ai-agents-multimodal-fallback--multimodal-request-with-image-url-fails-on-primary-8f579596

THEN Deep copy the messages payload before passing to each provider in the fallback chain. In the router's fallback logic (e.g., `_acompletion` / `run_async_fallback`), use `copy.deepcopy(messages)` to create an independent copy for each provider, preventing mutations from one provider's transformation (e.g., converting image_url to base64 for OpenAI) from affecting subsequent fallback requests.

import copy

async def _acompletion_with_fallback(...):
    for provider in fallback_providers:
        messages_copy = copy.deepcopy(messages)
        result = await provider.acompletion(model, messages_copy, ...)