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

Annotations already declare readOnlyHint=true, idempotentHint=true, and destructiveHint=false, so the safety profile is covered. The description adds valuable behavioral context: it's an 'astronomical calculation (SunCalc), exact and instant,' and notes the price is free. It also clarifies the date defaults to today (UTC) via the schema, and that tz affects local time output.

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 description is two sentences with no wasted words. It front-loads the core outputs, then adds the calculation method, the tz hint, and the price. Every sentence earns its place.

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 read-only calculation tool with no output schema, the description covers the inputs (place, date, tz), the outputs (sun, moon, twilight values), and the behavioral traits (exact, instant, free). It doesn't specify the exact return format, but the absence of an output schema and the tool's simple nature make this a minor gap.

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 67%: place has no description, while tz and date are described. The description compensates by explaining that tz is for local times and that date defaults to today (UTC), which adds meaning beyond the raw schema. The place parameter remains minimally documented, but its purpose is obvious from the tool's name and description.

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 ('Sunrise, sunset...') and enumerates the exact resources and computed values: sunrise, sunset, golden hour, twilight, day length, solar noon, moon phase, illumination, moonrise, and moonset. It clearly distinguishes itself from sibling tools like weather or time_convert by focusing on astronomical calculations for any place and date.

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?

The description states the tool is for 'any place and date' and mentions passing tz for local times, which gives clear context for when to use it. It doesn't explicitly name alternatives or exclusions, but the sibling list contains no other astronomy tool, so the usage context is sufficiently clear.

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