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

The annotations already cover the safety profile (readOnlyHint, idempotentHint, destructiveHint=false), so the description's job is to add context -- and it does, disclosing the upstream source (Defra UK-AIR), a 1-credit cost, and the important caveat that a single monitor may not represent the user's street. It stops short of saying what happens when no working monitor falls inside max_km, which is the main unresolved behavioral question for a nearest-monitor lookup.

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 sentences, each doing distinct work: what is returned, when it is useful, provenance, and the caveat plus cost. The pollutant and time resolution are front-loaded and nothing is 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 it well by enumerating the reading, the 24-hour mean, and the DAQI band with health advice. The gap is edge-case coverage -- behavior when the nearest monitor is beyond max_km or offline is unstated.

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%, and the schema already documents lat/lon UK bounds and the max_km default of 25 with a 100 km cap. The description only restates the radius concept via 'nearest working ... monitor' and adds no format or tolerance detail beyond the schema, so the 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?

States a specific resource and measurement: the latest hourly PM10 reading in ug/m3 at the nearest working Defra AURN monitor, plus the 24-hour mean and the DAQI band with health advice. The pollutant is named outright, which lets an agent separate it from pm25_level_near, no2_level_near and ozone_level_near 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 contexts (Saharan dust episodes, construction and road dust) and an explicit exclusion, 'Not a forecast', which steers the agent away from forecast-style questions. What it does not do is name the specific alternative to call instead, so the routing is implied rather than spelled out.

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