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 99cf2a62..e9025bfd 100644
--- a/mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
+++ b/mcp/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
@@ -416,6 +416,7 @@ public Mono<Void> sendMessage(JSONRPCMessage message) {
 			String jsonText = this.objectMapper.writeValueAsString(message);
 			URI requestUri = Utils.resolveUri(baseUri, endpoint);
 			HttpRequest request = this.requestBuilder.uri(requestUri)
+				.setHeader("Content-Type", "application/json;charset=UTF-8")
 				.POST(HttpRequest.BodyPublishers.ofString(jsonText))
 				.build();