("/messages", async (req, res) => {
    const sessionId = req.query.sessionId;
    const transport = transports[sessionId];
    if (transport) {
      await transport.handlePostMessage(req, res);
    } else {
      res.status(400).send('No transport found for sessionId');
    }
  });
  
  const port = 3000;
  app.listen(port, () => {
    console.log(`HTTP server listening on port ${port}`);
  });

client.js

// Create an MCP client
const client = new Client(
    {
  name: "DemoClient",
  version: "1.0.0"
    },
    {
        capabilities: {}
      }
);

const transport = new SSEClientTransport(new URL(`http://localhost:3000/sse`));

// Connect to the server using stdio transport
await client.connect(transport);

const tools = await client.listTools();

console.log(tools)


const resp = await client.callTool({
    name: "pullTestResults",
    arguments: {
      a: "2024-12-04_18-13-21-190087751"
    }
  })

  console.log(resp)

Issue exist in both client or api way.

Api way

curl --location 'http://localhost:3000/messages?sessionId=40652e8f-2633-40b8-98f8-5258fbbcb06a' \
--header 'Content-Type: application/json' \
--data '{
        "jsonrpc": "2.0",
        "id": "a29ba38d-fed4-49f0-bbba-6c8b1f4606c9",
        "method": "tools/call",
        "params": {
            "name": "pullTestResults",
            "arguments": {
                "a": "2024-12-04_18-13-21-190087751"
            }
        }
        
    }'

Expected behavior
List out tools and execute pullTestResults tool.

Logs

throw new Error(`Error POSTing to endpoint (HTTP ${response.status}): ${text}`);
                      ^

Error: Error POSTing to endpoint (HTTP 400): InternalServerError: stream is not readable

Workaround for this issue is to send reqbody to handlePostMessage function.

..... same as index.js
      await transport.handlePostMessage(req, res, req.body);

[Rajesh-Narayanappa87]

Found problem.
Here is handlePostMessage implementation

 async handlePostMessage(req, res, parsedBody) {
        var _a, _b, _c;
        if (!this._sseResponse) {
            const message = "SSE connection not established";
            res.writeHead(500).end(message);
            throw new Error(message);
        }
        let body;
        try {
            const ct = contentType.parse((_a = req.headers["content-type"]) !== null && _a !== void 0 ? _a : "");
            if (ct.type !== "application/json") {
                throw new Error(`Unsupported content-type: ${ct}`);
            }
            body = parsedBody !== null && parsedBody !== void 0 ? parsedBody : await getRawBody(req, {
                limit: MAXIMUM_MESSAGE_SIZE,
                encoding: (_b = ct.parameters.charset) !== null && _b !== void 0 ? _b : "utf-8",
            });
        }
        catch (error) {
            res.writeHead(400).end(String(error));
            (_c = this.onerror) === null || _c === void 0 ? void 0 : _c.call(this, error);
            return;
        }
        try {
            await this.handleMessage(typeof body === 'string' ? JSON.parse(body) : body);
        }
        catch (_d) {
            res.writeHead(400).end(`Invalid message: ${body}`);
            return;
        }
        res.writeHead(202).end("Accepted");
    }

getRawBody expects buffer stream, since our case is json process throws error rightaway.
Actual json handling is done later after throwing error await this.handleMessage .

If dev confirms this is bug and not being looked upon, I can fix this quick

[Rajesh-Narayanappa87]

Raised PR for above issue: #238