diff --git a/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java b/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
index b9ed2711d..defaf7495 100644
--- a/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
+++ b/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
@@ -11,6 +11,7 @@
 import java.net.http.HttpResponse;
 import java.time.Duration;
 import java.util.List;
+import java.util.concurrent.CancellationException;
 import java.util.concurrent.CompletableFuture;
 import java.util.concurrent.atomic.AtomicReference;
 import java.util.function.Consumer;
@@ -399,9 +400,13 @@ else if (MESSAGE_EVENT_TYPE.equals(responseEvent.sseEvent().event())) {
 						logger.warn("SSE stream observed an error", t);
 						sink.error(t);
 					}
+					this.messageEndpointSink.tryEmitError(t);
 					return true;
 				})
 				.doFinally(s -> {
+					this.messageEndpointSink
+						.tryEmitError(new CancellationException("SSE stream ended before receiving the endpoint event"));
+
 					Disposable ref = this.sseSubscription.getAndSet(null);
 					if (ref != null && !ref.isDisposed()) {
 						ref.dispose();
diff --git a/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransportTests.java b/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransportTests.java
index a24805a30..ac9e6709f 100644
--- a/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransportTests.java
+++ b/mcp-test/src/test/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransportTests.java
@@ -244,6 +244,27 @@ void testRetryBehavior() {
 		failingTransport.closeGracefully().block();
 	}
 
+	@Test
+	void testSendMessageFailsWhenSseConnectionFails() {
+		// Create transport that connects to a non-existent host
+		HttpClientSseClientTransport failingTransport = HttpClientSseClientTransport.builder("http://non-existent-host")
+			.connectTimeout(Duration.ofSeconds(2))
+			.build();
+
+		// Attempt to connect (will fail)
+		failingTransport.connect(Function.identity()).subscribe();
+
+		// Create a test message
+		JSONRPCRequest testMessage = new JSONRPCRequest(McpSchema.JSONRPC_VERSION, "test-method", "test-id",
+				Map.of("key", "value"));
+
+		// Verify sendMessage fails with an error rather than hanging forever
+		StepVerifier.create(failingTransport.sendMessage(testMessage)).expectError().verify(Duration.ofSeconds(10));
+
+		// Clean up
+		failingTransport.closeGracefully().block();
+	}
+
 	@Test
 	void testMultipleMessageProcessing() {
 		// Simulate receiving multiple messages in sequence