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 d951349d..49a5d1c4 100644
--- a/mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
+++ b/mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
@@ -417,6 +417,7 @@ public Mono<Void> sendMessage(JSONRPCMessage message) {
 			URI requestUri = Utils.resolveUri(baseUri, endpoint);
 			HttpRequest request = this.requestBuilder.copy()
 				.uri(requestUri)
+        .setHeader("Content-Type", "application/json;charset=UTF-8")
 				.POST(HttpRequest.BodyPublishers.ofString(jsonText))
 				.build();