A forked subagent (subagent_type: "fork") inherits the whole conversation and its prompt cache. It is on by default, and CLAUDE_CODE_FORK_SUBAGENT=0 turns it off.
Use
In settings.json:
{
"env": { "CLAUDE_CODE_FORK_SUBAGENT": "0" }
}Turn it off when you want Claude to wait for a subagent’s result instead of spawning it in the background. Fork mode is on by default since v2.1.232 for interactive sessions.
Contrast with subagent context isolation: a fork shares your context, an isolated subagent starts clean.