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

Annotations already declare read-only, idempotent, open-world, non-destructive behavior, so the bar is lower. The description still adds real context beyond them: the data source and license (MET Norway CC BY 4.0, OpenStreetMap), the fact that no API key is needed, and the 9-day ceiling. It does not discuss rate limits or failure modes when geocoding fails.

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?

Four short blocks with no waste: capability statement first, then 'Use when:', a call example, and provenance/auth info. Front-loaded and easy to scan.

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 burden of telling the agent what comes back, and it does so by enumerating all returned fields plus the hourly 12-hour block. Combined with the annotations covering the safety profile and the fully documented schema, nothing an agent needs to call this correctly is missing.

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 schema already documents location, lat/lon override, days, lang and units in detail. The description adds only the 'up to 9 days' framing and an example invocation {"location":"Paris","days":3}, which is marginal beyond structured data — baseline 3 applies.

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?

The description names a specific verb and resource (current weather + forecast) and bounds the scope precisely: 'up to 9 days', 'any city, address or coordinates worldwide'. Enumerating the returned quantities (temperature, conditions, rain, wind, humidity, UV index, hourly for 12h) makes it unmistakably distinct from the pollen_and_air_quality and sun_times siblings.

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?

A dedicated 'Use when:' line gives concrete triggers (weather, forecast, rain, temperature, wind; today/tomorrow/this week). It stops short of explicit exclusions or naming the sibling tools as alternatives for air quality or sunrise/sunset questions, which keeps it below a 5.

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