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

Annotations already establish readOnly and openWorld, so the lower bar applies; the description goes beyond them by disclosing that results are reduced to closest approach, include where to look and visibility (above horizon, dark sky), and that the period cap is 7 days (1 day with a satellite). It does not describe result structure, ordering, or empty-result behavior, but the behavioral picture is substantially richer than the annotations alone.

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?

The core statement is front-loaded in the first clause and the satellite exception is placed last where it belongs. The enumerative star list ('Regulus, Spica, Antares, Aldebaran, Pollux…') is somewhat padded, but the sentences otherwise carry real information.

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?

For a 9-parameter read-only tool with no output schema, the description supplies the essential missing pieces: what the output conceptually contains (closest separation, sky position, visibility) and the temporal cap. It leaves return shape, ordering and the role of lang/time_zone to the schema, which is acceptable but not complete.

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 description coverage is 100%, so the schema already documents every parameter, including the default/max on days and the satellite-day interaction. The description only restates the 7-day default/cap and the one-day satellite case, adding essentially nothing beyond the schema on lat/long, start, height, max_separation, lang or time_zone.

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 names a concrete resource ('apparent close pairings' of Moon, planets and ecliptic bright stars) and a concrete scope (observer location, bounded period), plus a satellite variant. It is clearly a conjunction-finder rather than a general ephemeris or events tool, though it never explicitly contrasts itself with siblings like perigee_events or perigee_sky.

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 stated scope (pairings within a period, up to 7 days; satellite mode limited to one day), but there is no explicit when-to-use/when-not guidance and no sibling is named as an alternative. An agent can infer the case but must guess where the boundary with perigee_events or perigee_tonight lies.

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