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DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

← All of v2.1.247 Home All releases ←olderv2.1.246 v2.1.248newer→
Claude Code v2.1.247

A "Writing for the user" prompt block, off unless explicitly turned on

Use it now
Useful4 Signal5
System Prompt Notable not in their notes

A writing-style prompt block can be forced on with an environment variable; otherwise it stays off.

Feature flag
tengu_willow_tern Off in both readings

The flag server returned off for the account this site reads and for the anonymous baseline. A reading of off cannot rule out a rollout these two readings sit outside of.

This account: off · anonymous baseline: off · compiled default in v2.1.247: not a boolean we can read

Read once, for one account on one subscription tier, against v2.1.247. It isn't a statement about your account. What a flag value here can and cannot tell you

CLAUDE_CODE_WILLOW_TERN
What

A new system-prompt section named willow_tern tells the model how its final message should read: lead with the answer, one idea per sentence of roughly 20 words, no em-dashes or parentheticals, keep code and numbers out of prose, and no headers under about 500 words. Set CLAUDE_CODE_WILLOW_TERN to any truthy value to force it on. Otherwise it needs a server-side decision, and the flag it falls back to is compiled off.

Details
  • Three inputs are checked in order: the environment variable, a boolean tengu_willow_tern key in client data (non-boolean values are ignored with a debug line), then the flag itself with fallback false.
  • Even when the flag says yes, the section only applies to models carrying the opus_5_prompt_bundle tag.
  • When applied, a tengu_willow_tern_applied event records whether client data supplied the decision.
Evidence

- No em-dashes, no parentheticals, no arrows., tengu_willow_tern_applied

Strings lifted out of the shipped bundle, so the claim above can be checked against them.

Related

Other releases about the same thing. Found by shared names or similar wording; neither means one caused the other.

See this entry in the whole of v2.1.247 →