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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, so the safety profile is covered. The description adds genuinely useful behavior beyond that: today is the default day, exact transits are limited to Jupiter through Pluto and span twelve months, and no forecasting/interpretation is performed.

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, front-loaded with the core operation and then enumerating the three output groups, with the "no forecast" caveat closing it. Nothing is wasted, though the colon-list makes it slightly harder to scan than separate sentences would.

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?

An output schema exists, so return values need no explanation, and the safety profile is in annotations. However, for a nine-parameter tool the description leaves real gaps: it never says birth data (`date`, optionally `time`/`place`/coordinates) is required, never mentions resolving place names (a find_place sibling exists), and its "a day (today by default)" phrasing sits ambiguously next to the actually-required `date`.

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 89%, so the schema already documents nearly all nine parameters including formats, enums, and ranges. The description adds only the "today by default" default for the day and the twelve-month transit window; it does not clarify the distinction between the transit day (`on`) and the required birth `date`, which is the one semantic an agent could get wrong.

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 names a specific operation and resource: planets of a day computed against a birth chart, returning houses, aspects, and twelve-month outer-planet transits. It is clearly distinct from natal-only or sky-snapshot siblings, though it never names those siblings 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?

"Against a birth chart" implies the precondition (you need birth data), and "Calculations only, no forecast" rules out interpretation, but there is no explicit statement of when to choose this over birth_chart, sky_at, solar_return, or planet_returns. Usage must be inferred from the output content described.

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