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

Beyond the readOnlyHint and openWorldHint annotations, the description discloses failure behavior (404 for nonexistent postcodes), billing behavior (failed calls not charged), price ($0.002 per successful call), payment method (x402 on Base, Solana, or Arc), and source-specific output differences for GB versus other countries.

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 description is compact and front-loaded, starting with the core purpose and then adding source behavior, error handling, and pricing. Every sentence adds useful information without repetition.

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?

Given that an output schema exists and annotations cover read-only/open-world behavior, the description provides everything else an agent needs: country-specific source behavior, failure handling, and payment details. Nothing essential for correct invocation is missing.

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 both parameters are documented in the schema. The description adds meaningful context for the country parameter by explaining that GB uses postcodes.io and provides extra district/ward/constituency data, while other countries use Zippopotam.us for ZIP and postal codes.

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 clearly states the resource and returned data: place names and coordinates for a postcode. It implicitly differentiates from broader geocoding by focusing on postal-code lookups, but it does not explicitly name a sibling alternative or use a direct verb phrase.

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 explains country-specific behavior (GB gets district/ward/constituency via postcodes.io; other countries use Zippopotam.us) and notes that nonexistent postcodes return 404. However, it gives no explicit guidance on when to choose this over siblings such as geocode, places, or address_parse.

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