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Fix: Replace Mono.zip with Mono.when in StdioServerTransportProvider#sendMessage #846

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Fix: Replace Mono.zip with Mono.when in StdioServerTransportProvider#sendMessage #846

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2 changes: 1 addition & 1 deletion .../src/main/java/io/modelcontextprotocol/server/transport/StdioServerTransportProvider.java
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Original file line number Diff line number Diff line change
Expand Up @@ -174,7 +174,7 @@ public StdioMcpSessionTransport() {
@Override
public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message) {

return Mono.zip(inboundReady.asMono(), outboundReady.asMono()).then(Mono.defer(() -> {
return Mono.when(inboundReady.asMono(), outboundReady.asMono()).then(Mono.defer(() -> {
try {
outboundSink.emitNext(message, Sinks.EmitFailureHandler.busyLooping(Duration.ofMillis(100)));
return Mono.empty();
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30 changes: 30 additions & 0 deletions ...test/java/io/modelcontextprotocol/server/transport/StdioServerTransportProviderTests.java
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Original file line number Diff line number Diff line change
Expand Up @@ -13,6 +13,7 @@
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.PrintStream;
import java.lang.reflect.Field;
import java.nio.charset.StandardCharsets;
import java.time.Duration;
import java.util.Map;
Expand All @@ -30,6 +31,7 @@
import org.junit.jupiter.api.Test;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;
import reactor.core.publisher.Sinks;
import reactor.core.scheduler.Schedulers;
import reactor.test.StepVerifier;

Expand Down Expand Up @@ -307,6 +309,34 @@ void shouldHandleSessionClose() {
verify(mockSession).closeGracefully();
}

@Test
@SuppressWarnings("unchecked")
void sendMessageWaitsForBothInboundAndOutboundReadinessSignals() throws Exception {
transportProvider = new StdioServerTransportProvider(McpJsonDefaults.getMapper(), System.in,
testOutPrintStream);
Class<?> transportClass = Class
.forName(StdioServerTransportProvider.class.getName() + "$StdioMcpSessionTransport");
var constructor = transportClass.getDeclaredConstructor(StdioServerTransportProvider.class);
constructor.setAccessible(true);
McpServerTransport transport = (McpServerTransport) constructor.newInstance(transportProvider);

Field inboundReadyField = StdioServerTransportProvider.class.getDeclaredField("inboundReady");
inboundReadyField.setAccessible(true);
Sinks.One<Void> inboundReady = (Sinks.One<Void>) inboundReadyField.get(transportProvider);

Field outboundReadyField = transportClass.getDeclaredField("outboundReady");
outboundReadyField.setAccessible(true);
Sinks.One<Void> outboundReady = (Sinks.One<Void>) outboundReadyField.get(transport);

StepVerifier
.create(transport.sendMessage(
new McpSchema.JSONRPCNotification(McpSchema.JSONRPC_VERSION, "test/notification", Map.of())))
.then(() -> inboundReady.tryEmitValue(null))
.expectNoEvent(Duration.ofMillis(100))
.then(() -> outboundReady.tryEmitValue(null))
.verifyComplete();
}

@Test
void shouldHandleConcurrentSendMessage() throws Exception {
int messageCount = 500;
Expand Down
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