diff --git a/lib/crewai/src/crewai/llms/providers/openai/completion.py b/lib/crewai/src/crewai/llms/providers/openai/completion.py
index 042c13e0a..155d29d7d 100644
--- a/lib/crewai/src/crewai/llms/providers/openai/completion.py
+++ b/lib/crewai/src/crewai/llms/providers/openai/completion.py
@@ -771,15 +771,17 @@ class OpenAICompletion(BaseLLM):
                 items.append({"role": "assistant", "content": content})
             for call in message["tool_calls"]:
                 function = call.get("function", {})
-                args = function.get("arguments", "")
+                args = function.get("arguments")
+                if args is None or args == "":
+                    args = "{}"
+                elif not isinstance(args, str):
+                    args = json.dumps(args)
                 items.append(
                     {
                         "type": "function_call",
                         "call_id": call.get("id") or f"call_{id(call)}",
                         "name": function.get("name", ""),
-                        "arguments": args
-                        if isinstance(args, str)
-                        else json.dumps(args),
+                        "arguments": args,
                     }
                 )
             return items
@@ -810,7 +812,7 @@ class OpenAICompletion(BaseLLM):
         - Internally-tagged tool format (flat structure)
         """
         instructions: str | None = self.instructions
-        input_messages: list[Any] = []
+        input_messages: list[dict[str, Any] | LLMMessage] = []
 
         for message in messages:
             if message.get("role") == "system":
@@ -824,7 +826,7 @@ class OpenAICompletion(BaseLLM):
                 input_messages.extend(self._to_responses_input(message))
 
         # Prepend reasoning items for ZDR (zero-data-retention) chaining when configured
-        final_input: list[Any] = []
+        final_input: list[dict[str, Any] | LLMMessage] = []
         if self.auto_chain_reasoning and self._last_reasoning_items:
             final_input.extend(self._last_reasoning_items)
         final_input.extend(input_messages if input_messages else messages)
diff --git a/lib/crewai/tests/llms/openai/test_openai.py b/lib/crewai/tests/llms/openai/test_openai.py
index c0a772883..925a941cb 100644
--- a/lib/crewai/tests/llms/openai/test_openai.py
+++ b/lib/crewai/tests/llms/openai/test_openai.py
@@ -1017,6 +1017,7 @@ def test_openai_responses_api_preserves_assistant_content_with_tool_calls():
             "tool_calls": [
                 {
                     "type": "function",
+                    "id": "call_fetch_page",
                     "function": {
                         "name": "fetch_page",
                         "arguments": {"url": "https://example.com"},
@@ -1033,10 +1034,39 @@ def test_openai_responses_api_preserves_assistant_content_with_tool_calls():
         "content": "I'll fetch that page now.",
     }
     assert params["input"][1]["type"] == "function_call"
-    assert params["input"][1]["call_id"].startswith("call_")
+    assert params["input"][1]["call_id"] == "call_fetch_page"
     assert params["input"][1]["arguments"] == '{"url": "https://example.com"}'
 
 
+def test_openai_responses_api_defaults_missing_tool_call_arguments():
+    """Missing or empty tool-call arguments must become a valid JSON object."""
+    llm = OpenAICompletion(model="gpt-4o-mini", api="responses")
+
+    messages = [
+        {
+            "role": "assistant",
+            "content": None,
+            "tool_calls": [
+                {
+                    "id": "call_no_args",
+                    "type": "function",
+                    "function": {"name": "ping"},
+                },
+                {
+                    "id": "call_empty_args",
+                    "type": "function",
+                    "function": {"name": "ping", "arguments": ""},
+                },
+            ],
+        }
+    ]
+
+    params = llm._prepare_responses_params(messages)
+
+    assert params["input"][0]["arguments"] == "{}"
+    assert params["input"][1]["arguments"] == "{}"
+
+
 def test_openai_responses_api_converts_tool_result_message():
     """Regression: tool-role messages (Chat-Completions shape) must become
     function_call_output input items for the Responses API.