A Claude Code environment variable, read out of the shipped bundle.
It was in the build from v2.1.215 and was gone by v2.1.271.
Boolean (1, true, yes or on). When true, the system prompt guidance on actions that are hard to reverse drops the sentence telling Claude to surface a contradiction, or a target it did not create, instead of proceeding before deleting or overwriting. The same result also follows when the tengu_gault_kestrel flag is on, when another override source sets tengu_gault_kestrel to true, or when the model has the opus_5_prompt_bundle capability and tengu_fennel_godwit is off.
Not Anthropic's. Written by Claude Opus 5.5 from reading the v2.1.270 bundle, and it can be wrong.
First seen v2.1.215 19 Jul 2026
Last seen v2.1.270 12 Sep 2026, gone by v2.1.271
Builds47 / 149on this box, as of v2.1.308
Written about v2.1.215 first named in a changelog, 19 Jul 2026
v2.1.215in the build not in the build never minedv2.1.308
In the changelogs
1
Releases whose published page names CLAUDE_CODE_GAULT_KESTREL.
No release's published evidence quotes this name. The ledger indexes the Evidence lines of every changelog on this site, so this says nobody here has ever quoted it; it says nothing about how old the name is.
v2.1.271 – v2.1.30835 builds14 Sep 2026 → 22 Oct 2026 ○not in the build
Build by build
82
One 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.
