Vibe Coding
Pattern: the vendor is shipping harness engineering as prompt text you are expected to paste
Six of the Fable 5.1 guide's sections resolve to a block of prose you copy into a system prompt: autonomy, scope discipline, compaction summarization, search triggering, whole-file rewrites, and prose density. Anthropic knows the behaviors are wrong for agent loops and is fixing them in documentation rather than in the model or the harness. The practical read for anyone maintaining an agent: your system prompt is now versioned against the model, and a model bump means re-reading the guide, not just changing a string.
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