litellm - 💡(How to fix) Fix feat: Custom x- headers not captured in spend logs on Vertex AI pass-through

We add custom headers to our app making LiteLLM calls. The custom headers are "x-" prefixed and we use them for spend and security attribution. It works great except for Gemini pass-through — none of the headers show up in request_tags on those spend log rows.

Our custom headers work fine when we hit /chat/completions or /v1/messages (Anthropic).

This means all of our Vertex/Gemini traffic ends up unattributable to a user and makes it difficult for various downstream systems, runbooks and processes.