The closest alternative to Claude Opus 4.5 is GPT-5.1-Codex — comparable intelligence (43.1 vs 43.1); for a cheaper option, GPT-5.4 runs $2.50/M vs $5.00/M; for more speed, MiniMax M2.7 hits 273 t/s; and the best open-weight substitute is MiniMax M3.
GPT-5.1-Codex is the closest alternative to Claude Opus 4.5, with an Intelligence Index of 43.1 (Claude Opus 4.5 scores 43.1) at $1.25 per million input tokens.
Yes — GPT-5.4 costs $2.50 per million input tokens versus $5.00 for Claude Opus 4.5, while staying within ~8 points of its Intelligence Index.
MiniMax M3 is the strongest open-weight alternative to Claude Opus 4.5 — downloadable from Hugging Face and self-hostable, scoring 54.7 on intelligence.