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

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"
  }
}

從重現步驟中描述的 servlet 伺服器傳輸錯誤路徑開始，追蹤驗證失敗是如何序列化的。將目前的回應與預期的 JSON-RPC 信封進行比較，然後執行所述的 Accept 標頭驗證請求，以確認本文僅包含 JSON-RPC 錯誤結構。