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GPT-6.1 Sol replaced GPT-6 Sol after seven days: +4 index points, +12 on Terminal-Bench 4.0, and a cache rate halved to $0.10. It also dropped the "none" reasoning setting, uses 10-30% more output tokens, and is measurably slower.
GPT-6.1 Sol matches Claude Opus 5.5's default index score for 46% less per task and ties it at low effort for 76% less — then stops at index 52, below every Opus effort above default.
Across all six effort levels, GPT-6 Sol scores within a point of GPT-5.6 Sol and costs about half as much per task. Same-harness data on both, the full effort ladder, where coding improved, where knowledge work regressed, and the November price cliff.