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fix: keep stdio client reading after a malformed response #1158

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fix: keep stdio client reading after a malformed response #1158

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39 changes: 38 additions & 1 deletion mcp-core/src/main/java/io/modelcontextprotocol/client/transport/StdioClientTransport.java
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Original file line number Diff line number Diff line change
Expand Up @@ -11,7 +11,9 @@
import java.time.Duration;
import java.util.ArrayList;
import java.util.EnumSet;
import java.util.HashMap;
import java.util.List;
import java.util.Map;
import java.util.Set;
import java.util.concurrent.Executors;
import java.util.function.Consumer;
Expand All @@ -23,6 +25,7 @@
import io.modelcontextprotocol.spec.McpClientTransport;
import io.modelcontextprotocol.spec.McpSchema;
import io.modelcontextprotocol.spec.McpSchema.JSONRPCMessage;
import io.modelcontextprotocol.spec.McpSchema.JSONRPCResponse;
import io.modelcontextprotocol.util.Assert;
import org.slf4j.Logger;
import org.slf4j.LoggerFactory;
Expand Down Expand Up @@ -290,7 +293,15 @@ private void startInboundProcessing() {
if (!isClosing) {
logger.error("Error processing inbound message for line: {}", line, e);
}
break;
// One malformed message must not end the session. Fail the
// request it answers, if any, and keep reading.
JSONRPCResponse failure = failureForMalformedResponse(line, e);
if (failure != null && !this.inboundSink.tryEmitNext(failure).isSuccess()) {
if (!isClosing) {
logger.error("Failed to enqueue inbound message: {}", failure);
}
break;
}
}
}
}
Expand All @@ -311,6 +322,32 @@ private void startInboundProcessing() {
});
}

/**
* Builds an error response for a response that could not be deserialized, so that the
* request it answers fails right away instead of waiting for its timeout.
* @param line the raw message
* @param cause the deserialization failure
* @return the error response, or {@code null} if the line is not a response with a
* usable id
*/
private JSONRPCResponse failureForMalformedResponse(String line, Exception cause) {
try {
Map<String, Object> message = this.jsonMapper.readValue(line, new TypeRef<HashMap<String, Object>>() {
});
Object id = message.get("id");
boolean isResponse = !message.containsKey("method")
&& (message.containsKey("result") || message.containsKey("error"));
if (isResponse && (id instanceof String || id instanceof Integer || id instanceof Long)) {
return JSONRPCResponse.error(id, new JSONRPCResponse.JSONRPCError(McpSchema.ErrorCodes.INTERNAL_ERROR,
"Received a malformed JSON-RPC response", cause.getMessage()));
}
}
catch (Exception ignored) {
// not JSON, so there is no request to fail
}
return null;
}

/**
* Reads a single line, mirroring {@link BufferedReader#readLine()}, but aborting once
* more than {@code maxSize} characters have been read without encountering a line
Expand Down
54 changes: 54 additions & 0 deletions ...est/src/test/java/io/modelcontextprotocol/client/transport/StdioClientTransportTests.java
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Original file line number Diff line number Diff line change
Expand Up @@ -6,12 +6,20 @@

import java.io.ByteArrayOutputStream;
import java.io.PrintStream;
import java.nio.file.Files;
import java.nio.file.Path;
import java.time.Duration;
import java.util.List;
import java.util.concurrent.CopyOnWriteArrayList;

import io.modelcontextprotocol.spec.McpSchema;
import io.modelcontextprotocol.spec.McpSchema.JSONRPCMessage;
import io.modelcontextprotocol.spec.McpSchema.JSONRPCResponse;
import org.awaitility.Awaitility;
import org.junit.jupiter.api.AfterEach;
import org.junit.jupiter.api.BeforeEach;
import org.junit.jupiter.api.Test;
import org.junit.jupiter.api.io.TempDir;
import reactor.test.StepVerifier;

import static io.modelcontextprotocol.util.McpJsonMapperUtils.JSON_MAPPER;
Expand Down Expand Up @@ -67,4 +75,50 @@ void shouldRejectInboundMessageExceedingMaxSize() throws Exception {
}
}

@Test
void shouldFailMalformedResponsesAndKeepProcessing(@TempDir Path tempDir) throws Exception {
// A server process that answers with two malformed responses and a line that
// is not JSON, followed by a valid response. It then stays alive until its
// stdin is closed.
Path serverOutput = tempDir.resolve("server-output.jsonl");
Files.write(serverOutput,
List.of("{\"id\":\"missing-jsonrpc\",\"result\":{}}",
"{\"jsonrpc\":\"2.0\",\"id\":2,\"result\":{},\"error\":{\"code\":-32000,\"message\":\"boom\"}}",
"this is not json", "{\"jsonrpc\":\"2.0\",\"id\":\"valid\",\"result\":{}}"));
ServerParameters params = ServerParameters.builder("sh")
.args("-c", "cat '" + serverOutput.toString().replace('\\', '/') + "'; cat > /dev/null")
.build();

List<JSONRPCMessage> received = new CopyOnWriteArrayList<>();
StdioClientTransport transport = new StdioClientTransport(params, JSON_MAPPER);
try {
StepVerifier.create(transport.connect(msg -> msg.doOnNext(received::add))).verifyComplete();

Awaitility.await()
.atMost(Duration.ofSeconds(5))
.pollInterval(Duration.ofMillis(100))
.untilAsserted(() -> assertThat(received).hasSize(3));

// each malformed response fails the request it answers
assertThat(received.get(0)).isInstanceOfSatisfying(JSONRPCResponse.class, response -> {
assertThat(response.id()).isEqualTo("missing-jsonrpc");
assertThat(response.result()).isNull();
assertThat(response.error().code()).isEqualTo(McpSchema.ErrorCodes.INTERNAL_ERROR);
});
assertThat(received.get(1)).isInstanceOfSatisfying(JSONRPCResponse.class, response -> {
assertThat(response.id()).isEqualTo(2);
assertThat(response.result()).isNull();
assertThat(response.error().code()).isEqualTo(McpSchema.ErrorCodes.INTERNAL_ERROR);
});
// the transport is still reading, so the valid response gets through
assertThat(received.get(2)).isInstanceOfSatisfying(JSONRPCResponse.class, response -> {
assertThat(response.id()).isEqualTo("valid");
assertThat(response.error()).isNull();
});
}
finally {
StepVerifier.create(transport.closeGracefully()).verifyComplete();
}
}

}