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. The description adds meaningful context beyond annotations: the result contains centroid and administrative areas, and the pricing model (free within daily allowance, then $0.002 USDC per call via x402). It does not cover authentication details or response structure, but the output schema handles the latter.

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?

Two sentences with zero waste: the first front-loads the payload, the second front-loads the billing terms. Every sentence earns its place and the structure is easy to parse quickly.

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 simple single-parameter lookup with an output schema and rich annotations, the description covers purpose, input format, and billing. The only notable gap is usage guidance relative to the many sibling postcode tools, but that does not prevent a correct invocation.

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?

The schema fully documents the single parameter as 'Outward code', but the description adds a concrete example ('SW1A') and equates an outward code with a postcode district. This clarifies format and granularity beyond the schema's terse label, which is useful even at 100% schema coverage.

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 states exactly what is returned — centroid and administrative areas — and scopes it to a postcode district (outward code), with an example 'SW1A' that makes the resource granularity clear. It implicitly distinguishes itself from full-postcode siblings like uk-postcodes__lookup, but does not name an alternative or use an explicit verb like 'Get'.

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 description gives no guidance on when to choose this tool over siblings such as lookup, reverse, nearest, autocomplete, validate, places, or terminated. The input scope (outward code) is implied, but there are no exclusions, prerequisites, or alternative-routing instructions.

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