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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered. The description adds genuinely useful non-annotation context: the cost model (free within the daily allowance, then $0.002 USDC per call via x402), which tells the agent about quota and payment behaviour. It stops short of noting any caching or rate-limit specifics.

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 tight sentences: what you get first, then the pricing constraint. Every clause earns its place with no filler.

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?

An output schema exists, so return values need no elaboration, and the description still previews the key fields plus the billing model. For a single-parameter lookup with full annotation coverage, this is essentially complete; only sibling differentiation is absent.

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?

There is a single parameter with 100% schema description coverage including a worked example ('SW1A 1AA'), so the schema carries the semantics. The description adds nothing about the postcode format beyond what the schema already provides, making the baseline 3 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 ('Full ONS record for one UK postcode') and enumerates what the record contains (latitude/longitude, administrative areas), so an agent knows exactly what it gets back. It does not name or differentiate from siblings like uk-postcodes__validate, __reverse, or __nearest, which is the only thing keeping it from 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 only implied: the tool takes a postcode, so it is for looking up a postcode you already hold. There is no explicit when-to-use/when-not, and no routing to alternatives such as validate (check existence) or reverse (coords to postcode), even though several close siblings exist.

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