A new academic benchmark gives the industry its first real measure of "agentic abstention": whether an AI agent recognizes a task is infeasible and stops rather than keeps burning tool calls. Every frontier system tested fails most of the time, and neither Claude Fable 5 nor GPT-5.6, which OpenAI is taking to general availability this week, has been scored on it yet.

Claude Sonnet 5 lands as the most capable model you can actually deploy at scale today, pitched as Opus-class autonomy at a Sonnet price. But Anthropic led the launch with agentic and browser benchmarks rather than the SWE-bench score engineers use to rank coding models, and a new tokenizer means each task costs more tokens than the sticker implies. A review of what is new, what it costs, and whether you still need Opus.