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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds behavior beyond that: it discloses what data comes back (prices, threshold, dispatch times), that the postcode is only used for the local-area check, and that international delivery is unsupported — the kind of scoping an agent cannot infer from annotations.

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 tight sentences, front-loaded with what is returned, then the optional postcode behavior, then the scope exclusion. No filler and no restatement of the name or title.

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-value detail is not required in prose. Given two simple flat parameters and full annotation coverage, the description supplies everything an agent needs: what it returns, how item_count and postcode affect results, and the UK-only constraint.

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% and both parameters are already documented with bounds and an example postcode, so the baseline is 3. The description adds modest meaning by tying item_count to the quoted prices ('prices for an order of item_count slimes') and by framing the postcode as an optional area check, which clarifies the interaction between the two parameters.

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 names the specific resource (UK delivery methods and prices), the driving parameter (item_count slimes), the concrete methods covered (Royal Mail 48 / 24), and the extra data returned (free-delivery threshold, dispatch times). It also states the hard scope boundary — no delivery outside the UK — so an agent can tell at a glance this is the UK shipping-cost tool and not a catalog or basket tool.

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 gives a clear conditional instruction for the optional parameter ('Add a UK postcode to check the free local delivery area') and an implicit when-not via 'The shop does not deliver outside the UK.' It does not point to a sibling for non-UK or basket-related needs, but the exclusion is explicit enough that misuse is unlikely.

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