diff --git a/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/SSEClientTransport.kt b/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/SSEClientTransport.kt
index 19405c57..335e9a63 100644
--- a/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/SSEClientTransport.kt
+++ b/src/commonMain/kotlin/io/modelcontextprotocol/kotlin/sdk/client/SSEClientTransport.kt
@@ -113,6 +113,7 @@ public class SseClientTransport(
 
         try {
             val response = client.post(endpoint.getCompleted()) {
+                requestBuilder()
                 headers.append(HttpHeaders.ContentType, ContentType.Application.Json)
                 setBody(McpJson.encodeToString(message))
             }