In terms of benchmarks for agentic coding, it basically stacks up nearly 1:1 with Opus 5.5.

Terminal-Bench: 70.6 (Sonnet 5.5) vs. 66.4% (Opus 5.5)

FrontierCode: 52.1% (Sonnet 5.5 xHigh) vs. 54.4 (Opus 5.5)

CursorBench: 55.5% (Sonnet 5.5) vs. 57.8 (Opus 5.5)

Opus 5.5 might be the best model I've ever used and Sonnet 5.5 matches it and exceeds in some benchmarks. Clearly Anthropic have had some sort of breakthrough with not just performance but also cost with the 5.5 family