Two frontier labs cut prices on the same Tuesday. Only one of them is a drop in swap.
OpinionOn 22 September OpenAI released GPT-6 Sol at $2 input and $10 output per million tokens, and GPT-6 Luna at $0.10 and $0.50, for prompts up to 272K input tokens. GPT-5.6 Sol and GPT-5.6 Luna remain listed at $4 and $20, and $0.20 and $1.20. The same day Anthropic released Claude Opus 5.5 at $4 and $20, twenty percent below Opus 5 at $5 and $25, with cache hits at $0.20 because Opus 5.5 prices them at 0.05x base input rather than the usual 0.1x. Anthropic's release notes also list three request shapes that now return a 400 error on Opus 5.5.
The OpenAI cut costs you an eval run. The Anthropic one has a refactor attached: the thinking field is rejected in both its enabled and disabled forms, tool_choice types any and tool return 400, and computer use needs the computer_toolset_20260801 toolset on the Claude API and Google Cloud. Search your call sites for all three before you point a single request at Opus 5.5. And read what OpenAI does not say: GPT-5.6 Sol is explicitly promotional at least through 21 November 2026, while nothing on either page labels GPT-6 pricing as promotional at all. Budget on the number, not on the direction of travel.