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

Beyond the annotations (readOnlyHint=true, openWorldHint=true, destructiveHint=false), the description discloses two things the annotations do not: a 2-credit cost and a plan-tier gating of the date and phases capabilities. The one gap is that it never says what happens on a free plan when date or phases is passed — error, ignored, or silently defaulted — which matters for correct invocation.

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 purpose is front-loaded in the first clause and the cost is tucked into a parenthetical at the end, so nothing is buried. It is a single long run-on sentence packing many output items, which is dense but not wasteful.

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 zero-required-parameter, read-only calculator with a full output schema, the description covers purpose, outputs, cost, and tier constraints without needing to explain return values. The only residual ambiguity — free-plan handling of date/phases — is a minor edge, not a blocking omission.

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%, and the schema already documents the MM-DD-YYYY format and each parameter's meaning, so the baseline is 3. The description adds only a coarse restatement ("custom date queries," "dates of the next new moon, first quarter, full moon and last quarter") without clarifying the interaction between the date and today flags or their free-tier behavior.

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 a specific verb+resource ("calculates the current lunar phase, illumination percentage, and lunar age") and enumerates the concrete outputs, so an agent can tell this apart from adjacent astronomy tools like MoonPosition or MoonriseMoonset. The opening restates the name/title redundantly ("Moon Phases: Moon Phases calculates..."), and it never explicitly contrasts itself with those siblings, which keeps it from a 5.

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?

Usage context is only implied: the default is "for today," and custom date queries plus next-phase dates are gated behind paid plans. There is no explicit statement of when to pick this over MoonPosition, MoonriseMoonset, or SunriseSunset, and no guidance on what to do if the caller is on a free plan and supplies a date.

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