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

Annotations already declare readOnly, idempotent and openWorld, so the safety profile is covered. The description adds genuinely non-redundant behavior: results are in local time, the lookahead is capped at 7 days, and the tool explicitly reports midnight sun / polar night rather than failing silently. What it omits is how ambiguous or unresolvable locations are handled.

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 output payload is front-loaded, then a compact 'Use when' trigger list, then a single inline example. Every sentence carries information an agent needs and there is 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?

There is no output schema, so the description carries the burden of describing returns — and it does list the four values plus local-time semantics and the polar edge cases. The only shortfall is the absence of any statement about output shape or the location-versus-coordinates precedence noted above.

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 100%, so the schema already documents every parameter and the baseline is 3. The description reinforces the two input modes ('any city or coordinates', 'a date or up to 7 days') but does not resolve the real ambiguity in the schema: 'location' is listed as required while lat/lon are documented as an alternative to it, so an agent still cannot tell whether location may be omitted.

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 the exact resources returned (sunrise, sunset, solar noon, day length) and the scope (local time, any city or coordinates, one date or a 7-day window). Midnight-sun and polar-night handling further sharpens the boundary against a generic weather sibling. An agent can distinguish this from weather_forecast and pollen_and_air_quality without opening any schema.

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 explicit 'Use when:' block gives concrete trigger queries ('what time is sunrise in Oslo tomorrow?') and use cases (photo, prayer, outdoor planning), which is strong affirmative guidance. It stops short of naming alternatives or exclusions — e.g. no statement that general weather conditions belong to weather_forecast — so it does not reach the top band.

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