diff --git a/google/genai/_api_client.py b/google/genai/_api_client.py
index b998139e5..2fa2e727a 100644
--- a/google/genai/_api_client.py
+++ b/google/genai/_api_client.py
@@ -362,10 +362,10 @@ def _iter_response_stream(self) -> Iterator[str]:
           data_buffer = []
         continue
 
-      # In streaming mode, the response of JSON is prefixed with "data: " which
+      # In streaming mode, the response of JSON is prefixed with either "data:" or "data: " which
       # we must strip before parsing.
-      if line.startswith('data: '):
-        data_buffer.append(line[len('data: '):])
+      if line.startswith('data:'):
+        data_buffer.append(line[len('data:'):].lstrip())
         continue
 
       # When API returns an error message, it comes line by line. So we buffer
@@ -412,10 +412,10 @@ async def _aiter_response_stream(self) -> AsyncIterator[str]:
               yield '\n'.join(data_buffer)
               data_buffer = []
             continue
-          # In streaming mode, the response of JSON is prefixed with "data: "
+          # In streaming mode, the response of JSON is prefixed with either "data:" or "data: "
           # which we must strip before parsing.
-          if line.startswith('data: '):
-            data_buffer.append(line[len('data: '):])
+          if line.startswith('data:'):
+            data_buffer.append(line[len('data:'):].lstrip())
             continue
 
           # When API returns an error message, it comes line by line. So we buffer
@@ -465,10 +465,10 @@ async def _aiter_response_stream(self) -> AsyncIterator[str]:
               data_buffer = []
             continue
 
-          # In streaming mode, the response of JSON is prefixed with "data: "
+          # In streaming mode, the response of JSON is prefixed with either "data:" or "data: "
           # which we must strip before parsing.
-          if line.startswith('data: '):
-            data_buffer.append(line[len('data: '):])
+          if line.startswith('data:'):
+            data_buffer.append(line[len('data:'):].lstrip())
             continue
 
           # When API returns an error message, it comes line by line. So we