Moon phase

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 new behavior: the default-to-now semantics and, importantly, the full set of computed return fields — which matters because there is no output schema to carry that information.

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 sentence, front-loaded with the resource and the default, with no filler. The long tail enumerating return fields is justified given the absence of an output schema, though the comma list is dense and could be grouped more readably.

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 zero-required-parameter, read-only lookup with no output schema, the description supplies both the default behavior and the shape of the result, which is precisely the information the agent lacks from structured fields. Nothing needed to call it correctly is missing.

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 100%, so date/time/timezone formats and defaults are already fully documented in the schema. The description only echoes the 'default: now' behavior and adds no new syntax, format, or interaction details (e.g., how date and time combine, or timezone handling when only a date is given).

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 resource (the Moon's phase) and a specific moment in time, and enumerates exactly what is computed: phase name, illuminated fraction, Sun–Moon angle, days since new moon, signs, and upcoming quarter phases. It is clearly a distinct astronomical lookup versus the natal/transit/synastry siblings, though it never explicitly contrasts itself with them.

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: '(default: now)' signals that omitting the date yields the current moment, which is reinforced by the schema's 'Omit for the current moment.' There is no explicit when-to-use, when-not-to-use, or alternative-tool routing (e.g., versus transits for a time range).

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