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Anthropic removed over 80% of Claude Code's system prompt for Claude Opus 5 and Fable 5 with no measurable loss on coding evals. The popular reading — 'write shorter prompts' — is wrong. What got deleted was constraints, not information. Here is a usable dividing line, the API-layer mechanics the official post left out (cache prefixes, defer_loading, runtime injection), and the two real disagreements from 343 comments on Hacker News.