From 081605d2da00392c7de9971bff890b59e8970b47 Mon Sep 17 00:00:00 2001
From: Ramesh Reddy Adutla <rameshreddy.adutla@mnscorp.net>
Date: Mon, 28 Sep 2026 13:37:30 +0100
Subject: [PATCH] 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.
---
 .../StdioServerTransportProvider.java         |  2 +-
 .../StdioServerTransportProviderTests.java    | 30 +++++++++++++++++++
 2 files changed, 31 insertions(+), 1 deletion(-)

diff --git a/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/StdioServerTransportProvider.java b/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/StdioServerTransportProvider.java
index 8370e41ed..c702ebd23 100644
--- a/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/StdioServerTransportProvider.java
+++ b/mcp-core/src/main/java/io/modelcontextprotocol/server/transport/StdioServerTransportProvider.java
@@ -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();
diff --git a/mcp-test/src/test/java/io/modelcontextprotocol/server/transport/StdioServerTransportProviderTests.java b/mcp-test/src/test/java/io/modelcontextprotocol/server/transport/StdioServerTransportProviderTests.java
index 72d0dfcf8..859ad7c68 100644
--- a/mcp-test/src/test/java/io/modelcontextprotocol/server/transport/StdioServerTransportProviderTests.java
+++ b/mcp-test/src/test/java/io/modelcontextprotocol/server/transport/StdioServerTransportProviderTests.java
@@ -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;
@@ -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;
 
@@ -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;