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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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14 changes: 10 additions & 4 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 @@ -387,9 +388,14 @@ private static String sessionIdOrPlaceholder(McpTransportSession<?> transportSes
private Flux<McpSchema.JSONRPCMessage> responseFlux(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 (Utils.hasText(responseMessage) && !responseMessage.trim().equals("{}")) {
McpSchema.JSONRPCMessage jsonRpcResponse = McpSchema.deserializeJsonRpcMessage(objectMapper,
responseMessage);
s.next(List.of(jsonRpcResponse));
}
else {
logger.warn("Received empty response message: {}", responseMessage);
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@chemicL chemicL Jul 18, 2025 •
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I am a bit worried that if the request body was an MCP Request then we will never complete a pending operation that awaits for it. Please consider adding a check whether the original message was a request and if so send an error to the Flux (using handle's sink).

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@tzolov tzolov Jul 18, 2025

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Unfortunately if we receive an empty response message (e.g. null, " " or {}) there is no way to determine if it is a bogus response from a previous request, compliment notification response or bogus request.
Here we assume that if an empty even is received thread it as incorrect notification response.

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@chemicL chemicL Jul 21, 2025

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I think we have to cancel the request's sink though. We end up in this method if the server responded with an application/json content type. That means semantically we are to consume a response here. If there is nothing, we have to clean up the request sink, otherwise we are creating memory leaks and also not really doing the users of the client a favour not letting them know that the request has failed to fulfil the promise of a response.

}
}
catch (IOException e) {
s.error(e);
Expand Down
18 changes: 13 additions & 5 deletions ...main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java
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Expand Up @@ -358,7 +358,7 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sendMessage) {
String jsonBody = this.toString(sendMessage);

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 @@ -436,11 +436,19 @@ else if (contentType.contains(TEXT_EVENT_STREAM)) {
else if (contentType.contains(APPLICATION_JSON)) {
messageSink.success();
String data = ((ResponseSubscribers.AggregateResponseEvent) responseEvent).data();
try {
return Mono.just(McpSchema.deserializeJsonRpcMessage(objectMapper, data));
if (Utils.hasText(data) && !data.trim().equals("{}")) {

try {
return Mono.just(McpSchema.deserializeJsonRpcMessage(objectMapper, data));
}
catch (IOException e) {
return Mono.error(e);
}
}
catch (IOException e) {
return Mono.error(e);
else {
// No content type means no response body
logger.debug("No content type returned for POST in session {}", sessionRepresentation);
return Mono.empty();
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@chemicL chemicL Jul 18, 2025

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Same comment as for the WebClient implementation.

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@tzolov tzolov Jul 19, 2025

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it is the same problem as above

}
}
logger.warn("Unknown media type {} returned for POST in session {}", contentType,
Expand Down
58 changes: 34 additions & 24 deletions mcp/src/main/java/io/modelcontextprotocol/client/transport/ResponseSubscribers.java
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Expand Up @@ -135,36 +135,46 @@ 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) {
String eventData = this.eventBuilder.toString();
SseEvent sseEvent = new SseEvent(currentEventId.get(), currentEventType.get(), eventData.trim());

this.sink.next(new SseResponseEvent(responseInfo, sseEvent));
this.eventBuilder.setLength(0);
}
}
else {
if (line.startsWith("data:")) {
var matcher = EVENT_DATA_PATTERN.matcher(line);
if (matcher.find()) {
this.eventBuilder.append(matcher.group(1).trim()).append("\n");
if (this.responseInfo.statusCode() >= 200 && this.responseInfo.statusCode() < 300) {

if (line.isEmpty()) {
// Empty line means end of event
if (this.eventBuilder.length() > 0) {
String eventData = this.eventBuilder.toString();
SseEvent sseEvent = new SseEvent(currentEventId.get(), currentEventType.get(),
eventData.trim());

this.sink.next(new SseResponseEvent(responseInfo, sseEvent));
this.eventBuilder.setLength(0);
}
}
else if (line.startsWith("id:")) {
var matcher = EVENT_ID_PATTERN.matcher(line);
if (matcher.find()) {
this.currentEventId.set(matcher.group(1).trim());
else {
if (line.startsWith("data:")) {
var matcher = EVENT_DATA_PATTERN.matcher(line);
if (matcher.find()) {
this.eventBuilder.append(matcher.group(1).trim()).append("\n");
}
}
}
else if (line.startsWith("event:")) {
var matcher = EVENT_TYPE_PATTERN.matcher(line);
if (matcher.find()) {
this.currentEventType.set(matcher.group(1).trim());
else if (line.startsWith("id:")) {
var matcher = EVENT_ID_PATTERN.matcher(line);
if (matcher.find()) {
this.currentEventId.set(matcher.group(1).trim());
}
}
else if (line.startsWith("event:")) {
var matcher = EVENT_TYPE_PATTERN.matcher(line);
if (matcher.find()) {
this.currentEventType.set(matcher.group(1).trim());
}
}
}
}
else {
// If the response is not successful, emit an error
System.out.println("Received non-successful response: " + this.responseInfo.statusCode());
SseEvent sseEvent = new SseEvent(null, null, null);
this.sink.next(new SseResponseEvent(responseInfo, sseEvent));
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@chemicL chemicL Jul 18, 2025

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This looks unfinished. Did you intend to use the logger and call sink.error ?

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@tzolov tzolov Jul 18, 2025

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Yeh good catch. I've mistakenly commit some experiments. Above logic is incorrect.

}
}

@Override
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