Unclear What either environment variable changes when it is set is not known.
What
Two new environment variables have been added to the list of runtime tuning variables Claude Code accepts. An environment variable is a named setting passed to a program when it starts. Neither name appeared in the build before this release.
CLAUDE_CODE_SPARE_KEEPS_BOOT_SESSION_IDaffects spare processes. A spare process is a copy of Claude Code started ahead of time so it is ready when needed. When a spare process is claimed without asessionIdand this variable is not set, the claim now looks up a session id and applies it, recording the reasonspare_claim. A session is one continuous conversation, and its id is the label that identifies it.CLAUDE_CODE_DISABLE_STOP_OF_HELD_MESSAGESrelates to held messages. The members do not say more about what it does.
Why
In practice, a prewarmed spare process now takes on a looked-up session id when it is claimed, unless CLAUDE_CODE_SPARE_KEEPS_BOOT_SESSION_ID is set. Setting that variable keeps the id the process started with. Both variables look like internal switches, not everyday settings.
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What either environment variable changes when it is set is not known.