diff --git a/astrbot/core/provider/sources/openai_source.py b/astrbot/core/provider/sources/openai_source.py
index 6848c5d43a..48a6e222f7 100644
--- a/astrbot/core/provider/sources/openai_source.py
+++ b/astrbot/core/provider/sources/openai_source.py
@@ -307,6 +307,14 @@ async def _query_stream(
         state = ChatCompletionStreamState()
 
         async for chunk in stream:
+            # Fix for #6661: Add missing 'index' field to tool_call deltas
+            # Gemini and some OpenAI-compatible proxies omit this field
+            if chunk.choices:
+                for choice in chunk.choices:
+                    if choice.delta and choice.delta.tool_calls:
+                        for idx, tc in enumerate(choice.delta.tool_calls):
+                            if not hasattr(tc, "index") or tc.index is None:
+                                tc.index = idx
             try:
                 state.handle_chunk(chunk)
             except Exception as e: