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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), so the description's marginal value is the "UK only" coverage boundary, the "Free" cost signal, and the crucial warning that the key is a UPRN rather than a free-text address. It doesn't discuss return shape or error behavior, but output schema exists so that is largely covered elsewhere.

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?

Three short fragments, zero filler: purpose, cost/scope, then input semantics and escalation path. The routing instruction is front-loaded in the same breath as the key constraint.

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 single-parameter, read-only lookup with a published output schema, the description supplies everything an agent needs: scope, cost, what the key is, and how to obtain it. Nothing material is missing.

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% and the only property is a bare integer named "uprn", so the description carries the burden — it clarifies that the value must be a UPRN identifier and points to address_resolve as the way to procure one. It stops short of describing the UPRN's format or numeric range, which is the remaining gap.

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 and resource ("Look up an address by UPRN") plus a geographic scope constraint ("UK only"). It also explicitly distinguishes its input from address-based siblings, so an agent can tell it apart from address_autocomplete and address_resolve without opening any schema.

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 routing recipe: use address_autocomplete to narrow, then address_resolve to obtain the UPRN this tool requires. This names the alternatives and the condition that selects them, leaving nothing to inference.

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