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fix: handle concurrent sendMessage calls in StdioServerTransportProvider #834

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fix: handle concurrent sendMessage calls in StdioServerTransportProvider #834

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2715ab8
fix: handle concurrent sendMessage calls in StdioServerTransportProvider
dominikozi Feb 24, 2026
c7e71b3
test: replace delay based concurrency test with thread swarm
dominikozi Feb 24, 2026
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8 changes: 2 additions & 6 deletions .../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 @@ -147,12 +147,8 @@ public StdioMcpSessionTransport() {
public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message) {

return Mono.zip(inboundReady.asMono(), outboundReady.asMono()).then(Mono.defer(() -> {
if (outboundSink.tryEmitNext(message).isSuccess()) {
return Mono.empty();
}
else {
return Mono.error(new RuntimeException("Failed to enqueue message"));
}
outboundSink.emitNext(message, Sinks.EmitFailureHandler.busyLooping(java.time.Duration.ofSeconds(1)));
return Mono.<Void>empty();
}));
}

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43 changes: 43 additions & 0 deletions ...test/java/io/modelcontextprotocol/server/transport/StdioServerTransportProviderTests.java
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Expand Up @@ -220,4 +220,47 @@ void shouldHandleSessionClose() throws Exception {
verify(mockSession).closeGracefully();
}

@Test
void shouldHandleConcurrentMessages() throws Exception {
java.io.PipedOutputStream pipedOut = new java.io.PipedOutputStream();
java.io.PipedInputStream pipedIn = new java.io.PipedInputStream(pipedOut);

ByteArrayOutputStream outputStream = new ByteArrayOutputStream();
transportProvider = new StdioServerTransportProvider(McpJsonDefaults.getMapper(), pipedIn, outputStream);

McpServerSession.Factory realSessionFactory = transport -> {
McpServerSession session = mock(McpServerSession.class);
when(session.handle(any())).thenAnswer(invocation -> {
McpSchema.JSONRPCMessage incomingMessage = invocation.getArgument(0);
// Simulate async tool call processing with a delay
return Mono.delay(java.time.Duration.ofMillis(50))
.then(transport.sendMessage(new McpSchema.JSONRPCResponse(McpSchema.JSONRPC_VERSION,
((McpSchema.JSONRPCRequest) incomingMessage).id(), Map.of("result", "ok"), null)));
});
when(session.closeGracefully()).thenReturn(Mono.empty());
return session;
};

// Set session factory
transportProvider.setSessionFactory(realSessionFactory);

String jsonMessage1 = "{\"jsonrpc\":\"2.0\",\"method\":\"test1\",\"params\":{},\"id\":1}\n";
String jsonMessage2 = "{\"jsonrpc\":\"2.0\",\"method\":\"test2\",\"params\":{},\"id\":2}\n";
pipedOut.write(jsonMessage1.getBytes(StandardCharsets.UTF_8));
pipedOut.write(jsonMessage2.getBytes(StandardCharsets.UTF_8));
pipedOut.flush();

// Verify both concurrent responses complete without error
StepVerifier
.create(Mono.delay(java.time.Duration.ofSeconds(2))
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@chemicL chemicL Feb 24, 2026

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The test passes if Duration.ofSeconds(3) is provided. Preferably, instead of using delays, you'd launch a swarm of threads trying to process the messages in parallel and ensure none of them fail and all the messages are processed.

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@dominikozi dominikozi Feb 24, 2026

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Updated the test according to your suggestion. The delay-based test has been replaced with a swarm test, which sends 100 requests. The test verifies all response IDs are received, no failures occur, and that concurrency actually happened. Added a 15s Timeout, consistent with existing tests in the repository.

.then(Mono.fromCallable(() -> outputStream.toString(StandardCharsets.UTF_8))))
.assertNext(output -> {
assertThat(output).contains("\"id\":1");
assertThat(output).contains("\"id\":2");
})
.verifyComplete();

pipedOut.close();
}

}