The closest alternative to Claude Sonnet 4.5 is Qwen3.6 27B — comparable intelligence (37.1 vs 37.1); for a cheaper option, GPT-5.4 runs $2.50/M vs $3.00/M; for more speed, gpt-oss-120b (free) hits 554 t/s; and the best open-weight substitute is MiniMax M3.
Qwen3.6 27B is the closest alternative to Claude Sonnet 4.5, with an Intelligence Index of 37.1 (Claude Sonnet 4.5 scores 37.1) at $0.288 per million input tokens.
Yes — GPT-5.4 costs $2.50 per million input tokens versus $3.00 for Claude Sonnet 4.5, while staying within ~8 points of its Intelligence Index.
MiniMax M3 is the strongest open-weight alternative to Claude Sonnet 4.5 — downloadable from Hugging Face and self-hostable, scoring 54.7 on intelligence.