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

Annotations already declare read-only, idempotent, open-world and non-destructive behavior, so safety is covered. The description adds genuinely new behavioral context: the cost model ('free within the daily allowance, then $0.002 USDC per call via x402'), which an agent needs to reason about budget-limited invocation. It does not mention failure behavior for unmatched postcodes.

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?

A single front-loaded sentence: the returned data leads, and pricing follows. No wasted words, and nothing important is buried.

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, return values need not be explained, and the description adequately frames the lookup. The one gap is that all three parameters are optional yet the description/schema do not state that at least a postcode or a lat+lng pair must be supplied.

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 lat, lng and postcode, including the 'or give lat and lng' alternative. The description adds no format or constraint detail beyond what the schema provides, so it sits at the baseline of 3.

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 enumerating exactly what is returned (region, district, planning authority, ward, parish, built-up area plus ONS codes) for a postcode or point. This implicitly differentiates it from the per-area siblings (uk-planning__region, uk-planning__ward, uk-planning__parish), but no sibling is named explicitly, so it stops short of a 5.

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?

Usage is implied by 'for a postcode or point' – an agent can infer this is the point-to-all-admin-areas resolver – and the pricing sentence gives cost context for deciding to call it. However, there is no explicit when-to-use versus the individual uk-planning/uk-geography area tools, and no stated alternative or exclusion.

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