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

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#955 opened on May 12, 2026 3 comments 0 reactions 0 assignees View on GitHub

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Assessment

Difficulty: 3/5
Estimated time: 1-2 days
Newbie friendliness: 68/100

Issue type: Bug
Clarity: Mostly clear
Activity status: Active
Tech stack: java
Domain: api, backend

Research direction

Start at the servlet server transport error paths described in the reproduction and trace how validation failures are serialized. Compare the current response with the expected JSON-RPC envelope, then exercise the stated Accept-header validation request to confirm the body contains only the JSON-RPC error shape.

Written by the indexing model from the issue text.

Description

bug P2 ready for work

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

Run an MCP server with a servlet transport.
Send a request that triggers a servlet transport error path.
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"
  }
}

Dominant language: Java
Stars: 3.7k
Forks: 1.1k
Avg merge: 1d 15h
Merged PRs (30d): 9

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

Assessment

Research direction

Description

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