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

With no annotations, the description carries the full burden and does well: it discloses the paid x402 flow, the exact price ($0.002 USDC/call), the chain (eip155:8453), the upstream data source, and the no-charge rule for unknown postcodes. It omits whether lookups are cached or rate-limited and what an unknown postcode actually returns, which keeps it short of a 5.

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?

One dense sentence that front-loads the cost/transport tag, then the purpose, then the return surface. Efficient, though the bracketed payment prefix is visually noisy relative to the lookup itself.

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?

No output schema exists, and the description compensates by enumerating the returned fields (lat/lon, country, region, local authority, ward, constituency, NHS ICB, police force, LSOA). Combined with the payment disclosure, an agent has what it needs to decide and call, though the shape of an unknown-postcode response is left open.

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% and the single parameter already has a format example ('SW1A 2AA'), so the schema does the work. The description adds no normalization rules (spacing, casing, partial postcodes), so 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 ('UK postcode lookup') and backs it with the data source (ONS Postcode Directory via postcodes.io) and the exact field set returned. The UK qualifier implicitly separates it from the us_geocode sibling, but the distinction is never made explicit.

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?

There is no statement of when to choose this tool over us_geocode or any other geo tool, and no prerequisites. The only conditional language ('Unknown postcode = not charged') is billing behavior, not invocation guidance.

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