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1. JunJieLiu51520 commented on Sep 27, 2025

   

   JunJieLiu51520

   on Sep 27, 2025

   Last edited by JunJieLiu51520

   Contributor

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   Extending from this issue, I hope to refactor abs mcptransport, because i found so many responseError method in different child class, for example, why public interface McpStatelessServerTransport {
   child class not extends McpServerTransport ?

   @tzolov What do you think?
2. quaff commented on Sep 28, 2025

   

   quaff

   on Sep 28, 2025

   Contributor

   More actions

   See #519
3. chemicL

   added

   enhancementNew feature or requestNew feature or request

   waiting for triage

   on Feb 20, 2026
4. chemicL

   added the

   Enhancement

   issue type on Feb 20, 2026
5. AlanBurlison commented on Jul 1, 2026

   

   AlanBurlison

   on Jul 1, 2026

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   DISCLAIMER: Codex-generated comment:

   This also occurs in the SDK 2.0.0 stateless servlet transport, through a slightly different path.

   Environment

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

   The server advertises tools but not resources or prompts.

   Reproduction

   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' \
     -H 'MCP-Protocol-Version: 2025-06-18' \
     --data-binary '{
       "jsonrpc": "2.0",
       "id": 1,
       "method": "resources/list",
       "params": {}
     }'

   Actual response

   The response is HTTP 500 with JSON-RPC code -32603. The body serializes the complete McpError, including stackTrace, cause, localizedMessage, and other Java exception
   properties:

   HTTP/1.1 500 Server Error
   Content-Type: application/json;charset=utf-8

   {
     "jsonRpcError": {
       "code": -32603,
       "message": "Failed to handle request: Missing handler for request type: resources/list"
     },
     "cause": null,
     "localizedMessage": "Failed to handle request: Missing handler for request type: resources/list",
     "message": "Failed to handle request: Missing handler for request type: resources/list",
     "stackTrace": [
       {
         "className": "io.modelcontextprotocol.spec.McpError$Builder",
         "methodName": "build",
         "fileName": "McpError.java"
       },
       {
         "className": "io.modelcontextprotocol.server.transport.HttpServletStatelessServerTransport",
         "methodName": "doPost",
         "fileName": "HttpServletStatelessServerTransport.java"
       }
     ]
   }

   Expected response

   The request should complete as a normal JSON-RPC response with the original request ID and -32601 Method not found, without serialized exception internals. For example:

   HTTP/1.1 200 OK
   Content-Type: application/json;charset=utf-8

   {
     "jsonrpc": "2.0",
     "id": 1,
     "error": {
       "code": -32601,
       "message": "Method not found: resources/list"
     }
   }

   Apparent cause

   DefaultMcpStatelessServerHandler.handleRequest correctly constructs an McpError using METHOD_NOT_FOUND, but propagates it with Mono.error(...).

   HttpServletStatelessServerTransport.doPost blocks on that publisher, catches the exception, replaces it with INTERNAL_ERROR, and passes the exception object to its JSON error
   writer. Jackson then serializes the Java exception properties.

   The missing-handler branch could instead return a normal JSONRPCResponse.error(...), consistent with how errors from registered handlers are converted into JSON-RPC responses.
6. chemicL

   added 3 commits that reference this issue on Oct 12, 2026

   Add a stateless server for MCP 2026-07-28

   ...

   70a8318

   Add a stateless server for MCP 2026-07-28

   ...

   838ab3c

   Modern stateless server for MCP 2026-07-28 (#1170)

   ...

   09020ac
7. IdentityOverflow commented on Nov 4, 2026

   

   IdentityOverflow

   on Nov 4, 2026

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   Confirming this is still present in 2.1.0 and on main, and that it is not specific to the session-id path: every error the servlet transports write as a JSON body goes through responseError, which does jsonMapper.writeValueAsString(mcpError) on the exception itself. HttpServletStatelessServerTransport L269, HttpServletStreamableServerTransportProvider L738, HttpServletSseServerTransportProvider L442 and L496 (2.1.0 line numbers). On the stateless transport that covers 415 (Content-Type), 400 (Accept, malformed body, unexpected message type) and the handler-failure 500s. With a default Jackson 2 mapper the body is about 20 KB with 80+ frames.

   Reproduced on a Jetty 12 deployment of HttpServletStatelessServerTransport (Java 17, mcp-json-jackson2 2.1.0):

   curl -s -X POST http://host/mcp -H 'Content-Type: application/xml' -H 'Accept: application/json, text/event-stream' -d '{}'
   

   returns 415 with {"cause":null,"stackTrace":[.... Minimal reproducer without a server:

   McpJsonMapper mapper = new JacksonMcpJsonMapper(new ObjectMapper());
   String body = mapper.writeValueAsString(McpError.builder(McpSchema.ErrorCodes.INVALID_REQUEST).message("x").build());
   assertFalse(body.contains("stackTrace")); // fails

   Workaround that works today: register a Jackson serializer for McpError on the mapper you pass to the transport builder, writing {"jsonrpc":"2.0","id":null,"error":{"code":…,"message":…}} from getJsonRpcError(). One note for the fix: McpSchema.JSONRPCResponse's compact constructor rejects a null id, and on these paths the id is unknown, so the envelope has to be built separately (JSON-RPC 2.0 prescribes id: null there).