## Check for existing issues

- [x] I searched existing issues and did not find a duplicate.

#24004 is related, but covers mid-stream SSE errors rather than Fable's HTTP-200 refusal response.

## What happened?

Anthropic supports server-side fallback when Fable 5 returns `stop_reason: "refusal"`:

```http anthropic-beta: server-side-fallback-2026-06-01 ```

```json { "fallbacks": [{ "model": "claude-opus-4-8" }] } ```

Through LiteLLM's `/v1/messages` endpoint, `fallbacks` appears to be interpreted as LiteLLM router configuration instead of being forwarded to Anthropic.

With `claude-opus-4-8`, LiteLLM returns a model-authorization error. With the configured LiteLLM alias `anthropic/claude-opus-4-8`, the request succeeds but the original Fable refusal is returned without running the fallback.

I expected LiteLLM to pass Anthropic's native fallback parameter through when the server-side fallback beta header is present.

Anthropic documentation: https://platform.claude.com/docs/en/build-with-claude/refusals-and-fallback

## Steps to Reproduce

1. Configure both models:

```yaml model_list: - model_name: anthropic/claude-fable-5 litellm_params: model: anthropic/claude-…