Claude Code defaults code review, fork skills to background

Claude Code made background execution the default for /code-review and any skill declaring context: fork, per the 2.1.218 release notes posted July 22. That default flip matters most for anyone who built a multi-step skill assuming a fork step blocks until it returns a result. It doesn't anymore. The next step in the chain now fires immediately, before the forked skill's output exists. A skill opts back into the old behavior by setting background: false.

This pairs with 2.1.215, which stopped /verify and /code-review from firing automatically. That release put invocation back in your hands. Running them now costs no main-thread context, since output moves to a background subagent instead of the main conversation.

The failure mode is quiet. A chained skill that reads its own fork step's output will run against nothing, or against a stale result. The error that follows will look unrelated to the frontmatter flag that caused it.

Anyone maintaining custom skills should grep their files for context: fork and check what fires right after. The declaration's meaning changed on July 22, without a rename.