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

Annotations already declare readOnly/idempotent/openWorld/non-destructive, and the description adds substantial context on top: the MET Norway 3.0 source for core times versus NOAA-derived twilight and golden hour flagged as derived, explicit polar day/night handling, and the important caveat that daylight saving is not applied automatically (pass +01:00 for BST). It also discloses the 1-credit cost.

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?

Front-loaded with the payload, then progressively adds sourcing, timezone caveat, and prerequisite. Dense but every sentence carries operational information; the trailing parenthetical cost is the only near-redundant element.

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 still enumerates what comes back (rise/set/noon/day length, bearings, noon altitude, twilight and golden-hour windows) and covers edge cases (polar day/night) plus the timezone trap. Nothing an agent needs to call it correctly is missing.

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 baseline is 3, but the description adds real meaning: utc_offset governs the returned times, defaults to +00:00, and DST must be supplied manually; lat/lon must come from geocoding for place names. Only 'date' semantics are left entirely to the schema.

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+resource with full scope: 'Sunrise, sunset, solar noon and day length for any place on Earth and any date', then enumerates the derived outputs (compass bearing at rise/set, noon height, civil twilight, golden hour). No sibling tool covers this, and the agent can tell instantly what it returns.

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?

Gives concrete use contexts (photographers, walkers, anglers, prayer and event timing) and an explicit prerequisite workflow: for UK place names or postcodes, call geocode or postcode_lookup first to get lat/lon. It does not explicitly exclude or compare against any sibling beyond that prerequisite routing, so it falls short of a full when/when-not statement.

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