Servlet-based server transports read request body with wrong charset encoding

Bug description
The servlet-based server transports call request.getReader() without first calling request.setCharacterEncoding("UTF-8"). Per the Jakarta Servlet spec, getReader() defaults to ISO-8859-1 when the request's Content-Type doesn't include an explicit charset parameter. Since Content-Type: application/json (without charset) is standard and correct per RFC 8259, any non-ASCII characters in the JSON-RPC request body are silently corrupted.

This affects everything in the request payload — tool names, argument values, notification data — for any MCP client that doesn't redundantly declare charset=utf-8 in its Content-Type header. The analogous issue in StdioServerTransportProvider was fixed in https://github.com/modelcontextprotocol/java-sdk/pull/826.

Environment

• MCP Java SDK: 1.0.0
• Java: 21
• No Spring AI or vector store involved — this is a bug in the core servlet request reading logic, not specific to any framework or integration.

Steps to reproduce

1. Start an MCP server using any of the servlet-based transports
2. Send a tool call with a non-ASCII string argument, e.g. a unicode escape sequence like \u2014 (em dash —).
3. The server receives mojibake (e.g. — → â).

Expected behavior
Non-ASCII characters in the JSON-RPC request body (tool names, argument values, etc.) should be preserved correctly. JSON is UTF-8 by definition, so the server should decode request bodies as UTF-8 regardless of whether the client includes charset=utf-8 in the Content-Type header.

Related

• https://github.com/modelcontextprotocol/java-sdk/issues/260

Xác định các triển khai transport dựa trên servlet gọi request.getReader(), sau đó so sánh cách xử lý request body của chúng với bản sửa lỗi StdioServerTransportProvider trong PR 826. Thêm kiểm thử hồi quy sử dụng một giá trị JSON-RPC không phải ASCII và xác minh rằng các request không có charset tường minh vẫn giữ nguyên các ký tự ban đầu.