Skip to content

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

New issue

Have a question about this project? Sign up for a free GitHub account to open an issue and contact its maintainers and the community.

Sign up for GitHub

By clicking “Sign up for GitHub”, you agree to our terms of service and privacy statement. We’ll occasionally send you account related emails.

Already on GitHub? Sign in to your account

Open
rameshreddy-adutla wants to merge 1 commit into
base: main
Choose a base branch
from
Open

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

Show file tree
Hide file tree
Changes from all commits
Commits
Show all changes
1 commit
Select commit
File filter

Filter by extension

Filter by extension
Conversations
Failed to load comments.
Loading
Jump to
Jump to file
Failed to load files.
Loading
Diff view
Diff view
Fix stdio readiness barrier to wait for both Mono<Void> signals 
Mono.zip cancels the slower Mono<Void> source when the faster one
completes empty, so sendMessage could proceed before both inbound and
outbound readiness signals fired. Mono.when waits for both. Fixes #303.
  • Loading branch information
@rameshreddy-adutla
rameshreddy-adutla committed Sep 28, 2026
commit 081605d2da00392c7de9971bff890b59e8970b47
2 changes: 1 addition & 1 deletion .../src/main/java/io/modelcontextprotocol/server/transport/StdioServerTransportProvider.java
View file
Open in desktop
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();
Expand Down
30 changes: 30 additions & 0 deletions ...test/java/io/modelcontextprotocol/server/transport/StdioServerTransportProviderTests.java
View file
Open in desktop
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
Loading