fix(vertex/gemini): keep tool_call reference when a text-only assistant message follows

litellm/llms/vertex_ai/gemini/transformation.py

@@ -996,7 +996,19 @@ def _gemini_convert_messages_with_history(  # noqa: PLR0915
                             excluded_keys=["thoughtSignature"],
                         ):
                             assistant_content.append(gemini_tool_call_part)
-                    last_message_with_tool_calls = assistant_msg
+                    # Only record this as the active tool-call message when it actually
+                    # carries tool calls. The `if` guard above is also entered for a
+                    # text-only assistant message (`assistant_msg.get("tool_calls", [])
+                    # is not None` is True for an empty list), so without this check a
+                    # later assistant message with no tool calls would clobber the
+                    # reference. The following tool result would then be matched against
+                    # an assistant message that has no tool_calls, raising "Missing
+                    # corresponding tool call for tool response message".
+                    if (
+                        assistant_msg.get("tool_calls")
+                        or assistant_msg.get("function_call") is not None
+                    ):
+                        last_message_with_tool_calls = assistant_msg
 
                 ## HANDLE SERVER-SIDE TOOL INVOCATIONS (context circulation)
                 _psf = assistant_msg.get("provider_specific_fields")

tests/test_litellm/llms/vertex_ai/gemini/test_tool_call_followed_by_text_assistant.py

@@ -0,0 +1,57 @@
+"""
+Regression test for tool-call / tool-result matching in the Gemini message converter.
+
+When an assistant message that contains tool_calls is followed by a *second* assistant
+message that has no tool_calls (e.g. the model emits a short narration turn after the
+tool call but before the tool result), the converter used to overwrite its
+`last_message_with_tool_calls` reference with the text-only assistant message. The
+subsequent tool result could then no longer be matched to its tool call, and conversion
+failed with:
+
+    Exception: Missing corresponding tool call for tool response message.
+
+This happens for any OpenAI-style history with that shape, independent of provider/model.
+"""
+
+import pytest
+
+from litellm.llms.vertex_ai.gemini.transformation import (
+    _gemini_convert_messages_with_history,
+)
+
+
+def _messages_with_text_assistant_between_tool_call_and_result():
+    return [
+        {"role": "user", "content": "list the files"},
+        {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [
+                {
+                    "id": "call_abc123",
+                    "type": "function",
+                    "function": {"name": "shell", "arguments": '{"command": ["ls"]}'},
+                }
+            ],
+        },
+        # text-only assistant message in between (no tool_calls)
+        {"role": "assistant", "content": "Running the command now."},
+        {"role": "tool", "tool_call_id": "call_abc123", "content": "math.py"},
+    ]
+
+
+def test_tool_result_matches_tool_call_with_text_assistant_in_between():
+    messages = _messages_with_text_assistant_between_tool_call_and_result()
+
+    # Should not raise "Missing corresponding tool call for tool response message".
+    contents = _gemini_convert_messages_with_history(messages=messages)
+
+    # The function response must be present and carry the correct tool name.
+    function_responses = [
+        part["function_response"]
+        for content in contents
+        for part in content["parts"]
+        if isinstance(part, dict) and part.get("function_response")
+    ]
+    assert function_responses, f"expected a functionResponse part, got: {contents}"
+    assert function_responses[0]["name"] == "shell"