Claude Sonnet 5.5 is here. This prompt stops it from wasting your usage

The latest version of Claude Sonnet—Anthropic’s workhorse AI model—has just landed, and with it comes a prompt that could help keep any AI agent from leaping into action before making a plan first.

Available now on the web, the Claude desktop app, and other Claude platforms, Claude Sonnet 5.5 is Anthropic’s middle-of-the-road model, It’s a step below Opus 5.5 but apparently just as capable as Fable 5.1 (Anthropic’s pricey frontier flagship) and a step above Haiku 5 (a fast but basic Claude model that’s due for a 5.5 upgrade).

In Anthropic’s benchmarks, Sonnet 5.5 achieves scores rivaling Opus 5.5 in areas such as agentic coding and computer use, although the latest Opus model is still on top in terms of “complex, open-ended work requiring sustained judgement,” Anthropic notes. Still, Sonnet 5.5 appears surprisingly powerful and affordable compared to Claude Opus, and it also has the distinction of being the default Claude model for free users.

In any event, with a new Claude model comes a new prompting guide, and the guide for the latest Sonnet contains a gem that could come in handy for other AI models, particularly those capable of building things.