### Check for existing issues

- [x] I have searched the existing issues and checked that my issue is not a duplicate.

### What happened?

LiteLLM's [document understanding documentation](https://docs.litellm.ai/docs/completion/document_understanding) explicitly shows that passing an HTTPS URL in `file_id` is the supported way to send a file by URL through the Chat Completions API:

```python file_url = "https://www.w3.org/WAI/ER/tests/xhtml/testfiles/resources/pdf/dummy.pdf" file_content = [ {"type": "text", "text": "What's this file about?"}, {"type": "file", "file": {"file_id": file_url, "format": "application/pdf"}} ] response = completion(model="...", messages=[{"role": "user", "content": file_content}]) ```

This pattern works correctly for Gemini, Bedrock, and Anthropic bridges. However, when the model routes through the **chat completions → Responses API bridge** (i.e. a model with `mode: "responses"`), the bridge passes the URL through as `input_file.file_id` unchanged, which the Responses API rejects:

```json { "error": { "message": "Invalid 'input[0].content[1].file_id': 'https://arxiv.org/pdf/1706.03762'. Expected an ID that contains letters, numbers, un…