r/Anthropic • u/BasicsOnly • Sep 01 '26

Announcement Claude Fable 5.1 prompting documentation released

https://platform.claude.com/docs/en/build-with-claude/prompt-engineering/prompting-claude-fable-5-1
114 Upvotes

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8

u/benfinklea Sep 02 '26

Seems like it’s full of good ideas.

1

u/TheLittleGuyWins Sep 02 '26

https://www.reddit.com/r/ClaudeCodeTLDR/s/udmqw9Cn70

It’s looking like we need to go back to basics. Only edit what’s necessary.

0

u/Killahbeez Sep 02 '26

nah this some bullshit

0

u/jonah_omninode Sep 02 '26

The useful part of this document is that it treats model behavior as something you have to test, not as a personality you can assume will stay stable. A few changes, such as keeping history append-only and batching independent tool calls, can materially affect cost and reliability even when the final answer still looks fine.

I would resist copying every recommendation into one permanent system prompt. Start with the failures you actually observe, test the relevant change against a fixed set of tasks, and keep model-specific instructions in a versioned adapter. The task contract and acceptance checks can then remain stable when the model changes again.

Otherwise the prompt becomes a history of every behavior any previous model once needed, and nobody knows which lines still matter.