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

Annotations already establish read-only, idempotent, non-destructive behavior. The description adds useful operational context beyond the annotations: cost (1 credit / 1p), free access on marketcode.ai, and the UK-only constraint.

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 short and front-loaded: purpose, scope, cost, then the key usage distinction/alternative workflow. There is no wasted text.

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 one-parameter read-only lookup with an output schema and rich annotations, the description covers the important missing context: cost, geography, UPRN keying, and the alternative lookup path.

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 0%, so the description must carry the burden. It does add meaning by clarifying that the key is a UPRN rather than an address and by pointing to `address_resolve` as a way to obtain one, though it does not give an example format or range.

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 a specific verb and resource: look up an address by UPRN. It adds scope ('UK only') and distinguishes this from address-based lookup by saying it is 'Keyed on a UPRN, not an address.'

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 explicitly tells the agent what to do when it does not already have a UPRN: use `address_autocomplete` to narrow and then `address_resolve` to obtain one. It also names the relevant alternative tools directly.

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