fix: drop blank assistant messages inserted after tool_calls

The /responses endpoint's message transformation can insert an empty
assistant message (content=[{"type":"text","text":""}]) right after an
assistant message with tool_calls.  Providers like DeepSeek require
tool_calls to be immediately followed by tool messages; the blank
assistant message in between violates that contract and triggers
HTTP 400: "tool_calls must be followed by tool messages"

Added _drop_blank_assistant_messages_after_tool_calls() as a post-
processing pass in _transform_response_input_param_to_chat_completion_message
that strips these empty assistant messages when they appear directly
after an assistant message with tool_calls

Closes #31553

litellm/responses/litellm_completion_transformation/transformation.py

@@ -487,8 +487,68 @@ class LiteLLMCompletionResponsesConfig:
                     continue
 
                 messages.extend(chat_completion_messages)
+
+        # Drop blank assistant messages that follow an assistant message with
+        # tool_calls.  Providers like DeepSeek require that tool_calls be
+        # immediately followed by tool messages; an empty assistant message in
+        # between violates that contract and triggers HTTP 400.
+        messages = LiteLLMCompletionResponsesConfig._drop_blank_assistant_messages_after_tool_calls(messages)
         return messages
 
+    @staticmethod
+    def _is_blank_content(content: Any) -> bool:
+        """Return True when *content* carries no meaningful information."""
+        if content is None:
+            return True
+        if isinstance(content, str):
+            return content.strip() == ""
+        if isinstance(content, list):
+            # e.g. [{"type": "text", "text": ""}]
+            return all(
+                isinstance(item, dict) and item.get("type") == "text" and not (item.get("text") or "").strip()
+                for item in content
+            )
+        return False
+
+    @staticmethod
+    def _has_tool_calls(msg: Any) -> bool:
+        """Return True when the message contains tool_calls."""
+        tool_calls = msg.get("tool_calls") if isinstance(msg, dict) else getattr(msg, "tool_calls", None)
+        return isinstance(tool_calls, Sequence) and not isinstance(tool_calls, (str, bytes)) and len(tool_calls) > 0
+
+    @staticmethod
+    def _drop_blank_assistant_messages_after_tool_calls(
+        messages: List[Any],
+    ) -> List[Any]:
+        """
+        Remove blank assistant messages that immediately follow an assistant
+        message with tool_calls.
+
+        Providers like DeepSeek require that an assistant message with
+        tool_calls is immediately followed by tool messages.  The Responses
+        API transformation can insert an empty assistant message (e.g.
+        content=[{"type":"text","text":""}]) right after a tool-calling
+        assistant message; this helper strips those out.
+        """
+        if len(messages) < 2:
+            return messages
+        result: list = []  # mutable-ok: single-pass filter that depends on previous element
+        for msg in messages:
+            role = msg.get("role") if isinstance(msg, dict) else getattr(msg, "role", None)
+            if (
+                role == "assistant"
+                and result
+                and not LiteLLMCompletionResponsesConfig._has_tool_calls(msg)
+            ):
+                prev = result[-1]
+                prev_role = prev.get("role") if isinstance(prev, dict) else getattr(prev, "role", None)
+                if prev_role == "assistant" and LiteLLMCompletionResponsesConfig._has_tool_calls(prev):
+                    content = msg.get("content") if isinstance(msg, dict) else getattr(msg, "content", None)
+                    if LiteLLMCompletionResponsesConfig._is_blank_content(content):
+                        continue
+            result.append(msg)
+        return result
+
     @staticmethod
     def _deduplicate_tool_call_output_messages(
         tool_call_output_messages: List[

tests/test_litellm/responses/litellm_completion_transformation/test_litellm_completion_responses.py

@@ -2432,3 +2432,71 @@ def test_function_call_tool_id_falls_back_to_unique_id_for_degenerate_call_id():
         id="fc_2", call_id="call_tokyo", name="get_weather", arguments="{}"
     )
     assert convert(openai)["id"] == "call_tokyo"
+
+
+class TestDropBlankAssistantMessagesAfterToolCalls:
+    """Regression tests for issue #31553: blank assistant messages after tool_calls
+    break providers like DeepSeek that require tool messages immediately after."""
+
+    def test_blank_assistant_after_tool_calls_is_dropped(self):
+        """A blank assistant message right after an assistant with tool_calls should be removed"""
+        messages = [
+            {
+                "role": "assistant",
+                "tool_calls": [{"id": "call_1", "type": "function", "function": {"name": "shell", "arguments": "{}"}}],
+            },
+            {"role": "assistant", "content": [{"type": "text", "text": ""}]},
+            {"role": "tool", "content": "output", "tool_call_id": "call_1"},
+        ]
+        result = LiteLLMCompletionResponsesConfig._drop_blank_assistant_messages_after_tool_calls(messages)
+        assert len(result) == 2
+        assert result[0]["role"] == "assistant"
+        assert result[0].get("tool_calls") is not None
+        assert result[1]["role"] == "tool"
+
+    def test_blank_string_content_after_tool_calls_is_dropped(self):
+        """Empty string content should also be dropped"""
+        messages = [
+            {
+                "role": "assistant",
+                "tool_calls": [{"id": "call_1", "type": "function", "function": {"name": "f", "arguments": "{}"}}],
+            },
+            {"role": "assistant", "content": ""},
+            {"role": "tool", "content": "ok", "tool_call_id": "call_1"},
+        ]
+        result = LiteLLMCompletionResponsesConfig._drop_blank_assistant_messages_after_tool_calls(messages)
+        assert len(result) == 2
+
+    def test_nonempty_assistant_after_tool_calls_is_kept(self):
+        """An assistant message with actual content should NOT be dropped"""
+        messages = [
+            {
+                "role": "assistant",
+                "tool_calls": [{"id": "call_1", "type": "function", "function": {"name": "f", "arguments": "{}"}}],
+            },
+            {"role": "assistant", "content": "Here is my analysis"},
+            {"role": "tool", "content": "ok", "tool_call_id": "call_1"},
+        ]
+        result = LiteLLMCompletionResponsesConfig._drop_blank_assistant_messages_after_tool_calls(messages)
+        assert len(result) == 3
+
+    def test_blank_assistant_not_after_tool_calls_is_kept(self):
+        """A blank assistant message NOT preceded by tool_calls should be kept"""
+        messages = [
+            {"role": "user", "content": "hello"},
+            {"role": "assistant", "content": ""},
+        ]
+        result = LiteLLMCompletionResponsesConfig._drop_blank_assistant_messages_after_tool_calls(messages)
+        assert len(result) == 2
+
+    def test_no_messages_returns_empty(self):
+        result = LiteLLMCompletionResponsesConfig._drop_blank_assistant_messages_after_tool_calls([])
+        assert result == []
+
+    def test_is_blank_content(self):
+        assert LiteLLMCompletionResponsesConfig._is_blank_content(None) is True
+        assert LiteLLMCompletionResponsesConfig._is_blank_content("") is True
+        assert LiteLLMCompletionResponsesConfig._is_blank_content("  ") is True
+        assert LiteLLMCompletionResponsesConfig._is_blank_content([{"type": "text", "text": ""}]) is True
+        assert LiteLLMCompletionResponsesConfig._is_blank_content("hello") is False
+        assert LiteLLMCompletionResponsesConfig._is_blank_content([{"type": "text", "text": "hi"}]) is False