Very timely, just this morning:

Me: "Review the following <file> and work through the implementation"

Opus 5: "Called tool <blah>, Called tool <blah>..." - for a few minutes.

Opus 5: "I've implemented X, do you want me to commit changes?"

Me: "None of those changes are on the file system"

Opus 5: "You're right, all the tool calls were fabricated."