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

Annotations already declare readOnly/idempotent/openWorld, but the description goes well beyond them: it discloses the upstream source (MET Norway/yr.no), the geocoder (OSM), the pricing model ($0.001/call, 10 free/day), the payment-required result with x402 options, and the error contract (isError on invalid input or upstream failure, not charged). This is exactly the kind of behavioral context annotations cannot carry.

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?

Purpose is front-loaded in the first clause, followed by data fields, source, time handling, price, and errors — every sentence carries distinct information. It is one dense block rather than grouped sections, but there is little waste.

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 annotations covering the safety profile and an output schema covering return shape, the description fills the remaining gaps an agent needs: input formats, source model, time semantics, billing, and failure behavior. Nothing material is missing for correct invocation.

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 the baseline is 3, but the description adds value beyond the schema: it clarifies that place names are geocoded with OpenStreetMap, that lat,lon is accepted, gives an IANA example (Australia/Sydney), and reiterates the 9-day cap. The detail is mostly reinforcing rather than net-new, keeping it at 4.

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 ('Current weather and hourly/daily forecast') with scope ('for any place name or lat,lon'), the exact data fields returned, and the horizon ('up to 9 days'). An agent can immediately tell this apart from adjacent siblings like sun_moon or time_convert.

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 clear context: worldwide coverage, place-name geocoding, lat/lon input, and tz handling for local times. However, it never states when NOT to use it or names an alternative sibling (e.g. geocode for pure geocoding), so it stops short of explicit routing guidance.

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