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

Goes well past the annotations: pricing ($0.003/call via x402 USDC or card key), first 25 free, failed calls free, local computation, and an explicit rationale for why readOnlyHint is false (each call is charged). It also clarifies the tricky 'local compute vs remote read' and billing-on-failure semantics an agent needs.

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 returned values, then usage, then example, then billing. Dense but every clause carries a distinct fact; the pricing sentence is long but necessary for a paid tool.

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 fully enumerates the return fields and the extra next-new/full-moon values, so an agent knows exactly what it gets. Cost and free-tier terms complete the picture for invocation decisions.

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% — the single date parameter already documents 'Date or ISO time (default now)'. The description adds only an example value and the UTC framing, which is consistent with the baseline 3 when the schema does the heavy lifting.

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?

Begins with the exact payload — phase name, illuminated percentage, age in days, plus next new and full moon (UTC) — and states the computation method (mean synodic month). It is unambiguous against siblings like sun, time, or holidays.

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" plus a concrete example (date=2026-10-18) gives clear context for invocation. It stops short of naming alternatives or exclusions, e.g. when to prefer the sun tool for a combined sky-planning query.

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