Servlet transports serialize McpError exceptions instead of JSON-RPC error envelopes

[ultramancode]

Servlet transports serialize McpError exceptions instead of JSON-RPC error envelopes

Bug description

Some servlet transport error paths serialize McpError directly instead of returning a JSON-RPC error response envelope.

Because McpError extends RuntimeException, the response body can expose exception-shaped fields such as:

• stackTrace
• cause
• localizedMessage
• jsonRpcError

For example, on current main, a servlet transport validation error returns a body like this:

This is not the JSON-RPC response shape clients expect.

Environment

• MCP Java SDK version: current main
• Java version: 21
• Transport: servlet server transports

Steps to reproduce

1. Run an MCP server with a servlet transport.
2. Send a request that triggers a servlet transport error path.
3. Inspect the response body.

Expected behavior

The response body should look like:

{
  "jsonrpc": "2.0",
  "error": {
    "code": -32601,
    "message": "Both application/json and text/event-stream required in Accept header"
  }
}

[ultramancode]

I opened a follow-up PR for this issue: #956

[yuchen-liu-everlaw]

I ran into a similar issue. A malformed payload such as below, emits a ServerResponse of McpError in WebMvcStatelessServerTransport. But McpError does not get translate into a message by DefaultEntityResponse from spring-webmvc (7.0.8). It emits a HttpMediaTypeNotAcceptableException instead of a JSON-PRC error. Can be reproduced by:

curl -i -sS -X POST http://localhost:8080/mcp \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/list"'

[ultramancode]

@yuchen-liu-everlaw
Thanks for sharing this. The WebMVC transport used to be part of the MCP Java SDK, but it moved to the Spring AI project for its 2.0
release. I opened an issue and a PR for this:

Issue: spring-projects/spring-ai#6855
PR: spring-projects/spring-ai#6856