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

Annotations already declare a safe open-world read, but the description adds genuinely load-bearing context the annotations cannot: the $0.002 USDC per-call price, the x402 payment path and supported chains (Base, Solana, Polygon, Arbitrum), that x402 MCP clients pay automatically, and that a free sample is available. That is exactly the behavioral disclosure an agent needs before invoking a paid tool.

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 purpose, then return fields, then params, legacy note, price, and sample — a sensible order with no filler sentences. It is dense but every block carries information; the only bloat is the self-contradictory days range, which is an accuracy problem rather than verbosity.

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 carries the return-value burden and does so well, enumerating current fields (temperature, feels like, humidity, wind/gusts, precipitation, cloud cover, pressure, conditions) and daily fields (min/max, precipitation sum/probability, max wind, UV, sunrise/sunset). The only gap is the unresolved 10-vs-16 day range, which leaves the caller unsure of the true limit.

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?

The description restates all four parameters, but its 'days (1 to 16)' claim directly contradicts the schema, which sets maximum=10 and notes the model only reaches ~9.5 days, and it also conflicts with the description's own opening 'up to 10 days'. The location and units explanations add useful format detail, but an agent following the stated 1-16 range would send invalid values.

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 ('Weather forecast and current weather'), the global scope ('any city or lat,lon worldwide'), the data source (MET Norway), and the horizons (10-day daily, 48h hourly). No sibling tool covers weather, so it is trivially distinguishable, and an agent knows exactly what it will get.

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 invocation guidance: location accepts a city name or 'lat,lon', it names the legacy forecast_3d tool for older clients, and it offers a free sample call with an example input. It stops short of explicit when-not-to-use or alternative-selection guidance, which is minor here since nothing else in the sibling list competes.

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