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

Annotations already cover read-only, idempotent, closed-world behavior, and the description adds genuinely new details: shadows use calculated direct-motion longitude returns rather than fixed added days, the search extends up to 366 days each way within provider coverage, and missing boundaries come back null. That is substantive behavioral disclosure beyond structured fields, with only return shape left unstated (and an output schema exists).

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 definition is front-loaded with purpose and then behaviour, with no filler sentences. It is dense and slightly overloaded with qualifiers ('not personal effects, harm or eclipse visibility'), which strain readability but do add 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 four-parameter, read-only, output-schema-backed tool, the description covers prerequisites, search bounds, and null handling — enough for an agent to call it confidently. Alternatives among the numerous timing/chart siblings remain unaddressed.

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 only 25%, so the description must carry weight. It clarifies includeShadows semantics (calculated returns, not fixed added days), the 'today' vs civil-date option, and timezone precision — though much of the timezone wording is duplicated in the schema. The body enum is self-explanatory from 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?

The description opens with a specific verb and resource: 'Returns one Mercury–Pluto planet's current motion, nearest previous and next refined stations and active retrograde boundaries.' The 'Mercury–Pluto' scoping and 'one planet' restriction signal it is narrower than broad sky tools like get_sky_overview, though 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?

It gives prerequisites ('Requires a confirmed viewing timezone and today or a civil date') and a scope exclusion ('collective facts, not personal effects, harm or eclipse visibility'), which implies when it is not appropriate. However, it never states when to choose this over get_sky_overview, get_timing_month, or get_daily_focus, so routing is left to inference.

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