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.
Names in the bundleCLAUDE_CODE_SPARE_KEEPS_BOOT_SESSION_ID
One source agreesOne thing we can check says the same as this entry.
The name it cites is new in this build
New in this build: CLAUDE_CODE_SPARE_KEEPS_BOOT_SESSION_ID