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

Annotations already cover read-only, idempotent and open-world, so the bar is lower; the description still adds real value by disclosing DST handling via UTC instants, polar day/night edge cases, and the caveat that values are geometric and ignore terrain and weather. It does not say what happens for out-of-catalogue slugs or how polar cases are represented in the 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?

Three short sentences, front-loaded with verb and resource, then edge-case behavior, then a caveat. No filler and nothing repeated from the schema or annotations.

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?

An output schema exists, so return values need no explanation, and annotations carry the safety profile; the description covers the notable behavioral edge cases (DST, polar days). The one gap is that it never explains that the required slug supplies the location, which is essential to interpreting a sunrise/sunset result.

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 67% and the schema already documents slug and timeZone well, so baseline 3 applies. The description clarifies the date semantics ('exact calendar date') and zone behavior (defaults handled via UTC) but never mentions the required slug or that the observation point comes from a peak, leaving the most important parameter implicit.

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?

States a specific verb and resource plus the precise outputs: 'Compute sunrise, sunset and civil twilight for the exact calendar date and IANA zone.' This clearly separates it from get_weather and the search/get_* siblings. It stops short of naming an alternative tool explicitly, so it is a 4 rather than 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 is implied by the domain (ask for daylight at a peak on a date) and the closing line hints that weather/terrain are separate concerns, but there is no explicit 'use this when' or 'use get_weather instead for X' routing. Nothing misleading, just no guidance for choosing between daylight and weather lookups.

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