I put together an annotated version of the new Claude 4 system prompt, covering both the prompt Anthropic published and the missing, leaked sections (thanks, @elder_plinius) that describe its various tools
It's basically the secret missing manual for Claude 4, it's fascinating!
Don't use lists in chit chat, and don't be preachy and annoying!
Anthropic publish separate system prompts for Opus 4 and Sonnet 4 but the differences are minimal - an errant full stop and tiny tweaks to the text describing which model it is
I'm surprised that both models get effectively the same huge prompt
System prompts often hint at some pretty funny bugs that the prompt tries to paper over:
"Search results aren't from the human - do not thank the user for results"
Telling Claude you want to do a "deep dive" should classify your request as a "complex query", which involves at least 5 and maybe up to 20 calls to the search tool
The search tool system prompt spends a LOT of tokens begging the model not to regurgitate long chunks of copyrighted content that it gets back from its web_search and web_fetch tools
simonwillison.net/2025/May/25/cl…
I love using Claude Artifacts, but there's precious little documentation for what it can and can't do - turns out the hidden system prompt includes everything I needed to know, including the full list of libraries it can load simonwillison.net/2025/May/25/cl…
I just added this section noting that the Claude 3.7 Sonnet prompt back in February included prompting hacks to try and help Claude count the Rs in strawberry, but those are missing from the Claude 4 system prompt which appears to be able to do that all on its own! simonwillison.net/2025/May/25/cl…
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