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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the description's job is lighter. It usefully adds the behavioral fact that both key inputs must be supplied explicitly ("Neither is defaulted") and states the chart contents, which is genuine context beyond the annotations.

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: the first front-loads the output contents, the second the input requirements. No filler, no restating the title, and the most decision-relevant constraint ('Neither is defaulted') lands last for emphasis.

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 10 params, an existing output schema, and annotations carrying the safety profile, the description only needs to cover essential inputs, which it does for facing and period. Minor gaps remain for the auxiliary fields (fields, precision, language, timezoneOffset), but none of these are decision-critical for selecting or calling the tool.

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 30% across 10 params, so the description must compensate, and it does for the critical fields: it explains that facing can be degrees or one of the 24 mountains (mapping the facing/facingMountain duality) and that period can be given directly or derived from the occupation date (period/occupiedDate). This resolves the main input ambiguity not documented in the schema, though language/precision/fields/timezoneOffset remain unaddressed.

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 the specific artifact (nine-palace Flying Star natal chart of a building) and enumerates its distinctive contents (mountain/period/facing star per sector, the named arrangement, special patterns), which clearly separates it from feng-shui siblings like annual_stars, bagua, kua, and lucky_directions. It is explicit about what it produces, though it never directly contrasts itself with those siblings.

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 rather than stated: it tells you to supply the facing as degrees or a named mountain and the period directly or via the occupation date, and warns that neither is defaulted. There is no explicit 'when to use this vs. alternatives' or prerequisite guidance, so an agent must infer the scenario.

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