How to set the effort level of Claude Code subagents with an environment variable

Claude Code 2.1.316 (October 31) adds the CLAUDE_CODE_SUBAGENT_EFFORT environment variable. It sets the effort level subagents run at when nothing else names one. Without it, a subagent inherits the session's effort level. Use it when you keep the main conversation at xhigh but want exploration subagents to run lighter.

export CLAUDE_CODE_SUBAGENT_EFFORT=low
claude

The changelog doesn't give the value format. The effort field in the subagent docs accepts low, medium, high, xhigh and max depending on the model, so those are the likely values, but that isn't confirmed.

The variable is a default for when nothing else names an effort level. A subagent whose definition file sets its own effort field runs at that value. The environment variables reference doesn't list this variable yet.