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

No annotations exist, so the description carries the burden and does substantial work: it discloses the data source (Norwegian Meteorological Institute), the exact model run, declared units, the per-day return fields, and the pricing model ($0.01 per call, paid over x402 in USDC). This is unusually rich for a tool with zero annotation coverage. It stops short of describing latency, error behavior, or rate limits.

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?

Three dense sentences, front-loaded with the capability and scope before elaborating on return fields and pricing. Every sentence earns its place, though the return-field enumeration is slightly list-like and could be tightened.

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?

For a 3-parameter tool with no output schema and no annotations, the description covers source, scope, return contents, units, model provenance, and payment mechanism. What's missing is the output shape (forecast array vs daily objects) and error behavior, but the coverage is strong given the tool's complexity.

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% and the schema already documents lat/lon ranges, rounding behavior, the days 1-9 range, the default of 5, and the partial-final-day caveat. The description adds no parameter-specific syntax or format that isn't already in the schema, so baseline 3 is appropriate.

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 ('Day-by-day forecast for any point on Earth, up to nine days out') and immediately distinguishes the source from a 'commercial aggregator'. The sibling weather_now is implicitly differentiated by the multi-day versus current scope.

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 'up to nine days out' and 'any point on Earth' scope implicitly tell the agent when this applies, and the mention of commercial aggregators hints at source preference. However, the description never explicitly names weather_now as the alternative for current conditions or states when-not-to-use this tool.

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