diff --git a/playwright/_impl/_transport.py b/playwright/_impl/_transport.py
index 2ca84d459..89928459d 100644
--- a/playwright/_impl/_transport.py
+++ b/playwright/_impl/_transport.py
@@ -176,3 +176,4 @@ def send(self, message: Dict) -> None:
         self._output.write(
             len(data).to_bytes(4, byteorder="little", signed=False) + data
         )
+        self._output.drain()