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

With readOnlyHint, idempotentHint and destructiveHint already declaring a safe, repeatable read, the description still adds real context: the JPL DE421 ephemeris source, the UTC-to-the-minute precision, and the fact that a year with no retrogrades reports that and names the next one. Only the ordering and pagination/shape of the returned period list is left unstated.

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?

A single dense sentence pair, front-loaded with the verb and scope, then precision and provenance, then the domain caveat. Every clause carries information an agent needs; nothing is padding.

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 two-parameter read tool with no output schema, the description covers the returned content (sign, shadows, cazimi/opposition, stations), precision, source, and the empty-result case. The only gap is the structural form of the response, which is minor at this complexity.

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 100%, so the baseline is 3, but 'in or overlaps a year' adds genuine filtering semantics beyond the schema — a period beginning in the prior year still counts, which the bare 'Year, 2025–2030' description does not convey. The planet enum and range are restated but not enriched.

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 opens with a specific verb and resource ('Lists every retrograde period') and enumerates the eight supported planets plus the year window, so the scope is unmistakable. It does not explicitly distinguish itself from the near-identical sibling mercury_retrograde_dates, leaving the agent to infer that this is the all-planet variant.

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 rather than stated: the 2025–2030 window and 'astronomical timing, not a horoscope' frame the intended context, and the empty-year behavior tells the agent what a null result looks like. There is no explicit guidance on when to prefer this over mercury_retrograde_dates, retrograde_planets_today, or is_planet_retrograde.

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