CLAUDE_CODE_VELVET_FALCON
A Claude Code environment variable, read out of the shipped bundle. It was in the build from v2.1.163 and was gone by v2.1.174.
Tri-state override for which models count as the falcon class, which claude-fable-5 and claude-mythos-5 always belong to. A true value puts every model in the class and a false value (such as 0 or false) leaves only those two; when unset, a model is also in the class if its name contains the non-empty velvet_falcon_model from cached client data or tengu_velvet_falcon_model. Falcon-class models report the family falcon and get extra system prompt guidance such as the # Communicating with the user section.
Not Anthropic's. Written by Claude Opus 5.5 from reading the v2.1.173 bundle, and it can be wrong.
In the changelogs
1Releases whose published page names CLAUDE_CODE_VELVET_FALCON.
-
v2.1.163
Velvet Falcon Model Detection [In Development]
found in this entry's text
First cited
1The earliest release whose published evidence quoted CLAUDE_CODE_VELVET_FALCON was v2.1.163, 4 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
126One 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 environment variable does. All environment variables.