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

Beyond the annotations, it discloses the billing model ($0.001 USDC on Base per successful call via x402) — which explains the otherwise puzzling readOnlyHint=false/idempotentHint=false — and the computation method (local mean synodic month), signaling approximate accuracy.

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?

Three tight sentences: outputs first, method second, cost last. Every clause carries information an agent needs, with no restatement of the tool name.

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 compensates by enumerating the returned fields and timezone (UTC), and it flags the payment requirement. 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 coverage is 100% and the single date parameter already documents its format and default. The description adds only 'for any date' and the UTC framing, so the schema does the heavy lifting; 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?

The description names the exact resource (moon phase) and enumerates the concrete outputs: phase name, illuminated percentage, age in days, plus next new and full moon. It is clearly distinguishable from neighboring astronomy/temporal siblings like sun, time, and weather.

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?

'Use for calendars and night-sky planning' gives an application context but no when-not conditions and names no alternative tool for date/time work (sun, date_info, time). Usage is implied rather than routed.

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