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

Annotations only declare the safety profile (readOnly, idempotent, closed-world); the description goes well beyond that by naming the data source and coverage window (JPL DE421, 2025–2030), the time standard and precision (UTC, to the minute), and the default-date behavior. It also enumerates the returned fields (sign, pre-shadow, station retrograde, cazimi/opposition, station direct, post-shadow end, days to next transition) and disclaims astrological interpretation, which is meaningful scoping for a niche tool.

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?

Two sentences, front-loaded with the core answer, then the field enumeration and provenance. The long parenthetical lists (planets, phases) are dense but each token carries information; no filler sentences.

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 compensates by naming the fields returned and their time basis, which is enough for an agent to decide if the response fits its need. The remaining gap is a precise statement of failure behavior outside the 2025–2030 ephemeris window, which is only hinted at.

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% — both parameters are documented, including the YYYY-MM-DD-as-12:00-UTC rule and the enum of eight planets — so the baseline is 3. The description adds only the UTC-to-the-minute framing and repeats the planet list; it contributes no new syntax or constraint beyond the schema.

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 and resource ('Says whether Mercury, Venus, ... Pluto is retrograde on a date') plus the secondary output (phase). The eight-planet, single-date scope implicitly separates it from is_mercury_retrograde and retrograde_planets_today, but no sibling is named explicitly.

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: use it to check one planet on one date. There is no explicit 'use X instead when...' routing to the six sibling tools, nor any statement about what happens for dates outside the 2025–2030 range beyond the parenthetical source note.

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