Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
Annotations already declare the safety profile (non-readOnly, idempotent, non-destructive, closed-world), so the bar is lower. The description usefully discloses what the operation actually does - recompute/solve constraints and reposition components - which is real behavioral context, but says nothing about failure modes (over-constrained or unsolvable assemblies) or what state must exist beforehand.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.