Unclear It is not stated what causes the built-in artifact tool to be hidden from MCP servers, so it is unclear which sessions this applies to.
When a session (one conversation with Claude) is moved to the background, Claude Code hands over a list of artifacts it is still watching. Previously that list held only artifacts being watched live. It can now also hold artifacts watched only so they can wake the session, and those subscriptions stay open.
This applies to sessions set up to react automatically to artifact activity, with a watch on that activity already armed, and only when the built-in artifact tool is hidden from MCP servers (outside programs that give Claude Code extra tools).
In practice, someone commenting on an artifact can wake a backgrounded session so it reacts, instead of the comment going unnoticed until you return.
It is not stated what causes the built-in artifact tool to be hidden from MCP servers, so it is unclear which sessions this applies to.