A Claude Code environment variable, read out of the shipped bundle.
It has been in the build since v2.1.138, including the newest one read.
Gates the extra subagent system prompt: when this is truthy and a prompt was supplied by the hidden --append-subagent-system-prompt option, its file variant, or the SDK's appendSubagentSystemPrompt initialize field, the text is appended to a Task-tool subagent's system prompt (skipped for agents with an isolated context and in one other case). Claude Code sets it to 1 itself when either command-line option supplies a prompt.
Not Anthropic's. Written by Claude Opus 5.5 from reading the v2.1.286 bundle, and it can be wrong.
First seen v2.1.138 9 May 2026
Last seen v2.1.308 22 Oct 2026
Builds149 / 149on this box, as of v2.1.308
Written about v2.1.111 first named in a changelog, 16 Apr 2026
v2.1.138in the build not in the build never minedv2.1.308
In the changelogs
2
Releases whose published page names CLAUDE_CODE_ENABLE_APPEND_SUBAGENT_PROMPT.
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.
Presence across releases
1
BuildsHow manyDatesReading
v2.1.138 – v2.1.308149 builds9 May 2026 → 22 Oct 2026 ◆in the build
Build by build
149
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.
v2.1.29914 Oct 2026·presentverdict: present · bool
v2.1.30014 Oct 2026·presentverdict: present · bool
v2.1.30115 Oct 2026·presentverdict: present · bool
v2.1.30216 Oct 2026·presentverdict: present · bool
v2.1.30318 Oct 2026·presentverdict: present · bool
v2.1.30419 Oct 2026·presentverdict: present · bool
v2.1.30520 Oct 2026·presentverdict: present · bool
v2.1.30621 Oct 2026·presentverdict: present · bool
v2.1.30721 Oct 2026·presentverdict: present · bool
v2.1.30822 Oct 2026·presentverdict: present · bool
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.
