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 non-obvious behavior: the tool checks 'live' postcode data (not just syntax) and has an explicit pricing/allowance model. It omits whether input is normalized or trimmed, but the pricing disclosure is real value beyond the 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?

Two tight sentences: the semantic meaning ('valid, live') is front-loaded, and the cost model follows. Nothing extraneous.

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, boolean-style validation tool with an output schema present, the description covers purpose, semantics ('live'), and cost. Return values need not be explained since an output schema exists. Minor gap: no statement about input normalization or expected postcode format.

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?

One parameter with 100% schema description coverage, so the schema already documents it. The description adds only 'a string' as the input type, which is a marginal restatement of the schema's `type: string`. Baseline 3 is 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 check (validity of a UK postcode string) with the added qualifier 'live', which distinguishes it from a pure format check. It is clearly distinguishable from lookup/autocomplete siblings by implication, though no sibling is named explicitly.

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 implied: this is the pre-flight check you run before feeding a postcode to lookup, reverse, or nearest. The cost condition ('free within the daily allowance, then $0.001 USDC') gives practical context for when to call. However, no alternative is named and no when-not guidance is given (e.g., 'use lookup if you also need location data').

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