What
The claude plugin marketplace add command adds a plugin marketplace, which is a catalog Claude Code can install plugins from. It now checks at the very start whether you asked for the reserved name anthropic-plugin-directory. The check ignores upper and lower case. If the name matches, the command stops with an error. It also reports a new failure code to telemetry, the usage reporting Claude Code sends back.
A refusal for this reserved name already existed further along in the add process. This adds an earlier one.
Why
You find out immediately that the name is reserved. Previously you found out only after the command had already started work on the add.