SSE Example Fails to Run - GET Requests Not Initialized Properly

[Hooper93]

Description:​​

I'm trying to run the SSE example from the official documentation, but encountering issues when making GET requests for SSE connections. The stdio example works fine, but the SSE implementation seems to have initialization problems.

​​Steps to Reproduce:​​

• Cloned the repository and copied examples from src/examples/server
• Ran simpleStatelessStreamableHttp.ts MCP server
• Started MCPI using npx @modelcontextprotocol/inspector
• Attempted to connect via SSE by making a GET request to http://localhost:3000/mcp
  ​​

Expected Behavior:​​

The SSE connection should establish successfully and stream data as described in the documentation.

Actual Behavior:​​

browser shows 500
and MCPI console show error:

400 - Error in /sse route: SseError: SSE error: Non-200 status code (400)

SSE transport: url=http://localhost:3000/mcp, headers=Accept
Error in /sse route: SseError: SSE error: Non-200 status code (400)
    at _eventSource.onerror (file:///Users/dxm/.npm/_npx/5a9d879542beca3a/node_modules/@modelcontextprotocol/sdk/dist/esm/client/sse.js:69:31)
    at EventSource.failConnection_fn (file:///Users/dxm/.npm/_npx/5a9d879542beca3a/node_modules/eventsource/dist/index.js:234:53)
    at file:///Users/dxm/.npm/_npx/5a9d879542beca3a/node_modules/eventsource/dist/index.js:76:74
    at process.processTicksAndRejections (node:internal/process/task_queues:95:5) {
  code: 400,
  event: {
    type: 'error',
    message: 'Non-200 status code (400)',
    code: 400,
    defaultPrevented: false,
    cancelable: false,
    timeStamp: 111063.962667
  }
}

Investigation Findings:​​

In transport.handleRequest, when the request method is GET, this._initialized is never set
Initialization appears to require a POST request first, seems like this issue
but: transport doesn't expose handlePostMessage publicly
No clear way to perform this initialization step

Similar issue occurs when trying to use the server with Cursor - it fails to recognize the MCP server
​​

Additional Context:​​

BUT The stdio example works correctly

Could you clarify if I'm missing a configuration step or if MCPI requires special parameters to support GET request initialization for SSE? The documentation doesn't mention any required setup for this scenario

[Hooper93]

[Meandmybadself]

I recently went through this same issue (I think).

In the protocol Transport spec, you'll see there are multiple transports (STDIO, Streamable & SSE

Services like MCP Inspector & Cursor are still using the spec from 2024-11-05 (and tell the MCP server their version when they connect to the server). Streamable HTTP requires a client that uses the newer spec.

To get things working now, look into using the SSE-based approaches for MCPs.

[Meandmybadself]

Found an example service that implements both Streamable HTTP and SSE:
https://github.com/modelcontextprotocol/typescript-sdk/blob/main/src/examples/server/sseAndStreamableHttpCompatibleServer.ts

[Hooper93]

Found an example service that implements both Streamable HTTP and SSE: https://github.com/modelcontextprotocol/typescript-sdk/blob/main/src/examples/server/sseAndStreamableHttpCompatibleServer.ts

I’ve tried the demo you suggested, but I’m still encountering the same issue when connecting via MCPI.

Here’s what I did:

• Cloned the example demo again to ensure a clean setup.
• Ran tsc and node build.js without modifications.
• Verified that the MCP server is running.
• Tested with MCPI, but the MCPI client still fails.
  ​​
  Error details:​​

Error from MCP server: Error: Error POSTing to endpoint (HTTP 400): InternalServerError: stream is not readable
at SSEClientTransport.send (file:///Users/dxm/.npm/_npx/5a9d879542beca3a/node_modules/@modelcontextprotocol/sdk/dist/esm/client/sse.js:159:23)
at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
Error from MCP server: Error: Error POSTing to endpoint (HTTP 400): InternalServerError: stream is not readable
at SSEClientTransport.send (file:///Users/dxm/.npm/_npx/5a9d879542beca3a/node_modules/@modelcontextprotocol/sdk/dist/esm/client/sse.js:159:23)
at process.processTicksAndRejections (node:internal/process/task_queues:105:5)
​​
Cursor also not work

It is really frustrating, which the official demo doesn't work

Environment:​​

Node.js: v20.18.1
TypeScript: ^5.8.3
@modelcontextprotocol/sdk: ^1.10.1

[jes5199]

I'm having the same problem, I was very confused about the docs and the server being out of sync

is it rude to tag @ihrpr who committed that example

[ihrpr]

@Hooper93, simpleStatelessStreamableHttp is for StreamableHttp transport, which was introduced in 25-03-26 version spec. This transport can be used with or without standalone SSE stream. It works quite differently to SSE transport.

I'll take a look what's up with sseAndStreamableHttpCompatibleServer/

To test older spec SSE transport in Inspector you can use npx tsx src/examples/server/simpleSseServer.ts - note the endpoint is mcp/ - code is here

For the new StreamableHttp transport, you can use src/examples/client/simpleStreamableHttp.ts client, not many apps support the new transport yet. Inspector is still in progress to support the new transport.

[ihrpr]

#339 adds support for StreamableHttp transport to inspector