Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations only cover the read/write/destructive/open-world tri-state, but the description adds auth requirements (forms:write scope), a processing constraint (on-server parser only, Workers AI off), the side-effect boundary ('Does not publish the form'), and a data-handling guarantee ('File contents are not logged'). That is exactly the extra behavioral context annotations cannot carry.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.