CLAUDE_CODE_PRINT_ENGINE_LOOP
A Claude Code environment variable, read out of the shipped bundle. It was in the build from v2.1.238 and was gone by v2.1.257.
Set one variable and print mode keeps the model, tools and caches warm between prompts instead of rebuilding.
Not Anthropic's. This is the line from the earliest changelog entry here that named it, v2.1.238.
In the changelogs
4Releases whose published page names CLAUDE_CODE_PRINT_ENGINE_LOOP.
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v2.1.257
Renamed/relaunched print-engine-loop flag as 'humble hammock'
The env var controlling print and engine loop behavior was renamed to CLAUDE_CODE_HUMBLE_HAMMOCK and still defaults to off.
found in this entry's text
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v2.1.246
Headless runs halt at the spend cap and stop background agents
Headless runs now stop at your spend cap and shut down background agents instead of spending on.
found in this entry's text
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v2.1.246
Headless startup logs which turn loop it is running
Headless runs log which turn loop they're using, making non-interactive problems easier to diagnose.
found in this entry's text
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v2.1.238
Print mode can reuse one engine across prompts
Set one variable and print mode keeps the model, tools and caches warm between prompts instead of rebuilding.
named in this entry, found in this entry's text
First cited
1The earliest release whose published evidence quoted CLAUDE_CODE_PRINT_ENGINE_LOOP was v2.1.238, 20 Aug 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
60One 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.