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

Annotations already establish readOnly, idempotent, non-destructive and closed-world, and the description adds substantive context beyond that: UTC timing to the minute, provenance (JPL DE421 ephemeris) and a stated validity window of 2025–2030, which tells the agent when results may not cover a requested date. It stops short of saying what happens for out-of-range dates or how a missing/ambiguous date is handled, which keeps it off a 5.

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?

Purpose is front-loaded in the first clause, with supporting detail following, and there is no filler. The parenthetical enumeration of the five cycle moments is long but carries real information, though it makes the middle of the sentence dense to parse.

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 one-optional-parameter read tool with full schema coverage and complete annotations, the description covers purpose, units, data source, temporal validity and return content, so an agent can call it correctly without an output schema. The missing piece is sibling routing, which matters here because six other tools return overlapping retrograde information.

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 single 'date' parameter is already fully documented including the YYYY-MM-DD read-as-noon-UTC behavior and the now-default. The description reinforces the UTC convention ('Times are UTC, to the minute') but adds no new parameter-level semantics; baseline 3 is appropriate.

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 gives a specific verb and resource ('Says whether Mercury is retrograde on a date') and even enumerates the phase values it can return, so its function is unambiguous. What it does not do is distinguish itself from siblings such as is_planet_retrograde (the all-planet version), mercury_retrograde_dates, or retrograde_planets_today, so an agent still has to infer why this one is Mercury-specific.

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?

There is no explicit when-to-use or when-not-to-use guidance, and none of the seven overlapping siblings are named, including the near-identical is_planet_retrograde and retrograde_planets_today. The only usage signal is implicit in the verb phrase 'on a date', which an agent must extrapolate into a routing decision.

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