tengu_sunset_penguin_opus46
A Claude Code feature flag, read out of the shipped bundle. It was in the build from v2.1.174 and was gone by v2.1.193.
Default "2026-06-29". Date (parsed with Date.parse) on which Opus 4.6 fast mode is removed: when fast mode is turned on while the current model is claude-opus-4-6 and the date is still in the future, Claude Code shows the warning Opus 4.6 fast mode is deprecated and will be removed on <date>, with the date formatted like Jun 29 in UTC. An unparseable or past date shows no warning.
Not Anthropic's. Written by Claude Opus 5.5 from reading the v2.1.191 bundle, and it can be wrong.
In the changelogs
1Releases whose published page names tengu_sunset_penguin_opus46.
-
v2.1.174
Opus 4.6 Sunset Date Notification
gate cited by this entry, found in this entry's text
Gate readings
1| Release | Resolved | This account | Anonymous | Compiled default |
|---|---|---|---|---|
| v2.1.174 read on another version | Not enough to say | no value returned | no value returned | 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.
First cited
1The earliest release whose published evidence quoted tengu_sunset_penguin_opus46 was v2.1.174, 11 Jun 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
2Build by build
117One 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.