You'll notice
Forking a conversation now refuses fork points that would split a tool call from its result
What
When forking a conversation at a specific message, Claude Code now checks whether that fork point would separate a tool call (tool_use) from its result (tool_result). If it would, the fork is refused with an error instead of proceeding.
Why
This prevents creating a forked conversation whose transcript would be invalid because a tool call is missing its matching result.