Streamable servlet transport does not close SSE response for unknown methods

[AlanBurlison]

DISCLAIMER: Codex-generated bug report

Bug description

With HttpServletStreamableServerTransportProvider in Java SDK 2.0.0, a request for an unknown or unsupported JSON-RPC method receives the correct -32601 Method not found
response, but the POST response's SSE stream remains open indefinitely.

Supported methods close their SSE streams normally after returning a response.

The MCP Streamable HTTP specification says that after the JSON-RPC response has been sent, the server should close the SSE stream:

https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#sending-messages-to-the-server

Environment

• MCP Java SDK: 2.0.0
• Transport: HttpServletStreamableServerTransportProvider
• Jetty: 12.1.10
• Jakarta Servlet API: 6.1
• Java: 25.0.3
• Negotiated MCP protocol: 2025-06-18

The server advertises tools but not resources or prompts.

Steps to reproduce

Initialize a session:

curl --max-time 5 -i -X POST http://127.0.0.1:8080/mcp \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  --data-binary '{
    "jsonrpc": "2.0",
    "id": 1,
    "method": "initialize",
    "params": {
      "protocolVersion": "2025-06-18",
      "capabilities": {},
      "clientInfo": {
        "name": "reproducer",
        "version": "1.0"
      }
    }
  }'

Copy the returned Mcp-Session-Id, send notifications/initialized, and then call an unsupported method:

curl --max-time 3 -i -X POST http://127.0.0.1:8080/mcp \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -H 'MCP-Protocol-Version: 2025-06-18' \
  -H 'Mcp-Session-Id: SESSION_ID' \
  --data-binary '{
    "jsonrpc": "2.0",
    "id": 2,
    "method": "resources/list",
    "params": {}
  }'

The server sends an SSE event promptly:

event: message
data: {"jsonrpc":"2.0","id":2,"error":{"code":-32601,"message":"Method not found: resources/list"}}

However, the HTTP response does not reach end-of-stream. Curl eventually reports:

curl: (28) Operation timed out after 3002 milliseconds

Calling a supported method such as tools/list returns its response and closes the stream immediately.

Expected behavior

After sending the -32601 JSON-RPC response, the server should complete the asynchronous servlet context and close the POST response's SSE stream.

Actual behavior

The JSON-RPC error is delivered, but the SSE stream and HTTP request remain open.

Apparent cause

In McpStreamableServerSession.responseStream, the missing-handler branch returns only:

return transport.sendMessage(
    McpSchema.JSONRPCResponse.error(...));

The registered-handler branch ends with:

.flatMap(transport::sendMessage)
.then(transport.closeGracefully());

The missing-handler branch appears to need the same transport closure:

return transport.sendMessage(
    McpSchema.JSONRPCResponse.error(...))
    .then(transport.closeGracefully());

Impact

Clients that probe an unsupported method may wait indefinitely unless they impose a timeout. The server also retains the asynchronous servlet request until the client disconnects.

[Kehrlann]

Hey @AlanBurlison ! Thanks for the report.

It is a duplicate of #1040 , which has been fixed in #1041. It will be shipped in 2.0.1 .

[AlanBurlison]

Thanks for the quick reply! 🤖 didn't find that bug unfortunately (I did ask...) A win for the humans! 😉