What it is
Set to 1 to write terminal output through a second non-blocking file descriptor, so a terminal that stops reading, such as a paused tmux control-mode pane or a stalled SSH connection, can't freeze Claude Code mid-session. Applies on macOS, Linux, and WSL when stdout is a terminal. Requires Claude Code v2.1.261 or later
Anthropic's own wording. Read off Environment variables, captured 2026-09-22.
Presence across releases
1Build by build
16One 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.
First cited
1The earliest release whose published evidence quoted CLAUDE_CODE_NONBLOCKING_STDOUT was v2.1.261, 4 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 above.
Names beside it
Every environment variable in the inventory starting claude_code, up to twelve of them.
CLAUDE_CODE_3P_PROBE_WROTE_OPUS_DEFAULTNo description from anybody. v2.1.207
CLAUDE_CODE_3P_PROBE_WROTE_SONNET_DEFAULTNo description from anybody. v2.1.207
CLAUDE_CODE_ACCESSIBILITYSet to 1 to keep the native terminal cursor visible and disable the inverted-text cursor indicator. Allows screen magnifiers like macOS Zoom to track cursor position v2.1.138
CLAUDE_CODE_ACCOUNT_TAGGED_IDNo description from anybody. v2.1.138
CLAUDE_CODE_ACCOUNT_UUIDThe device bridge is skipped entirely when the host's account UUID doesn't match the stored one. v2.1.138
CLAUDE_CODE_ACT_DONT_REDERIVEThree existing feature gates moved position; their defaults are unchanged. v2.1.162
CLAUDE_CODE_ACTIONNo description from anybody. v2.1.138
CLAUDE_CODE_ADDITIONAL_DIRECTORIES_CLAUDE_MDSet to 1 to load memory files from directories specified with --add-dir. Loads CLAUDE.md, .claude/CLAUDE.md, .claude/rules/*.md, and CLAUDE.local.md. By default, additional directories do not load memory files v2.1.138
CLAUDE_CODE_ADDITIONAL_PROTECTIONNo description from anybody. v2.1.138
CLAUDE_CODE_ADOPT_UNDERIVABLE_PARKED_PERMISSIONA resumed headless session could hand you back a permission prompt left unanswered, but three variables must be set. v2.1.238
CLAUDE_CODE_AGENTNo description from anybody. v2.1.138
CLAUDE_CODE_AGENT_COST_STEERNo description from anybody. v2.1.138
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.