CLAUDE_CODE_SPARE_KEEPS_BOOT_SESSION_ID
A Claude Code environment variable, read out of the shipped bundle. It has been in the build since v2.1.318, including the newest one read.
Claude Code now accepts CLAUDE_CODE_SPARE_KEEPS_BOOT_SESSION_ID and CLAUDE_CODE_DISABLE_STOP_OF_HELD_MESSAGES, but their effect is not described
Not Anthropic's. This is the line from the earliest changelog entry here that named it, v2.1.318.
In the changelogs
2Releases whose published page names CLAUDE_CODE_SPARE_KEEPS_BOOT_SESSION_ID.
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v2.1.318
Two new environment variables for held messages and spare sessions
Claude Code now accepts
CLAUDE_CODE_SPARE_KEEPS_BOOT_SESSION_IDandCLAUDE_CODE_DISABLE_STOP_OF_HELD_MESSAGES, but their effect is not describedfound in this entry's text
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v2.1.318
Prewarmed spare processes now pick up a session ID when claimed
A prewarmed spare process with no session ID now looks one up when claimed, unless CLAUDE_CODE_SPARE_KEEPS_BOOT_SESSION_ID is set
found in this entry's text, named in this entry
First cited
1The earliest release whose published evidence quoted CLAUDE_CODE_SPARE_KEEPS_BOOT_SESSION_ID was v2.1.318, 3 Nov 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.