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

Annotations already cover safety (readOnly, idempotent, non-destructive, closed-world), and the description adds real context beyond them: data source (JPL DE421), validity window 2025–2030, UTC-to-the-minute precision, and the 'astronomical timing, not a horoscope' framing. It does not say what happens for dates outside 2025–2030, which is the one behavioral 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?

Front-loaded with the use-case trigger, then the return payload, then scope/provenance caveats — a sensible order. The middle sentence is dense (phase, sign, cycle moments, two flag types, summary sentence) but every element maps to an actual return field, so nothing is wasted.

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 the return-value burden and does so thoroughly, enumerating phases, sign, cycle timestamps, cazimi/opposition flags, and the summary sentence. Missing only edge-case behavior (out-of-range dates, whether all eight planets are always present).

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% and the single 'date' parameter is fully documented there (format, 12:00 UTC reading, defaults to now). The description only says 'on a date' and 'Times are UTC,' adding no format or default details beyond the schema, so the baseline 3 applies.

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?

States a specific verb+resource ('returns the phase... for Mercury, Venus, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto on a date'), which makes it clearly the all-planets bulk lookup. It does not explicitly name the single-planet siblings (is_planet_retrograde, is_mercury_retrograde) as the narrower alternative, so the differentiation is implied rather than stated.

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?

Opens with an explicit trigger: 'Use for "which planets are retrograde" questions,' which maps directly to the user intent. It gives no when-not guidance or named alternative for single-planet queries, so an agent must infer the boundary from the enumerated planet list.

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