From 759a4c18811a140d8680eb532d9114278e1b7c1d Mon Sep 17 00:00:00 2001
From: logan <logan@querypie.com>
Date: Thu, 19 Jun 2025 06:21:42 +0900
Subject: [PATCH] fix(client): Fixes a bug in McpClientSession.sendRequest()
 where it waited until timeout when receiving error responses such as 4xx or
 5xx.

Returning failedFuture() when an error is detected in HttpClientSseClientTransport.sendMessage() resolves the issue.
---
 .../transport/HttpClientSseClientTransport.java   | 15 +++++++++------
 1 file changed, 9 insertions(+), 6 deletions(-)

diff --git a/mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java b/mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
index 99cf2a625..6005c4496 100644
--- a/mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
+++ b/mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
@@ -420,12 +420,15 @@ public Mono<Void> sendMessage(JSONRPCMessage message) {
 				.build();
 
 			return Mono.fromFuture(
-					httpClient.sendAsync(request, HttpResponse.BodyHandlers.discarding()).thenAccept(response -> {
-						if (response.statusCode() != 200 && response.statusCode() != 201 && response.statusCode() != 202
-								&& response.statusCode() != 206) {
-							logger.error("Error sending message: {}", response.statusCode());
-						}
-					}));
+					httpClient.sendAsync(request, HttpResponse.BodyHandlers.discarding())
+							.thenCompose(response -> {
+								int status = response.statusCode();
+								if (status != 200 && status != 201 && status != 202 && status != 206) {
+									return CompletableFuture.failedFuture(new McpError("Error sending message: " + status));
+								}
+								return CompletableFuture.completedFuture(null);
+							})
+			);
 		}
 		catch (IOException e) {
 			if (!isClosing) {