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

Annotations already declare readOnly, idempotent, non-destructive, closed-world. The description adds behavior beyond them: it requires an API key and a paid Ephemerides plan, and discloses the default window (today, 7 days, or the plan's lower ceiling). It does not cover rate limits or output format, so it stops short of a 5.

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 returned quantities are front-loaded in the first sentence, followed immediately by usage routing, an actionability hint about elevation, and defaults. Every clause carries information an agent needs to call the tool correctly; there is no padding.

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 carries the burden of describing returns and does so exhaustively (all solar/lunar event types, one entry per day). Auth prerequisites and default time window are also stated, so an agent has everything needed for correct invocation.

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%, so the baseline is 3. The description still adds meaning: it quantifies the days default ('7 days or the plan's limit if lower'), which the schema only leaves as 'plan-dependent', and it re-emphasizes that supplying elevation shifts sunrise/sunset at altitude.

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 (sun and moon rise/set times for a location) and enumerates the exact quantities returned: sunrise, sunset, solar noon, the three twilights, golden and blue hour, moonrise/set and phase, one entry per day. It explicitly distinguishes itself from siblings by noting get_solar_position is for the sun's angle at a moment and get_tide_window for tides.

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 states the when-to-use condition ('any question about when the sun or moon rises, sets or gets dark') and names two alternatives with the exact conditions that select them (sun's angle at a moment -> get_solar_position; tides alongside the sun -> get_tide_window). Explicit exclusions plus routing leave nothing to inference.

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