Sonnet 5 self-checks its output without being asked.

Elvis Saravia's roundup characterizes it as making plans, driving browsers and terminals, and finishing multi-step tasks where prior Sonnets stopped short, notably verifying its own output unprompted. Anthropic puts it near Opus 4.8 on reasoning, tool use, coding, and knowledge work at a lower price, with introductory pricing of $2/$10 per 1M through August 31. Default on Free and Pro. Test the self-checking behavior specifically. If it holds, it changes how much verification scaffolding your agent loop actually needs, which is the single biggest source of harness complexity.