## What happened

When a request comes in on `/v1/messages` (Anthropic shape) whose last message is `role:"assistant"` — i.e. the [Anthropic-spec prefill semantic](https://docs.anthropic.com/en/api/messages) — and is routed to a `hosted_vllm/*` model, **vLLM never receives `continue_final_message: true`**. The model treats the prefill as a completed prior turn and starts a fresh response instead of continuing.

This silently breaks the Anthropic prefill contract for the entire self-hosted ecosystem behind LiteLLM. Claude Code (and any other Anthropic-SDK client) cannot use prefill against a vLLM backend through LiteLLM, even though it works fine when calling vLLM directly via `/v1/chat/completions` with `continue_final_message: true`.

Verified on **LiteLLM v1.82.6**, vLLM serving `qwen3.6-35b-a3b`.

## Repro

### A — broken (Anthropic shape, current behavior) ```bash curl -sS -X POST "https://<proxy>/v1/messages" \ -H "x-api-key: $KEY" -H "anthropic-version: 2023-06-01" -H "content-type: application/json" \ -d '{"model":"qwen3-coder","max_tokens":50,"system":"You are precise.", "messages":[{"role":"user","content":"Count to three, just the numbers comma-separated."}, …