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

Beyond the annotations, the description discloses the computation method (mean synodic month), pricing and free-call structure, failed-call behavior, and the fact that repeats are charged. It also clarifies that the tool only reads or computes and changes nothing, which reinforces the safety 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?

The description is front-loaded with what the tool returns and stays reasonably concise. The final sentence partially repeats the pricing and read-only points, keeping it from being maximally tight.

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 simple one-parameter computation tool, the description covers return values, method, usage context, cost, failure behavior, and side-effect profile. No output schema exists, but the description adequately enumerates what comes back.

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 100% and the schema already documents the single date parameter, including its default-now behavior. The description only adds an example value, without extra constraints or format details, so baseline 3 is appropriate.

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 clearly identifies the resource and return fields: moon phase name, illumination percentage, age in days, and next new/full moon dates. It is unmistakably about lunar data, though it does not explicitly differentiate itself from nearby siblings such as sun or date_info.

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?

It gives clear usage contexts (calendars and night-sky planning) and a concrete example date. It does not state when not to use it or name alternatives, so it falls short of full when/when-not guidance.

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