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

Annotations already declare readOnly, openWorld, non-idempotent and non-destructive, so the safety profile is covered; "reads or computes only and changes nothing" largely restates that. The description does add genuinely new behavior: cost per call, the 25 free calls, that failed calls are free, and explicitly that repeats are charged — which explains the practical consequence of idempotentHint=false.

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 returned, then usage, then an example, then commercial terms — a sensible ordering. The pricing sentence is dense but every clause is actionable; only the "reads or computes only" clause is filler against the 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?

With no output schema, the description does the job of enumerating what comes back (current index, daily max, category, advice). For a two-parameter lookup that is essentially complete; it omits only units/scale of the UV index value, which is minor.

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%, with both lat and lon documented including valid ranges, so the schema carries the semantics. The description only adds a concrete coordinate example (lat=59.33, lon=18.07), which is mildly helpful but does not add format or meaning beyond 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 resource and scope: current UV index plus today's maximum at a point, along with the WHO exposure category and sun-protection advice, sourced from Open-Meteo. The resource is unambiguous against every sibling (the nearest, air_quality, returns different measurements), so an agent can select it on the first read.

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?

"Use for outdoor planning and health reminders" gives clear positive usage context. It stops short of naming when not to use it or pointing to an alternative (e.g., air_quality for other atmospheric data), so it misses the exclusion criterion for a 5.

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