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

Goes well beyond annotations: discloses the pricing model ($0.003/call via x402 USDC or card key, first 25 free, failed calls free), that no outside service is used, and that polar day/night is handled. Crucially it explains the unusual readOnlyHint=false annotation ('each call is charged'), resolving a contradiction an agent would otherwise have to guess at.

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 what is computed, followed by edge cases, use cases, example, then cost. Dense but every clause earns its place; the pricing sentence is long but necessary given the annotation rationale it supports.

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 the returned quantities (sunrise, sunset, solar noon, civil dawn/dusk, day length) and their UTC basis. Edge-case handling, cost, and auth path are all covered, leaving nothing essential for a correct invocation unspecified.

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 three parameters are already documented, putting the baseline at 3. The description adds an end-to-end worked example that shows valid lat/lon/date values together and confirms output timezone is UTC, which is real practical value above 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?

Names a specific verb+resource ('Sunrise, sunset, solar noon, civil dawn and dusk (UTC) and day length') and scopes it to any place/date. An agent can distinguish it from siblings like moon, time, and weather without opening schemas, and the stated computation method ('standard solar equations') pins the semantics.

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?

Explicit use cases ('photography, events and outdoor planning') and a concrete worked example (lat=59.33, lon=18.07, date=2026-12-21) give clear context. However, no sibling alternatives are named (e.g. when to prefer moon or time instead), so the routing guidance is contextual rather than exclusionary.

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