Some skills, which are packaged instructions Claude can use, can only be started by Claude and not typed by you as a slash command. If you type one anyway, Claude Code now shows This skill can only be invoked by Claude, not directly by users. as the command's output and also returns it as the command's result text.
When a command that sends a prompt hits an error, Claude Code now reports the error as error output, marks the command's outcome as failed, and includes the error text as the result. Before, both cases produced a plain message with no result text.
If you run Claude Code without its interactive screen, for example from a script or through the SDK (the library for driving Claude Code from your own code), you now receive the error text in the result instead of having to find it elsewhere.