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

Annotations already declare readOnly/idempotent/openWorld/non-destructive, and the description adds genuinely new behavioral facts: the x402 payment requirement and cost ($0.001 USDC per call), the error contract (400 invalid format, 404 not found), and input normalization ('Spaces optional'). This is meaningful context an agent could not derive from the annotations alone.

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?

Front-loads the purpose and return fields, then the when-to-use cases, then the format/error/billing details. Every sentence carries distinct information (outputs, triggers, normalization, errors, price) with no filler or 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?

With no output schema, the description carries the burden of describing results and does so by listing the returned geography fields. It also covers error codes and the payment model, so an agent has everything needed to call and interpret this tool.

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% for the single postcode parameter, so the baseline is 3, and the description adds the non-obvious normalization rule that spaces are optional (e.g. 'SW1A1AA' vs 'SW1A 1AA'). That is real added meaning about accepted input formats, though no further constraints are described.

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 ('look up any UK postcode') and enumerates the returned attributes (lat/long, council, county, ward, parish, constituency, region, country). It also names the data source (official ONS data), which cleanly separates it from the unrelated siblings like uk_company_check.

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?

Gives explicit usage contexts: 'place a UK address on a map, work out which council or MP covers it, or check a postcode is real.' That is clear when-to-use guidance, though no when-not-to-use case or named alternative is offered (none of the siblings are plausible substitutes anyway).

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