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Sweep 28 Aug 2026 · 16:06Z Build v2.1.251 479 read Stable v2.1.236 Latest v2.1.251 Next v2.1.251 Feeds RSS JSON llms.txt

DisclaimerUnofficial, and not affiliated with Anthropic. Nearly all of this is read straight out of what ships: npm bundles, captured prompts, published docs. Anthropic's own notes go in verbatim, marked as theirs. The rest is my reading, and every entry carries the strings behind it. If one looks wrong, vote it down and say why.

Reference / Environment variables

CLAUDE_CODE_FORK_SUBAGENT

A Claude Code environment variable, read out of the shipped bundle. It has been in the build since v2.1.138, including the newest one read.

What it is

Controls [fork mode](/docs/en/sub-agents#turn-fork-mode-on-or-off), which lets Claude spawn [forked subagents](/docs/en/sub-agents#fork-the-current-conversation) itself and is on by default in interactive sessions only. Set to 1 to turn it on in claude -p and the Agent SDK as well, or 0 to turn it off in every kind of session. You can run /subtask whether or not fork mode is on. The interactive default requires Claude Code v2.1.232 or later; on earlier versions, set the variable to 1 to turn fork mode on

Anthropic's own wording. Read off Environment variables, captured 2026-08-28.

Presence across releases

ReleasesReadingDeclared typeDefault
2.1.245 – 2.1.251 6 releases never mined — —
2.1.210 – 2.1.243 33 releases in the build triBool —
2.1.160 – 2.1.209 42 releases in the build bool —
2.1.138 – 2.1.159 20 releases in the build — —

A row reading never mined is a release with no archived bundle behind it. Nothing was read out of it, so nothing here says whether the name was in it. A declared type or a default is only ever what that release's own bundle stated.

Changelog entries naming it

  1. v2.1.232Three more variables can be set from settings.json

    You can now set OTEL_SERVICE_NAME, OTEL_TRACES_EXPORT_INTERVAL and CLAUDE_CODE_FORK_SUBAGENT in settings.json.

    named in this entry

  2. v2.1.232Fork-subagent setting is resolved once per session, with the environment checked first

    Setting CLAUDE_CODE_FORK_SUBAGENT decides fork subagents up front, and the answer is cached per session.

    named in this entry

  3. v2.1.232Forked subagents are on by default

    Forked subagents are on by default in interactive sessions; set CLAUDE_CODE_FORK_SUBAGENT to false to opt out.

    named in this entry

Documentation pages

A reference table page is one whose own table defines this name, and it is where the description above came from. A page that mentions it carries the name somewhere in its text and may say nothing about it at all. Every page carrying it.

Names beside it

Every environment variable in the inventory starting claude_code_fork, up to twelve of them.

CLAUDE_CODE_FORK_SUBAGENT_DEFAULT_ONNo description from anybody. 2.1.160

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.