CLAUDE_CODE_APPEND_PROMPT_HEAD
A Claude Code environment variable, read out of the shipped bundle. It has been in the build since v2.1.284, including the newest one read.
In cloud sessions, CLAUDE_CODE_APPEND_PROMPT_HEAD can move a verified opening of --append-system-prompt text into the cacheable system prompt
Not Anthropic's. This is the line from the earliest changelog entry here that named it, v2.1.284.
In the changelogs
1Releases whose published page names CLAUDE_CODE_APPEND_PROMPT_HEAD.
-
v2.1.284
Cloud sessions can keep a verified start of the appended prompt in the cached part
In cloud sessions,
CLAUDE_CODE_APPEND_PROMPT_HEADcan move a verified opening of--append-system-prompttext into the cacheable system promptfound in this entry's text, named in this entry
First cited
1The earliest release whose published evidence quoted CLAUDE_CODE_APPEND_PROMPT_HEAD was v2.1.284, 28 Sep 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
1Build by build
1One 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.