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fix: propagate SSE stream errors to messageEndpointSink to prevent sendMessage() hang #786

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fix: propagate SSE stream errors to messageEndpointSink to prevent sendMessage() hang #786

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5 changes: 5 additions & 0 deletions .../src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
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Original file line number Diff line number Diff line change
Expand Up @@ -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;
Expand Down Expand Up @@ -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();
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21 changes: 21 additions & 0 deletions ...test/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransportTests.java
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Expand Up @@ -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
Expand Down