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

Goes well past the annotations (which only declare read-only/idempotent/open-world) by disclosing data provenance (MET Norway Sunrise 3.0 for rise/set and phase angle; Meeus formulas for full/new moon times), an accuracy caveat (a few minutes, illumination approximate), and the 1-credit cost. Those are exactly the traits an agent needs to decide whether the answer is trustworthy for the question asked.

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-loads the resource and its outputs, then usage, then provenance and cost. Dense and mostly waste-free, though the long output enumeration in the first sentence is heavy and could be trimmed since the schema and call site carry part of that load.

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 no output schema, the description correctly compensates by listing the returned fields, and it covers the coordinate prerequisite and accuracy limits. It does not specify the format of bearings or rise/set times (aside from utc_offset handling), leaving a small gap for an agent reasoning about the response.

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 lat, lon, date and utc_offset are already fully documented with ranges and defaults. The description reinforces that lat/lon are mandatory and routes the agent to geocode/postcode_lookup for UK place names, but adds no new meaning about utc_offset or the date semantics beyond the schema. Baseline 3 applies.

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 specific verb+resource (moon phase for a place and date) and then enumerates exactly what comes back: phase name, waxing state, illuminated fraction, distance from full moon, next full/new moon dates, moonrise/moonset with bearings, and culmination. No sibling tool covers lunar data, and the enumeration makes the tool unmistakable.

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 concrete question-shaped use cases ("is it a full moon tonight", "when is the next full moon") plus the audience (stargazers, anglers, photographers), and crucially states the prerequisite chain: get coordinates from geocode or postcode_lookup before calling. It stops short of naming an explicit when-not or contrasting with sunrise_sunset, so it falls just short of a 5.

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