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

Annotations already cover readOnly, idempotent, openWorld and non-destructive, so the bar is lower; the description still adds real context: the upstream source (MET Norway Locationforecast 2.0), that values are model forecast percentages of sky covered, that fog is only available for the first few days, and that a call costs 1 credit. It does not mention rate limits or forecast-update cadence, which keeps it below a 5.

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 the tool returns, then use cases, then source and constraints; every sentence carries information. The parenthetical cost note and the mixed prose list make it slightly dense, but nothing is truly redundant.

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: it names the cloud layers returned, the fog extra, the clearest-hour pick, and the units (percent of sky covered). Combined with the prerequisite routing and the fog time limitation, an agent has everything needed to call and interpret it.

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 all three parameters (lat, lon, hours) are already documented with types and ranges. The description restates the lat/lon requirement and the 1-48 hour window but adds no new syntax, format, or edge-case guidance beyond the schema, which is the baseline-3 case.

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 and resource: hourly cloud cover forecast for the next 1-48 hours at a point, broken into total/low/medium/high cloud plus fog, with the clearest hour identified. That level of specificity makes it distinguishable from generic siblings like weather, rain_forecast_next_hours, or aurora_chance_at_location without opening any schema.

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 use cases (stargazing, astrophotography, aurora, sunset photography, solar panel output) and an explicit prerequisite chain: get coordinates from geocode or postcode_lookup for UK place names or postcodes. It does not explicitly say when to prefer this over the sibling `weather` tool or how it complements aurora_chance_at_location, so it stops short of full alternative routing.

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