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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive). The description adds value annotations don't: it is materially more expensive than valuation_estimate, it is free, and it is UK-only. It does not discuss rate limits or failure modes, but cost and geographic scope are exactly the kind of behavioral context that matters here.

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?

Heavily front-loaded: the output contents come first, then the alternative, then prerequisites. Most sentences earn their place, though the parenthetical explaining the back-series mechanics ('scaled by the local £/m² index, terminating at the headline figure') is dense and slightly more than an agent needs to select the tool.

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 an output schema present, the description need not explain return values, yet it still usefully previews the payload components. Combined with the cost note, UK-only scope, and the UPRN-acquisition path, an agent has everything required to select and call it correctly.

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% for the single uprn parameter, so the description carries the burden. It does so well by stating the tool is 'keyed on a UPRN, not an address' and describing how to obtain one via sibling tools, which prevents the common error of passing an address string. It stops short of documenting the UPRN format or integer bounds.

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 resource (full valuation block for a property) and enumerates what it returns: value and range, monthly back-series, rental estimate, and trust_score. It explicitly distinguishes itself from valuation_estimate ('if you only need the number'), so the agent can separate it from a sibling without reading schemas.

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 an explicit alternative and the condition selecting it ('Use valuation_estimate if you only need the number; this is materially more expensive'). It also spells out the prerequisite pipeline (address_autocomplete then address_resolve) and the UK-only constraint, so the agent knows when and how to invoke it.

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