The source field on SessionStart hook events now accepts a new "fork" value, in addition to the existing "startup", "resume", "clear", and "compact".
- Fires when a session is loaded with
forkSession: true— the code path activated when a skill carriescontext: "fork"or when the fork-subagent experiment (tengu_copper_fox/tengu_fork_subagent_enabled) spawns a subagent by resuming an existing conversation under a new session ID - The hook JSON payload is
{ hook_event_name: "SessionStart", source: "fork", agent_type: <string>, model: <string>, session_title: <string> }— same fields as"resume"events - Unlike
"clear"and"compact", a"fork"event does propagate the session title returned by the hook (same behaviour as"startup"and"resume"): the title is stored so the forked session can be labelled distinctly - The new session ID passed in the hook is the forked child's fresh ID (not the parent's); the hook fires after the fork's conversation history is loaded but before the first turn runs
- Skills with
context: "fork"are excluded from being re-dispatched inside the fork they created, preventing recursive expansion - Hook output (additional context messages,
reloadSkills,watchPaths) works identically to otherSessionStartsources — hooks can inject context or block the fork by returning a blocking error
Schema enum (search for "fork" in SessionStart hook schema alongside "startup", "resume", "clear", "compact"); fork dispatch (search for forkSession, x$e("fork", "fork" : "resume", aye = "fork", tengu_copper_fox)
Strings lifted out of the shipped bundle, so the claim above can be checked against them.
tengu_copper_fox Off in both readingsThe flag server returned off for the account this site reads and for the anonymous baseline. A reading of off cannot rule out a rollout these two readings sit outside of.
This account: off · anonymous baseline: off · compiled default in v2.1.213: not a boolean we can read
These values were read against a different version of Claude Code, so treat them as the nearest reading available instead of one taken on this release.
Read once, for one account on one subscription tier, against v2.1.213. It isn't a statement about your account. What a flag value here can and cannot tell you
A reading is one sample. Claude Code evaluates its flags remotely, so no client sees the targeting rule behind a value and this says nothing about your account.
A reading is one sample. Claude Code evaluates its flags remotely, so no client sees the targeting rule behind a value and this says nothing about your account.