Client request handlers that complete empty send no JSON-RPC response

[1fanwang]

Bug description

When a server sends a request to a Java SDK client, an asynchronous sampling or elicitation handler can return Mono.empty(). McpClientSession then sends no JSON-RPC response, so the server waits until its request timeout.

JSON-RPC 2.0 requires a response for every request with an ID. An empty handler completion should produce an error response with the original request ID.

This is the client-side counterpart to #1081. The open fix at #1099 changes only server-side dispatch paths.

Environment

• Java SDK v2.0.1, commit c7e1cfe90edcd9cbe030924310a194dc5492eab2
• Java SDK main, commit 39c225e46c51311eaf82192f15b950e421580fb5
• Temurin 25.0.3, compiling with the repository's Java 17 target

Steps to reproduce

Register a client request handler that returns Mono.empty(), then deliver a JSON-RPC request with an ID:

var transport = new MockMcpClientTransport();
var session = new McpClientSession(TIMEOUT, transport,
		Map.of("test.method", params -> Mono.empty()), Map.of(), Function.identity());

transport.simulateIncomingMessage(new McpSchema.JSONRPCRequest("test.method", "test-id"));

assertThat(transport.getLastSentMessage()).isInstanceOf(McpSchema.JSONRPCResponse.class);

Both v2.0.1 and main fail because getLastSentMessage() is null:

Tests run: 1, Failures: 1, Errors: 0, Skipped: 0
Expecting actual not to be null

Expected behavior

The client sends one JSON-RPC error response with ID test-id and code -32603.

Minimal Complete Reproducible example

The regression test is in McpClientSessionTests.testEmptyRequestHandlerSendsErrorResponse.

[ZYZ666-RGB]

Hi, I'd like to work on this issue.

I verified the behavior against the current main branch: when the client request handler returns Mono.empty(), handleIncomingRequest() completes without producing a JSONRPCResponse, so transport.sendMessage() is never invoked.

I plan to:

1. Add a focused regression test in McpClientSessionTests covering an empty request-handler completion.
2. Verify that exactly one JSON-RPC error response is sent with the original request ID and INTERNAL_ERROR (-32603).
3. Update the client request dispatch path so an empty completion becomes an explicit error response.
4. Preserve the existing success, handler-error, and method-not-found behavior.

I also checked #1099, which only covers the server-side dispatch path, so this would address the client-side counterpart described here.

Could you please assign this issue to me if this approach looks good? Thank you!