diff --git a/src/agentscope/model/_gemini_model.py b/src/agentscope/model/_gemini_model.py
index 6dfeddc8de..18f99fa4e3 100644
--- a/src/agentscope/model/_gemini_model.py
+++ b/src/agentscope/model/_gemini_model.py
@@ -303,6 +303,36 @@ async def __call__(
 
         return parsed_response
 
+    def _extract_usage(
+        self,
+        usage_metadata: Any,
+        start_datetime: datetime,
+    ) -> ChatUsage | None:
+        """Extract ChatUsage from usage_metadata safely, returning None if
+        unavailable or if token counts are None.
+
+        Args:
+            usage_metadata:
+                The usage metadata object from the Gemini response.
+            start_datetime (`datetime`):
+                The start datetime of the generation.
+
+        Returns:
+            `ChatUsage | None`:
+                A ChatUsage object, or None if data is unavailable.
+        """
+        if not usage_metadata:
+            return None
+        prompt_tokens = usage_metadata.prompt_token_count
+        total_tokens = usage_metadata.total_token_count
+        if prompt_tokens is not None and total_tokens is not None:
+            return ChatUsage(
+                input_tokens=prompt_tokens,
+                output_tokens=total_tokens - prompt_tokens,
+                time=(datetime.now() - start_datetime).total_seconds(),
+            )
+        return None
+
     async def _parse_gemini_stream_generation_response(
         self,
         start_datetime: datetime,
@@ -383,14 +413,7 @@ async def _parse_gemini_stream_generation_response(
             if text and structured_model:
                 metadata = _json_loads_with_repair(text)
 
-            usage = None
-            if chunk.usage_metadata:
-                usage = ChatUsage(
-                    input_tokens=chunk.usage_metadata.prompt_token_count,
-                    output_tokens=chunk.usage_metadata.total_token_count
-                    - chunk.usage_metadata.prompt_token_count,
-                    time=(datetime.now() - start_datetime).total_seconds(),
-                )
+            usage = self._extract_usage(chunk.usage_metadata, start_datetime)
 
             # The content blocks for the current chunk
             content_blocks: list = []
@@ -502,16 +525,7 @@ def _parse_gemini_generation_response(
         if response.text and structured_model:
             metadata = _json_loads_with_repair(response.text)
 
-        if response.usage_metadata:
-            usage = ChatUsage(
-                input_tokens=response.usage_metadata.prompt_token_count,
-                output_tokens=response.usage_metadata.total_token_count
-                - response.usage_metadata.prompt_token_count,
-                time=(datetime.now() - start_datetime).total_seconds(),
-            )
-
-        else:
-            usage = None
+        usage = self._extract_usage(response.usage_metadata, start_datetime)
 
         return ChatResponse(
             content=content_blocks + tool_calls,