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

Annotations already cover readOnly/idempotent/openWorld/non-destructive, so the bar is lower. The description adds genuinely non-structured behavior: free within the daily allowance, then $0.002 USDC per call via x402, plus a capped-geometry caveat. It does not detail error or return behavior, but the pricing/quota context is valuable.

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?

Three short sentences, front-loaded with the resource identity, then input modes, then pricing. No filler; every sentence carries information.

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 an output schema present and annotations covering the safety profile, the description need not explain return values. It supplies input modes and cost behavior, leaving only explicit sibling routing unaddressed.

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 each of the 5 params is already documented. The description only restates the postcode-or-latlng input choice, adding no syntax or format detail beyond the schema. Baseline 3 is appropriate.

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: returns the ONS built-up area (town/city) containing a point. This is distinct from the many sibling constraint datasets (green-belt, flood-zone, etc.), but the description never explicitly contrasts itself with them, so an agent relies on the name to disambiguate.

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?

It says how to supply the point ('by UK postcode or lat/lng') and discloses the pricing threshold, which is real usage context. However there is no explicit when-to-use-vs-alternatives guidance against the sibling designation tools.

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