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fix: improve compatibility with non-compliant MCP servers #413

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fix: improve compatibility with non-compliant MCP servers #413

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7d1ce52
fix: improve compatibility with non-compliant MCP servers (#413)
tzolov Jul 18, 2025
2d05308
addres review comments
tzolov Jul 18, 2025
521c2cc
Address review comments
tzolov Jul 22, 2025
18e10f0
address review
tzolov Jul 22, 2025
babb566
another round
tzolov Jul 22, 2025
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21 changes: 15 additions & 6 deletions .../main/java/io/modelcontextprotocol/client/transport/WebClientStreamableHttpTransport.java
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Original file line number Diff line number Diff line change
Expand Up @@ -30,6 +30,7 @@
import io.modelcontextprotocol.spec.McpTransportSessionNotFoundException;
import io.modelcontextprotocol.spec.McpTransportStream;
import io.modelcontextprotocol.util.Assert;
import io.modelcontextprotocol.util.Utils;
import reactor.core.Disposable;
import reactor.core.publisher.Flux;
import reactor.core.publisher.Mono;
Expand Down Expand Up @@ -244,7 +245,7 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage message) {

Disposable connection = webClient.post()
.uri(this.endpoint)
.accept(MediaType.TEXT_EVENT_STREAM, MediaType.APPLICATION_JSON)
.accept(MediaType.APPLICATION_JSON, MediaType.TEXT_EVENT_STREAM)
.headers(httpHeaders -> {
transportSession.sessionId().ifPresent(id -> httpHeaders.add("mcp-session-id", id));
})
Expand Down Expand Up @@ -287,7 +288,7 @@ else if (mediaType.isCompatibleWith(MediaType.APPLICATION_JSON)) {
logger.trace("Received response to POST for session {}", sessionRepresentation);
// communicate to caller the message was delivered
sink.success();
return responseFlux(response);
return directResponseFlux(message, response);
}
else {
logger.warn("Unknown media type {} returned for POST in session {}", contentType,
Expand Down Expand Up @@ -384,14 +385,22 @@ private static String sessionIdOrPlaceholder(McpTransportSession<?> transportSes
return transportSession.sessionId().orElse("[missing_session_id]");
}

private Flux<McpSchema.JSONRPCMessage> responseFlux(ClientResponse response) {
private Flux<McpSchema.JSONRPCMessage> directResponseFlux(McpSchema.JSONRPCMessage sentMessage,
ClientResponse response) {
return response.bodyToMono(String.class).<Iterable<McpSchema.JSONRPCMessage>>handle((responseMessage, s) -> {
try {
McpSchema.JSONRPCMessage jsonRpcResponse = McpSchema.deserializeJsonRpcMessage(objectMapper,
responseMessage);
s.next(List.of(jsonRpcResponse));
if (sentMessage instanceof McpSchema.JSONRPCNotification && Utils.hasText(responseMessage)) {
logger.warn("Notification: {} received non-compliant response: {}", sentMessage, responseMessage);
s.complete();
}
else {
McpSchema.JSONRPCMessage jsonRpcResponse = McpSchema.deserializeJsonRpcMessage(objectMapper,
responseMessage);
s.next(List.of(jsonRpcResponse));
}
}
catch (IOException e) {
// TODO: this should be a McpTransportError
s.error(e);
}
}).flatMapIterable(Function.identity());
Expand Down
14 changes: 10 additions & 4 deletions ...main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java
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Expand Up @@ -342,9 +342,9 @@ public String toString(McpSchema.JSONRPCMessage message) {
}
}

public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sendMessage) {
public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sentMessage) {
return Mono.create(messageSink -> {
logger.debug("Sending message {}", sendMessage);
logger.debug("Sending message {}", sentMessage);

final AtomicReference<Disposable> disposableRef = new AtomicReference<>();
final McpTransportSession<Disposable> transportSession = this.activeSession.get();
Expand All @@ -355,10 +355,10 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sendMessage) {
requestBuilder = requestBuilder.header("mcp-session-id", transportSession.sessionId().get());
}

String jsonBody = this.toString(sendMessage);
String jsonBody = this.toString(sentMessage);

HttpRequest request = requestBuilder.uri(Utils.resolveUri(this.baseUri, this.endpoint))
.header("Accept", TEXT_EVENT_STREAM + ", " + APPLICATION_JSON)
.header("Accept", APPLICATION_JSON + ", " + TEXT_EVENT_STREAM)
.header("Content-Type", APPLICATION_JSON)
.header("Cache-Control", "no-cache")
.POST(HttpRequest.BodyPublishers.ofString(jsonBody))
Expand Down Expand Up @@ -436,10 +436,16 @@ else if (contentType.contains(TEXT_EVENT_STREAM)) {
else if (contentType.contains(APPLICATION_JSON)) {
messageSink.success();
String data = ((ResponseSubscribers.AggregateResponseEvent) responseEvent).data();
if (sentMessage instanceof McpSchema.JSONRPCNotification && Utils.hasText(data)) {
logger.warn("Notification: {} received non-compliant response: {}", sentMessage, data);
return Mono.empty();
}

try {
return Mono.just(McpSchema.deserializeJsonRpcMessage(objectMapper, data));
}
catch (IOException e) {
// TODO: this should be a McpTransportError
return Mono.error(e);
}
}
Expand Down
9 changes: 9 additions & 0 deletions mcp/src/main/java/io/modelcontextprotocol/client/transport/ResponseSubscribers.java
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Original file line number Diff line number Diff line change
Expand Up @@ -12,6 +12,7 @@
import org.reactivestreams.FlowAdapters;
import org.reactivestreams.Subscription;

import io.modelcontextprotocol.spec.McpError;
import reactor.core.publisher.BaseSubscriber;
import reactor.core.publisher.FluxSink;

Expand Down Expand Up @@ -135,6 +136,7 @@ protected void hookOnSubscribe(Subscription subscription) {

@Override
protected void hookOnNext(String line) {

if (line.isEmpty()) {
// Empty line means end of event
if (this.eventBuilder.length() > 0) {
Expand Down Expand Up @@ -164,6 +166,13 @@ else if (line.startsWith("event:")) {
this.currentEventType.set(matcher.group(1).trim());
}
}
else {
// If the response is not successful, emit an error
// TODO: This should be a McpTransportError
this.sink.error(new McpError(
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I believe this should be a McpTransportError, please keep a note of it :)

"Invalid SSE response. Status code: " + this.responseInfo.statusCode() + " Line: " + line));

}
}
}

Expand Down
3 changes: 3 additions & 0 deletions mcp/src/main/java/io/modelcontextprotocol/util/Utils.java
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Expand Up @@ -69,6 +69,9 @@ public static boolean isEmpty(@Nullable Map<?, ?> map) {
* base URL or URI is malformed
*/
public static URI resolveUri(URI baseUrl, String endpointUrl) {
if (!Utils.hasText(endpointUrl)) {
return baseUrl;
}
URI endpointUri = URI.create(endpointUrl);
if (endpointUri.isAbsolute() && !isUnderBaseUri(baseUrl, endpointUri)) {
throw new IllegalArgumentException("Absolute endpoint URL does not match the base URL.");
Expand Down