Anthropic releases Claude Opus 5.5, first with an ECI score of 167

Anthropic released the closed-weight model Claude Opus 5.5 on September 22, 2026, with an ECI score of 167 that ranks first among 253 tracked models. It scores 77% on the software engineering benchmark MirrorCode and 83% on the multimodal Furniture Assembly Benchmark. Pricing is $4 per 1M input tokens and $20 per 1M output tokens.

Why it matters: Claude Opus 5.5 beats GPT-6 Astra on software engineering but trails it on the hardest math tests.