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

Annotations already establish read-only, closed-world, idempotent behavior, so the bar is lower. The description still adds genuinely new context: computed locally via Meeus, accurate to about 0.5 hour of JPL ephemerides, and no network call — useful for judging trustworthiness and latency instead of repeating safety hints.

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 clauses: what it returns, when to use it versus the sibling, and how it is computed. Capability and routing come first, with provenance last — no filler.

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 an output schema present, the description needn't explain return shape, and it covers scope, alternative routing, and computation fidelity. Only minor gaps remain, such as whether the "what it means for observing faint objects" narrative text is localized by `lang`.

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%, with both `time` (ISO-8601, default now, years 1900-2200) and `lang` (fr/en, fr default) fully documented. The description only restates the "default now" behavior and adds nothing about the language switch or accepted date range, so 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?

Uses a specific verb and resource ("Return the Moon's phase and illuminated percentage") and enumerates the extra outputs — next full/new Moon and its impact on observing faint objects. This is clearly distinct from siblings like solar, sky_events or observation_conditions.

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?

Explicitly states the trigger ("Use it for Moon questions") and routes the agent elsewhere when the question is broader: "for an overall observing score use observation_conditions." The named alternative makes the boundary unambiguous.

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