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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, open-world), and the description adds substantial context beyond them: the 1-credit cost, the DAQI band and averaging methodology, the use of the nearest working AURN monitors, the fact that different pollutants may come from different nearby sites, and the explicit caveat that this is not Defra's published index or the forecast. That is meaningful provenance and expectation-setting for a read 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?

The answer the tool provides is front-loaded, and the second clause packs provenance, methodology and caveats without filler. It is a dense run-on sentence with a parenthetical cost note, slightly harder to scan than a short set of bullet-style clauses, but every element earns its place.

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 read-only tool with no output schema, the description sufficiently describes what comes back (index, band, health advice, driving pollutant) and how it is computed. The main omission is behavior when no working monitor falls within max_km (error vs. degraded answer), and the required/optional status of lat and lon is left slightly ambiguous versus the schema.

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%, so the schema already documents lat, lon and max_km including ranges and the default of 25 km. The description still adds semantic value by explaining that monitors are 'nearest working' sites and that the km radius is applied per pollutant across possibly different sites, which clarifies how max_km actually behaves. It does not add anything for lat/lon, hence not a 5.

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-like outcome (return the overall UK DAQI for a location) and a concrete resource (Defra AURN monitor measurements of NO2, ozone, PM2.5, PM10), plus what the response contains (index 1-10, band, health advice, driving pollutant). It is distinguishable from per-pollutant siblings such as no2_level_near or pm25_level_near because it explicitly presents itself as the aggregate 'one answer' rather than a single measurement.

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 framing 'what is the air quality near me today' gives clear when-to-use context and implicitly routes agents wanting a single headline answer here rather than to the per-pollutant or history tools. It also excludes the forecast and Defra's own published index as alternatives. It never explicitly names those sibling tools or states when not to use this one, so it falls short of a 5.

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