diff --git a/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java b/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
index 2e639f3c5..275f7a22d 100644
--- a/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
+++ b/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientSseClientTransport.java
@@ -445,7 +445,7 @@ private Mono<HttpResponse<String>> sendHttpPost(final String endpoint, final Str
 		return Mono.deferContextual(ctx -> {
 			var builder = this.requestBuilder.copy()
 				.uri(requestUri)
-				.header(HttpHeaders.CONTENT_TYPE, "application/json")
+				.header(HttpHeaders.CONTENT_TYPE, "application/json; charset=UTF-8")
 				.header(MCP_PROTOCOL_VERSION_HEADER_NAME, MCP_PROTOCOL_VERSION)
 				.POST(HttpRequest.BodyPublishers.ofString(body));
 			var transportContext = ctx.getOrDefault(McpTransportContext.KEY, McpTransportContext.EMPTY);
diff --git a/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java b/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java
index 86acf4e99..2003666f9 100644
--- a/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java
+++ b/mcp-core/src/main/java/io/modelcontextprotocol/client/transport/HttpClientStreamableHttpTransport.java
@@ -477,7 +477,7 @@ public Mono<Void> sendMessage(McpSchema.JSONRPCMessage sentMessage) {
 
 				var builder = requestBuilder.uri(uri)
 					.header(HttpHeaders.ACCEPT, APPLICATION_JSON + ", " + TEXT_EVENT_STREAM)
-					.header(HttpHeaders.CONTENT_TYPE, APPLICATION_JSON)
+					.header(HttpHeaders.CONTENT_TYPE, APPLICATION_JSON + "; charset=UTF-8")
 					.header(HttpHeaders.CACHE_CONTROL, "no-cache")
 					.header(HttpHeaders.PROTOCOL_VERSION,
 							ctx.getOrDefault(McpAsyncClient.NEGOTIATED_PROTOCOL_VERSION,