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{"@context":"https://schema.org","@type":"DiscussionForumPosting","headline":"Streamable servlet transport does not close SSE response for unknown methods","articleBody":"# DISCLAIMER: Codex-generated bug report\n\n  ## Bug description\n\n  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`\n  response, but the POST response's SSE stream remains open indefinitely.\n\n  Supported methods close their SSE streams normally after returning a response.\n\n  The MCP Streamable HTTP specification says that after the JSON-RPC response has been sent, the server should close the SSE stream:\n\n  https://modelcontextprotocol.io/specification/2025-06-18/basic/transports#sending-messages-to-the-server\n\n  ## Environment\n\n  - MCP Java SDK: 2.0.0\n  - Transport: `HttpServletStreamableServerTransportProvider`\n  - Jetty: 12.1.10\n  - Jakarta Servlet API: 6.1\n  - Java: 25.0.3\n  - Negotiated MCP protocol: `2025-06-18`\n\n  The server advertises tools but not resources or prompts.\n\n  ## Steps to reproduce\n\n  Initialize a session:\n\n  ```sh\n  curl --max-time 5 -i -X POST http://127.0.0.1:8080/mcp \\\n    -H 'Content-Type: application/json' \\\n    -H 'Accept: application/json, text/event-stream' \\\n    --data-binary '{\n      \"jsonrpc\": \"2.0\",\n      \"id\": 1,\n      \"method\": \"initialize\",\n      \"params\": {\n        \"protocolVersion\": \"2025-06-18\",\n        \"capabilities\": {},\n        \"clientInfo\": {\n          \"name\": \"reproducer\",\n          \"version\": \"1.0\"\n        }\n      }\n    }'\n  ```\n\n  Copy the returned `Mcp-Session-Id`, send `notifications/initialized`, and then call an unsupported method:\n\n  ```sh\n  curl --max-time 3 -i -X POST http://127.0.0.1:8080/mcp \\\n    -H 'Content-Type: application/json' \\\n    -H 'Accept: application/json, text/event-stream' \\\n    -H 'MCP-Protocol-Version: 2025-06-18' \\\n    -H 'Mcp-Session-Id: SESSION_ID' \\\n    --data-binary '{\n      \"jsonrpc\": \"2.0\",\n      \"id\": 2,\n      \"method\": \"resources/list\",\n      \"params\": {}\n    }'\n  ```\n\n  The server sends an SSE event promptly:\n\n  ```text\n  event: message\n  data: {\"jsonrpc\":\"2.0\",\"id\":2,\"error\":{\"code\":-32601,\"message\":\"Method not found: resources/list\"}}\n  ```\n\n  However, the HTTP response does not reach end-of-stream. Curl eventually reports:\n\n  ```text\n  curl: (28) Operation timed out after 3002 milliseconds\n  ```\n\n  Calling a supported method such as `tools/list` returns its response and closes the stream immediately.\n\n  ## Expected behavior\n\n  After sending the `-32601` JSON-RPC response, the server should complete the asynchronous servlet context and close the POST response's SSE stream.\n\n  ## Actual behavior\n\n  The JSON-RPC error is delivered, but the SSE stream and HTTP request remain open.\n\n  ## Apparent cause\n\n  In `McpStreamableServerSession.responseStream`, the missing-handler branch returns only:\n\n  ```java\n  return transport.sendMessage(\n      McpSchema.JSONRPCResponse.error(...));\n  ```\n\n  The registered-handler branch ends with:\n\n  ```java\n  .flatMap(transport::sendMessage)\n  .then(transport.closeGracefully());\n  ```\n\n  The missing-handler branch appears to need the same transport closure:\n\n  ```java\n  return transport.sendMessage(\n      McpSchema.JSONRPCResponse.error(...))\n      .then(transport.closeGracefully());\n  ```\n\n  ## Impact\n\n  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.\n","author":{"url":"https://github.com/AlanBurlison","@type":"Person","name":"AlanBurlison"},"datePublished":"2026-07-01T11:03:28.000Z","interactionStatistic":{"@type":"InteractionCounter","interactionType":"https://schema.org/CommentAction","userInteractionCount":2},"url":"https://github.com/1050/java-sdk/issues/1050"}