tengu_willow_tern
A Claude Code feature flag, read out of the shipped bundle. It was in the build from v2.1.248 and was gone by v2.1.295.
A writing-style prompt block can be forced on with an environment variable; otherwise it stays off.
Not Anthropic's. This is the line from the earliest changelog entry here that named it, v2.1.247.
In the changelogs
2Releases whose published page names tengu_willow_tern.
-
v2.1.247
A "Writing for the user" prompt block, off unless explicitly turned on
A writing-style prompt block can be forced on with an environment variable; otherwise it stays off.
gate cited by this entry, found in this entry's text
-
v2.1.247
A model-capability toggle, tengu_willow_tern
A new toggle applies only to models advertising an opus 5 prompt bundle capability.
named in this entry, gate cited by this entry, found in this entry's text
Gate readings
1| Release | Resolved | This account | Anonymous | Compiled default |
|---|---|---|---|---|
| v2.1.247 | Off in both readings | off | off | not a boolean we can read |
Two arms, because they disagree: this account is the one account this site reads and anonymous is a baseline with no identity attached. A reading says nothing about your account, and off cannot rule out a rollout both readings sit outside of. What these readings can and cannot show.
Flag server, over time
10| Read as | Reading | From | Until | Version installed |
|---|---|---|---|---|
| anonymous | off | 2026-10-17T05:30:04Z | 2026-10-22T06:25:04Z | v2.1.302 to v2.1.308 |
| this account | off | 2026-09-10T21:47:12Z | 2026-10-22T06:25:04Z | v2.1.268 to v2.1.308 |
| anonymous | no longer carried | 2026-10-17T05:25:57Z | 2026-10-17T05:25:57Z | v2.1.302 |
| anonymous | off | 2026-08-26T02:30:04Z | 2026-10-17T05:10:04Z | v2.1.233 to v2.1.302 |
| this account | no longer carried | 2026-09-10T21:46:45Z | 2026-09-10T21:46:45Z | v2.1.268 |
| this account | off | 2026-09-08T20:36:15Z | 2026-09-10T17:40:05Z | v2.1.263 to v2.1.267 |
| this account | no longer carried | 2026-09-08T20:34:58Z | 2026-09-08T20:34:58Z | v2.1.265 |
| this account | off | 2026-09-06T02:40:05Z | 2026-09-08T18:45:05Z | v2.1.261 to v2.1.263 |
| this account | no longer carried | 2026-09-06T02:36:19Z | 2026-09-06T02:36:19Z | v2.1.263 |
| this account | off | 2026-08-26T03:25:04Z | 2026-09-06T01:55:04Z | v2.1.233 to v2.1.261 |
One row per unbroken stretch of one reading, newest first, from snapshots taken every twenty-five minutes or so. No longer carried is not off: it means the flag server stopped returning the gate at all, which is what a retired experiment looks like. The version column is what was installed when the reading was taken, not what caused it.
First cited
1The earliest release whose published evidence quoted tengu_willow_tern was v2.1.247, 26 Aug 2026. That is the oldest release this project wrote about it, so it is a floor on the name's age and not the release that introduced it. It is not a presence reading: which builds carried the name is the table below.
Presence across releases
6Build by build
51One row per release since the first build this name was read out of. Absent means the build was read and the name was not in it, never mined means no bundle for that release was ever archived, and a declared type or a default is only ever what that release's own bundle stated.
Read out of the published npm bundle release by release, and out of Anthropic's own documentation as this site captured it. Nothing on this page is a description anybody here wrote about what the feature flag does. All feature flags.