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

With no annotations provided, the description carries the full behavioral burden, and it delivers: it discloses the closed-form computation model, the non-astronomical nature, the start-of-day measurement assumption, and how to adjust the observation point. This is rich, non-obvious behavioral context beyond the bare schema.

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 dense but every sentence earns its place: it explains the output semantics, the computation model, the timing convention, and the input choice in a compact paragraph. Nothing is filler, and the most important phrase appears first.

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 tool with no output schema, the description does a good job defining returned values, and it covers the tricky day_fraction/time behavior. But it leaves meaningful gaps: the structure and semantics of the required 'calendar' and 'date' nested objects are not explained, and it does not specify what happens if both day_fraction and time are supplied despite 'Give one or the other.'

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 coverage is 0%, so the description must compensate. It explains day_fraction and time well, including default behavior and mutual exclusivity, and it defines the meaning of the returned fields. However, it gives almost no guidance on the required 'date' and 'calendar' object parameters, which are nested and opaque in the schema.

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 opening line, 'Every moon's phase on a date,' states a specific computation and resource, and the rest of the description defines the output fields (fraction, phase, illumination) in enough detail to distinguish it from the sun/calendar siblings. It is a clear, non-tautological statement of what the tool does.

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 gives explicit conditional guidance on when to pass 'day_fraction 0.5' versus 'time', and instructs 'Give one or the other.' It also draws a boundary around the model ('not simulated and not astronomical'), which prevents misuse. However, it does not explicitly name sibling tools such as sun_daylight or state when to choose them instead.

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