Anthropic's Claude Sonnet 5.5 Outperforms Opus 5.5 on Key Coding Benchmark at Half the Price

Anthropic released Claude Sonnet 5.5 on September 28, priced at $2/$10 per million tokens — exactly half the cost of Opus 5.5. On Terminal-Bench 4.0, a benchmark measuring real-world agentic coding performance, Sonnet 5.5 scored 70.6% against Opus 5.5's 66.4%, marking a rare instance where a mid-tier model outperformed its flagship sibling. Across most other benchmarks, however, Opus 5.5 holds narrow leads, and the two models are nearly indistinguishable on knowledge-work evaluations. A critical caveat is that the benchmark comparisons used different effort levels — Sonnet 5.5 was tested at Max effort while Opus 5.5 ran at a lower Xhigh setting, meaning per-task costs can actually favour Opus for complex workloads. The release also coincided with OpenAI's GPT-6 Sol launching at the same price point, signalling that mid-tier model pricing has effectively become a commodity.