fix(vertex): repair tool-call history for Gemini

litellm/llms/vertex_ai/gemini/transformation.py

@@ -305,6 +305,16 @@ def _gemini_convert_messages_with_history(  # noqa: PLR0915
     user_message_types = {"user", "system"}
     contents: List[ContentType] = []
 
+    # Pre-build tool_call_id → assistant message so we can recover
+    # last_message_with_tool_calls at the tool-response side when a provider
+    # (e.g. Codex CLI) sends assistant messages with an empty tool_calls list.
+    tool_call_map: dict = {}
+    for msg in messages:
+        if msg["role"] == "assistant":
+            for tc in msg.get("tool_calls") or []:
+                if "id" in tc:
+                    tool_call_map[tc["id"]] = msg
+
     last_message_with_tool_calls = None
 
     msg_i = 0
@@ -527,7 +537,10 @@ def _gemini_convert_messages_with_history(  # noqa: PLR0915
 
                 ## HANDLE ASSISTANT FUNCTION CALL
                 if (
-                    assistant_msg.get("tool_calls", []) is not None
+                    (
+                        assistant_msg.get("tool_calls", []) is not None
+                        and len(assistant_msg.get("tool_calls", [])) > 0
+                    )
                     or assistant_msg.get("function_call") is not None
                 ):  # support assistant tool invoke conversion
                     gemini_tool_call_parts = convert_to_gemini_tool_call_invoke(
@@ -589,6 +602,10 @@ def _gemini_convert_messages_with_history(  # noqa: PLR0915
                 msg_i < len(messages)
                 and messages[msg_i]["role"] in tool_call_message_roles
             ):
+                _tid = messages[msg_i].get("tool_call_id")
+                if _tid and _tid in tool_call_map:
+                    last_message_with_tool_calls = tool_call_map[_tid]
+
                 _part = convert_to_gemini_tool_call_result(
                     messages[msg_i], last_message_with_tool_calls  # type: ignore
                 )

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

@@ -1913,3 +1913,61 @@ def test_multi_turn_function_calling_roles():
                 assert (
                     content["role"] == "user"
                 ), f"Content block {i} with function_response has role='{content['role']}', expected 'user'"
+
+
+def test_tool_call_history_repair_empty_tool_calls():
+    """
+    Regression test for: Codex CLI sends assistant messages with empty tool_calls
+    list, causing 'Missing corresponding tool call for tool response message'.
+
+    Codex CLI sometimes emits:
+        {"role": "assistant", "content": "...", "tool_calls": []}
+    followed by a separate assistant message that has the actual tool_calls,
+    then a tool response referencing one of those calls.
+
+    Before fix: last_message_with_tool_calls pointed to the empty-tool_calls
+    message, name recovery failed, Vertex AI returned 400.
+
+    After fix: a pre-pass builds tool_call_id → assistant message so the
+    correct originating message is always found.
+    """
+    messages = [
+        {"role": "user", "content": "Run a shell command"},
+        {
+            "role": "assistant",
+            "content": "I'll run that for you.",
+            "tool_calls": [],  # empty — Codex CLI quirk
+        },
+        {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [
+                {
+                    "id": "call_abc123",
+                    "type": "function",
+                    "function": {
+                        "name": "shell",
+                        "arguments": '{"cmd": "echo hello"}',
+                    },
+                }
+            ],
+        },
+        {
+            "role": "tool",
+            "tool_call_id": "call_abc123",
+            "content": "hello",
+        },
+    ]
+
+    contents = _gemini_convert_messages_with_history(messages=messages)
+
+    # The function_response part must have the correct function name
+    found = False
+    for content in contents:
+        for part in content.get("parts", []):
+            if "function_response" in part:
+                assert part["function_response"]["name"] == "shell", (
+                    f"Expected function name 'shell', got {part['function_response']['name']}"
+                )
+                found = True
+    assert found, "No function_response part found in converted history"