diff --git a/pydantic_ai_slim/pydantic_ai/models/gemini.py b/pydantic_ai_slim/pydantic_ai/models/gemini.py
index b98bfb9b1..e49803d01 100644
--- a/pydantic_ai_slim/pydantic_ai/models/gemini.py
+++ b/pydantic_ai_slim/pydantic_ai/models/gemini.py
@@ -420,13 +420,13 @@ async def _get_gemini_responses(self) -> AsyncIterator[_GeminiResponse]:
             responses_to_yield = gemini_responses[:-1]
             for r in responses_to_yield[current_gemini_response_index:]:
                 current_gemini_response_index += 1
-                self._usage += _metadata_as_usage(r)
+                self._usage = _metadata_as_usage(r)
                 yield r
 
         # Now yield the final response, which should be complete
         if gemini_responses:
             r = gemini_responses[-1]
-            self._usage += _metadata_as_usage(r)
+            self._usage = _metadata_as_usage(r)
             yield r
 
     @property