### What happened?

When a tool-less request is sent through the **Responses API → Chat Completions bridge** (`LiteLLMCompletionTransformationHandler` / `litellm.responses.litellm_completion_transformation`), LiteLLM forwards `tools: []` to the upstream chat/completions endpoint. Strict OpenAI-compatible servers (e.g. **vLLM**) reject an empty tools array:

``` 422 Unprocessable Entity {"detail":[{"type":"value_error","loc":["body"], "msg":"Value error, `tools` must not be an empty array. Either provide at least one tool or omit the field entirely.", "input":{"model":"...","messages":[...],"stream":true,"tools":[]}}]} ```

This happens even when the caller sends **no** `tools` field at all — the bridge manufactures the empty list itself.

### Root cause

In `litellm/responses/litellm_completion_transformation/transformation.py`, `transform_responses_api_request_to_chat_completion_request`:

1. Tools are coerced to a list regardless of input: ```python tools, web_search_options = ( LiteLLMCompletionResponsesConfig.transform_responses_api_tools_to_chat_completion_tools( responses_api_request.get("tools") or [] # None / absent / [] all become [] ) …