Anthropic announced Claude Opus 5.5 on September 23, 2026, claiming Fable 5.1-level performance at 40% lower cost. Reporting has since established pricing at $4 per million input and $20 per million output tokens, output roughly 30% faster than Opus 5, a 66.4% score on Terminal Bench 4.0 against Fable 5.1's 55.8%, and a verified 285 tokens per second at max with 50% more tokens per dollar than OpenAI's fast mode. Every calls it the most readable prose yet from an Anthropic or OpenAI model. Anthropic now requires thinking blocks preserved across turns and shipped an API compaction feature, and Opus 5.5 does not carry Fable's data retention requirements. What remains contested is how much the benchmark lead translates into practice.