Streamable HTTP accepts unsupported or malformed MCP-Protocol-Version headers

Bug description

After a normal Streamable HTTP initialization flow, the server transport accepts requests with an unsupported or malformed MCP-Protocol-Version header and dispatches them normally with HTTP 200. The MCP Streamable HTTP specification for 2025-11-25 requires that if a server receives a request with an invalid or unsupported MCP-Protocol-Version, it MUST respond with HTTP 400 Bad Request.

This is a server-side validation issue. It is separate from client-side fixes that make the Java SDK client send the negotiated protocol version correctly.

Environment

• Java SDK: reproduced with v1.1.2
• Also reproduced with a main snapshot from 2026-05-11 (87e2c7d4dec60dc98a981ff24fedc2afc960de72)
• Transport: Streamable HTTP server
• Java: Java 17 target
• Spring MCP integration: not required for reproduction
• Vector store: not used

Steps to reproduce

1. Start a Java SDK Streamable HTTP server.
2. Complete a normal initialize followed by notifications/initialized flow.
3. Send a valid JSON-RPC request, such as tools/list, with MCP-Protocol-Version set to a well-formed but unsupported value such as 1900-01-01.
4. Repeat with a malformed value such as not-a-version.
5. Observe that the request is dispatched and returns HTTP 200.

Expected behavior

The server should reject the request before dispatching it:

HTTP/1.1 400 Bad Request

Minimal Complete Reproducible example

Set ENDPOINT to a Java SDK Streamable HTTP endpoint:

ENDPOINT=http://127.0.0.1:8080/mcp

Initialize and copy the returned Mcp-Session-Id header into SID:

curl -i -sS --http1.1 -X POST "$ENDPOINT" \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -H 'MCP-Protocol-Version: 2025-11-25' \
  --data '{"jsonrpc":"2.0","id":"init-1","method":"initialize","params":{"protocolVersion":"2025-11-25","capabilities":{},"clientInfo":{"name":"version-header-repro","version":"0.1.0"}}}'

Send the initialized notification:

curl -i -sS --http1.1 -X POST "$ENDPOINT" \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -H 'MCP-Protocol-Version: 2025-11-25' \
  -H "Mcp-Session-Id: $SID" \
  --data '{"jsonrpc":"2.0","method":"notifications/initialized","params":{}}'

Send an otherwise valid request with an unsupported protocol version:

curl -i -sS --http1.1 -X POST "$ENDPOINT" \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -H 'MCP-Protocol-Version: 1900-01-01' \
  -H "Mcp-Session-Id: $SID" \
  --data '{"jsonrpc":"2.0","id":"bad-version-1","method":"tools/list","params":{}}'

Observed response:

HTTP/1.1 200 OK

{"jsonrpc":"2.0","result":{"tools":[...]},"id":"bad-version-1"}

The same behavior is observed with a malformed header value:

curl -i -sS --http1.1 -X POST "$ENDPOINT" \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -H 'MCP-Protocol-Version: not-a-version' \
  -H "Mcp-Session-Id: $SID" \
  --data '{"jsonrpc":"2.0","id":"bad-version-2","method":"tools/list","params":{}}'

Observed response:

HTTP/1.1 200 OK

{"jsonrpc":"2.0","result":{...},"id":"bad-version-2"}

Additional context

• Related: #436 reports a client-side failure when a server responds with an unsupported protocol version during initialization. This is the opposite direction: the Java SDK server accepts unsupported or malformed protocol-version headers on subsequent HTTP requests.
• Related: #883 and #931 discuss client-side version emission on GET reconnect. Those are useful context, but this report is about server-side validation before dispatch.

Bắt đầu tại điểm vào request của server transport Streamable HTTP và tái hiện vấn đề bằng chuỗi curl được cung cấp, kiểm thử cả các giá trị MCP-Protocol-Version không được hỗ trợ lẫn các giá trị sai định dạng. Theo dõi quá trình xác thực trước khi dispatch request và thêm hoặc cập nhật các test của transport để cả hai trường hợp đều trả về HTTP 400 thay vì HTTP 200.