You'll notice
Reading an artifact that 404s now falls back to returning its type declaration if it's a frozen artifact type
What
When reading an artifact page or file returns a 404 (not found) error, Claude Code now checks whether the requested URL actually refers to a 'frozen artifact type' (an entry in a type catalog). If it does, it returns that type's declaration and instructions instead of just reporting the 404 as a failure. This applies to both the page read action and the single-file read_file action.
Why
This avoids a misleading failure when the URL was never a live artifact page in the first place but a reference to a known artifact type, giving useful information instead of a plain error.