#35053 [Bug]: /v1/messages skips supports_sampling_params gating — temperature/top_p/top_k 400 on claude-sonnet-5/opus-4-7/4-8/opus-5 while /chat/completions 200s; fallbacks silently downgrade the model
### What happened?
`POST /v1/messages` (Anthropic-native route) forwards `temperature`, `top_p` and `top_k` **raw** to Claude models whose model-map entry says `supports_sampling_params: false` (claude-sonnet-5, claude-opus-4-7, claude-opus-4-8, claude-opus-5). The provider rejects them with a 400 (``"`temperature` is deprecated for this model"``, same for `top_p` / `top_k`). The **identical request via `/v1/chat/completions` returns 200**, because that path drops the params.
The asymmetry is in the code, not the map — litellm already knows these params are unsupported:
- **Chat path:** sampling params go through `AnthropicModelInfo._apply_sampling_param` ([`litellm/llms/anthropic/common_utils.py#L268`](https://github.com/BerriAI/litellm/blob/v1.94.0/litellm/llms/anthropic/common_utils.py#L268)), which consults `_supports_sampling_params` (the `supports_sampling_params` map flag) and drops the param under `drop_params` → 200. - **`/v1/messages` path:** the request is built by `AnthropicMessagesRequestUtils.get_requested_anthropic_messages_optional_param` ([`litellm/llms/anthropic/experimental_pass_through/messages/utils.py#L24`](https://github.com/BerriAI/litellm/blob/v1.94.0/li…