We removed ~80% of the Claude Code system prompt — what we learned about writing system prompts, skills, and CLAUDE.mds
Source
x.com
Author
Thariq
Date
Context
Anthropic reports removing more than 80% of Claude Code's system promptThe standing instructions a model receives before any user input — defining its role, rules, tools, and tone for the whole conversation.Full definition → for models including Opus 5 and Fable 5 without measurable loss on its coding evaluations. Thariq's primary account describes removing conflicting or repeated guidance and relocating detailed verification and review instructions into skills loaded when needed.
A smaller permanent prompt can therefore coexist with detailed task guidance. The change concerns where instructions live and when they enter context windowThe maximum amount of text a model can consider at once — its working memory for the current conversation or task.Full definition →, as well as which rules remain useful for those models. The reported reduction applies to the system prompt, not all runtime context or overall operating cost.
Terms in this piece · Glossary
system prompt — The standing instructions a model receives before any user input — defining its role, rules, tools, and tone for the whole conversation.
context window — The maximum amount of text a model can consider at once — its working memory for the current conversation or task.
