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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
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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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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.

}
}
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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Same comment as for the WebClient implementation.

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it is the same problem as above

}
}
logger.warn("Unknown media type {} returned for POST in session {}", contentType,
Expand Down
8 changes: 8 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 +166,12 @@ else if (line.startsWith("event:")) {
this.currentEventType.set(matcher.group(1).trim());
}
}
else {
// If the response is not successful, emit an error
this.sink.error(new McpError(
"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
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