Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
With no annotations, the description carries the full burden, but it only reveals that the response includes the requested fields ('whichever fields you ask for'). It does not disclose error behavior (e.g., not found), authentication requirements, rate limits, or whether expansions/tweet.fields must be provided. The field-selection nuance is helpful but insufficient.
Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.