Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?

Annotations already declare readOnlyHint and openWorldHint, so the description is free to add value, which it does: output units (km, km/s), ICRF/J2000 axes, the default centre rule, and the presence of derived values like light time, sunlight flag and provenance. It does not say whether this is a single instant only (no series) or how large a response is, which is the remaining gap.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Is the description appropriately sized, front-loaded, and free of redundancy?

Position and velocity lead, with units and frame immediately attached, so it is front-loaded and dense. The trailing 'over Brazil' / 'over the Indian Ocean' examples add colour at some length, but they clarify the ground-point output rather than pad.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

With no output schema, the description carries responsibility for describing the return and does so thoroughly: units, frame, centre-relative geometry, and the conditional ground-point fields. Only the response shape boundaries (single instant, no series, no request limits) are left unstated.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

Schema coverage is 67%: date and centre are already documented in the schema. The description adds meaning beyond that by explaining what centre defaults to ('the body it orbits or stands on') and its effect on which extras appear, and by implying that id is an object identifier.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Does the description clearly state what the tool does and how it differs from similar tools?

The description states a concrete verb and resource ('position ... and velocity ... of an object at a date') plus the frame, units and centre semantics, which is far more specific than the title. It doesn't name a sibling it replaces, so the agent must infer that perigee_orbit or perigee_ground_track do related but different things.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Does the description explain when to use this tool, when not to, or what alternatives exist?

Usage is implied by the returned quantities ('at a date', default centre, ground point only around Earth/Moon/Mars), but there is no explicit when-to-use or when-not guidance. Nothing tells the agent to prefer perigee_orbit, perigee_distance or perigee_ground_track instead for their cases.

Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.