[2026-07-28] Java SDK 2.0.0 returns HTTP 500 for OpenAI server/discover requests #1072

Starting on 2026-07-30 at approximately 07:10 UTC, we observed OpenAI's hosted MCP client begin sending server/discover before tools/list when using a remote MCP server through the
Responses API.

Our server uses the latest released MCP Java SDK, 2.0.0, which supports protocol revision 2025-11-25 but does not currently implement server/discover.

When the request reaches HttpServletStatelessServerTransport, the SDK returns HTTP 500 with an error similar to:

Missing handler for request type: server/discover

OpenAI then returns HTTP 424 external_connector_error from the Responses API. This prevents the entire response from being generated, even when the user sends a message such as "hi" that
does not require an MCP tool call.

We understand that support for the new stateless lifecycle is already tracked in:

• SEP-2575: Make MCP Stateless #1011

We also found that the general unknown-handler behavior was fixed by:

• Fix error behavior on unregistered handlers in stateless server handler #800

However, that change was merged after the latest 2.0.0 release and does not appear to be available in a published version.

Could you please clarify:

1. Which Java SDK version is expected to support the 2026-07-28 protocol revision?
2. Is there an estimated release timeline?
3. Will there be an interim 2.x release containing the unknown-handler fix from Fix error behavior on unregistered handlers in stateless server handler #800?
4. Until full support is available, what is the recommended server behavior for server/discover so modern clients can safely fall back to the legacy initialize flow?

The 2026-07-28 Streamable HTTP specification requires an unsupported RPC to return HTTP 404 with JSON-RPC error -32601, rather than HTTP 500:

https://modelcontextprotocol.io/specification/2026-07-28/basic/transports/streamable-http

Environment

• MCP Java SDK: 2.0.0
• Maven artifacts:
  • io.modelcontextprotocol.sdk:mcp-core
  • io.modelcontextprotocol.sdk:mcp-json-jackson2
• Server transport: HttpServletStatelessServerTransport
• Server type: McpStatelessSyncServer
• Java: Amazon Corretto 25
• Spring Boot: 3.5.16
• Transport: Streamable HTTP
• Deployment: Stateless Kubernetes service
• MCP capabilities: Tools only
• Vector store: Not applicable
• Client: OpenAI hosted remote MCP client through the Responses API

Steps to reproduce

1. Start a stateless MCP server using Java SDK 2.0.0 and HttpServletStatelessServerTransport.
2. Register tools, but do not register a custom server/discover handler.
3. Send the following request:

curl -i http://localhost:8080/mcp \
  -H 'Content-Type: application/json' \
  -H 'Accept: application/json, text/event-stream' \
  -H 'MCP-Protocol-Version: 2026-07-28' \
  -H 'Mcp-Method: server/discover' \
  --data '{
    "jsonrpc": "2.0",
    "id": "discover-1",
    "method": "server/discover",
    "params": {
      "_meta": {
        "io.modelcontextprotocol/protocolVersion": "2026-07-28",
        "io.modelcontextprotocol/clientInfo": {
          "name": "reproduction-client",
          "version": "1.0.0"
        },
        "io.modelcontextprotocol/clientCapabilities": {}
      }
    }
  }'

4. Observe that the released SDK returns HTTP 500 with a missing-handler error.
5. Configure this server as a remote MCP tool in the OpenAI Responses API.
6. Observe that OpenAI returns HTTP 424 external_connector_error before producing an answer.

Expected behavior

Ideally, the Java SDK should support server/discover and the complete 2026-07-28 tools flow.

Until that support is available, an unsupported modern RPC should return a backward-compatible method-not-found response instead of HTTP 500, allowing clients that support both protocol
eras to fall back to the legacy initialize flow.