diff --git a/src/stan/callbacks/json_writer.hpp b/src/stan/callbacks/json_writer.hpp
index d791b8a7371..efc572e30ff 100644
--- a/src/stan/callbacks/json_writer.hpp
+++ b/src/stan/callbacks/json_writer.hpp
@@ -228,6 +228,7 @@ class json_writer final : public structured_writer {
     } else {
       *output_ << "\n";
     }
+    output_->flush();
   }
 
   /**