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

The annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false, and openWorldHint=false, so the safety profile is fully covered without the description. The description adds the one behavioral fact annotations cannot express — a 10-credit Tier 1 cost — but says nothing about determinism, input validation failures, or output units.

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?

A single tight sentence does the core work and is front-loaded, followed by compact bracketed metadata. The metadata is cheap and useful (cost is not in the schema), though the 'Required' line duplicates the schema's required array, which costs it a point.

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 need not explain return values, and annotations cover the safety profile; for a low-complexity computation tool the definition is close to sufficient. A minor gap remains around which twilight definitions (civil, nautical, astronomical) are returned and the time units emitted.

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 57%: fields, timezone, precision, and timezoneOffset carry rich schema-level text, while date, latitude, and longitude are bare. The description only restates the required trio, which the schema's required array already encodes, so it adds no meaning about coordinate sign convention, date range limits, or how timezone interacts with timezoneOffset.

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 gives a specific verb (Calculate) plus a concrete resource list (sunrise, sunset, twilight times, day length) and the scope (given date and geographic location). No sibling tool in the list computes solar day events, so the resource itself distinguishes it from astroway_calendar_moon_phase, astroway_horoscope_daily, and the rest.

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 bracketed metadata states the endpoint, the three required parameters, and the credit cost, which orients the agent, but there is no explicit when-to-use versus when-not, and no named alternative for related needs (e.g., planetary hours or moon phase). Usage is implied by the purpose statement rather than stated.

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