diff --git a/src/agents/models/chatcmpl_converter.py b/src/agents/models/chatcmpl_converter.py
index 87b55a9e26..86a7d9e841 100644
--- a/src/agents/models/chatcmpl_converter.py
+++ b/src/agents/models/chatcmpl_converter.py
@@ -790,10 +790,17 @@ def ensure_assistant_message() -> ChatCompletionAssistantMessageParam:
             elif func_call := cls.maybe_function_tool_call(item):
                 asst = ensure_assistant_message()
 
+                call_id = func_call.get("call_id")
+                if call_id is None:
+                    raise UserError(
+                        "Unpaired function calls are supported by Responses but cannot be "
+                        "converted to Chat Completions tool calls. "
+                        "Use a Responses model to preserve this input."
+                    )
                 tool_calls = list(asst.get("tool_calls", []))
                 arguments = func_call["arguments"] if func_call["arguments"] else "{}"
                 new_tool_call = ChatCompletionMessageFunctionToolCallParam(
-                    id=func_call["call_id"],
+                    id=call_id,
                     type="function",
                     function={
                         "name": func_call["name"],
diff --git a/tests/models/test_openai_chatcompletions.py b/tests/models/test_openai_chatcompletions.py
index cd3c34d32d..43917f8dc4 100644
--- a/tests/models/test_openai_chatcompletions.py
+++ b/tests/models/test_openai_chatcompletions.py
@@ -795,6 +795,58 @@ async def handler(request: httpx2.Request) -> httpx2.Response:
     assert requests == []
 
 
+@pytest.mark.allow_call_model_methods
+@pytest.mark.asyncio
+@pytest.mark.parametrize("call_id_fields", [{}, {"call_id": None}], ids=["omitted", "null"])
+@pytest.mark.parametrize("stream", [False, True], ids=["non_streaming", "streaming"])
+@pytest.mark.parametrize("strict_feature_validation", [False, True], ids=["default", "strict"])
+async def test_unpaired_function_call_rejected_before_chat_request(
+    call_id_fields: dict[str, None], stream: bool, strict_feature_validation: bool
+) -> None:
+    """A function call without a call ID cannot become a Chat Completions tool call."""
+    requests: list[httpx2.Request] = []
+
+    async def handler(request: httpx2.Request) -> httpx2.Response:
+        requests.append(request)
+        raise AssertionError("Unpaired function calls must not reach Chat Completions")
+
+    async with httpx2.AsyncClient(transport=httpx2.MockTransport(handler)) as http_client:
+        model = OpenAIChatCompletionsModel(
+            model="gpt-4",
+            openai_client=AsyncOpenAI(api_key="test-key", http_client=http_client),
+            strict_feature_validation=strict_feature_validation,
+        )
+        request_kwargs: dict[str, Any] = {
+            "system_instructions": None,
+            "input": [
+                {
+                    "type": "function_call",
+                    "name": "notifications",
+                    "namespace": "slack",
+                    "arguments": "{}",
+                    **call_id_fields,
+                }
+            ],
+            "model_settings": ModelSettings(),
+            "tools": [],
+            "output_schema": None,
+            "handoffs": [],
+            "tracing": ModelTracing.DISABLED,
+        }
+
+        with pytest.raises(
+            UserError,
+            match="Unpaired function calls.*Chat Completions.*Use a Responses model",
+        ):
+            if stream:
+                async for _ in model.stream_response(**request_kwargs):
+                    pass
+            else:
+                await model.get_response(**request_kwargs)
+
+    assert requests == []
+
+
 @pytest.mark.allow_call_model_methods
 @pytest.mark.asyncio
 async def test_get_response_rejects_non_text_tool_output_in_strict_mode() -> None: