5/10 reviewers in the last round flagged Kimi forwarding non-default frequency_penalty as a 400 risk for K2.5 / K2.6, mirroring the existing lock on temperature and top_p. Hide the slider on the frontend and add frequency_penalty to Kimi's body_omit so even stale clients have the field stripped before the request hits the wire.
service_tier on the generic OpenAI-compatible branch was forwarding whatever value the dispatcher received, so a stale frontend could send standard_only (Anthropic) or scale to providers like Mistral that do not document the field, producing 400s. Gate the forward on an explicit accepts_service_tier=True provider registry opt-in; Anthropic and OpenAI Responses already handle service_tier inside their own helpers.
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