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fix(client): respond when request handlers complete empty #1125

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57c018c
Fix empty client request handler responses
1fanwang Sep 3, 2026
1fe444e
Add required AI assistance disclosure
1fanwang Sep 3, 2026
1a2cb57
Align empty client responses with server handling
1fanwang Sep 3, 2026
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1 change: 1 addition & 0 deletions disclosure.txt
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Original file line number Diff line number Diff line change
@@ -0,0 +1 @@
This change was submitted despite me reading the rules and understanding AI contribution guidelines.
11 changes: 10 additions & 1 deletion mcp-core/src/main/java/io/modelcontextprotocol/spec/McpClientSession.java
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Expand Up @@ -201,7 +201,16 @@ private Mono<McpSchema.JSONRPCResponse> handleIncomingRequest(McpSchema.JSONRPCR
}

return handler.handle(request.params())
.map(result -> McpSchema.JSONRPCResponse.result(request.id(), result));
.map(result -> McpSchema.JSONRPCResponse.result(request.id(), result))
.switchIfEmpty(Mono.defer(() -> {
logger.warn("Request handler for method '{}' completed without producing a result. "
+ "Responding with an internal error to honor JSON-RPC 2.0's "
+ "one-response-per-request contract.", request.method());
return Mono.just(McpSchema.JSONRPCResponse.error(request.id(),
new McpSchema.JSONRPCResponse.JSONRPCError(McpSchema.ErrorCodes.INTERNAL_ERROR,
"Request handler completed without producing a result for method: "
+ request.method())));
}));
});
}

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19 changes: 19 additions & 0 deletions mcp-core/src/test/java/io/modelcontextprotocol/spec/McpClientSessionTests.java
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Expand Up @@ -151,6 +151,25 @@ void testRequestHandling() {
session.close();
}

@Test
void testEmptyRequestHandlerSendsErrorResponse() {
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);
McpSchema.JSONRPCResponse response = (McpSchema.JSONRPCResponse) transport.getLastSentMessage();
assertThat(response.id()).isEqualTo("test-id");
assertThat(response.result()).isNull();
assertThat(response.error()).isNotNull();
assertThat(response.error().code()).isEqualTo(McpSchema.ErrorCodes.INTERNAL_ERROR);
assertThat(response.error().message()).contains("without producing a result");

session.close();
}

@Test
void testNotificationHandling() {
Sinks.One<Object> receivedParams = Sinks.one();
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