Spawn Split

Spawn Split gives an agent a single tool, spawn_split, for fanning work out into child agent threads without leaving the conversation it started in. Call it with a prompt and it spawns one or more child threads - each parented to the calling thread - and opens every one as its own split pane right next to the caller, so the human running the session can watch each child work in real time.

The spawn_split tool

Arguments cover the common fan-out shapes: prompt (required), an optional title, provider, and model override per child, count (1 to 8) for how many children to spawn, split for which direction to open each pane (down, left, replace, right, top), and env for which environment each child runs in.

Worktree vs shared

env: "worktree" (the default) gives every child a fresh, isolated managed worktree, so parallel children never collide on the same files - ideal for independent reviews, parallel implementations, or exploratory branches. env: "shared" instead reuses the calling thread's own environment, so the child reads and writes the exact same working tree as the caller - useful when a child needs to continue or extend work already in progress rather than start clean.

Results are bounded: spawn_split reports which children spawned and opened successfully and which failed, rather than returning full transcripts, and partial failures are reported without discarding the successes.

Headless by design

This plugin ships no frontend surface. Spawning a child thread and opening its pane both happen from backend code in the same tool call, using bb.sdk.threads.spawn and bb.sdk.threads.open, so there is no page or component for a user to install or configure beyond the plugin itself.