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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint=false, so safety is covered. The description adds real behavioral context beyond that: 'No generated text' tells the agent this returns computed astronomy rather than written interpretation, and it discloses that a birth date alone yields a withheld-time chart context.

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 returned content, then the usage rule, in three sentences with no filler. The middle section is a dense enumerated list, which is efficient but slightly run-on; nothing is redundant with the annotations.

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?

There is no output schema, so the description must describe the return — and it does, enumerating the four payload components and flagging the absence of generated prose. The nested birth object and its null-time behavior are documented in the schema, so an agent has what it needs to call this correctly.

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?

With only 50% schema description coverage, the description carries useful extra meaning: the presence or absence of 'birth' switches the output between a personal transit report and a general sky, and 'with its orb' signals natal-transit precision. Date syntax and birth-field constraints are left to the schema, but the key semantic fork is explained.

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 concrete verb+resource: it returns 'the calculated day' — sky at noon UTC, Moon phase, coming week's sky events, and natal transits with orbs. The scope is specific enough to distinguish it from a generic chart tool, though it never names which sibling (sky, transits, natal_chart) it should be preferred over, so an agent must infer the boundary.

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?

Gives a direct usage rule: 'Use it for a day's horoscope or what is happening for someone now', plus the decisive branch — send birth for a personal answer, omit it for the general sky. No explicit alternatives or when-not-to-use cases are named, so routing against the overlapping 'sky' and 'transits' siblings 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.