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

Annotations already declare readOnlyHint, openWorldHint, idempotentHint, and destructiveHint=false. The description adds valuable behavioral context beyond that: it clarifies the output structure (phase value 0-1, illumination 0-100), explains polar flags (moon_always_up/moon_always_down), and notes no auth/free commercial use. It does not contradict annotations and provides meaningful additional insight.

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 description is well-structured and front-loaded with the primary purpose. It lists return fields compactly and includes relevant operational notes (no auth, source). It is slightly verbose with the full enumeration of moon phases, but each piece earns its place and there is no wasted wording.

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?

The description covers the core purpose, the required inputs, the key outputs (including polar edge cases), authentication, and licensing. With a rich output schema presumably present)Skip, the description is complete for an agent to decide whether to call it and what to expect. Minor missing details like error handling don't detract for a simple read-only astronomy 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?

The input schema has 100% description coverage for all 4 parameters (lat, lng, date, timezone). The description does not add further meaning about the input parameters themselves; it only reiterates that location and date are used. Baseline of 3 is appropriate because the schema carries the parameter documentation burden fully.

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 uses a specific verb 'Get' and names the exact resource: moon phase, lunar illumination, and moonrise/moonset times for any location and date. It also lists the precise return fields (phase name, value, illumination, times, polar flags), which distinguishes it from sibling tools like sun.position that deal with solar data.

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?

The description implies usage ('Get moon phase... for any location and date') and adds context like 'No auth required; free commercial use.' However, it does not explicitly state when to prefer this over sibling tools such as sunrisesunset.astronomy.daily or range, nor does it mention exclusions. The guidance is implicit only.

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