diff --git a/tools/h3_request/h3_request.py b/tools/h3_request/h3_request.py
index 17a2209aa969..3fcf5013adb2 100644
--- a/tools/h3_request/h3_request.py
+++ b/tools/h3_request/h3_request.py
@@ -92,10 +92,13 @@ async def request(
             ],
             end_stream=not post_stdin,
         )
+        self.transmit()
         if post_stdin:
             async for data in _stream_stdin_generator():
                 self._http.send_data(stream_id=stream_id, data=data, end_stream=False)
+                self.transmit()
             self._http.send_data(stream_id=stream_id, data=b'', end_stream=True)
+            self.transmit()
 
         await future