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

Annotations already carry the safety profile (readOnlyHint, idempotentHint, destructiveHint=false), and the description still adds behavior the annotations cannot express: the outcome is a format/checksum result rather than a registration confirmation, and the cost model is 'free within the daily allowance, then $0.001 USDC per call via x402.' That quota and pricing disclosure is genuinely useful for deciding whether and how often to call.

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 sentences with no filler, and the capability plus scope statement is front-loaded ahead of the pricing note. Every clause carries information an agent needs.

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 one-parameter, read-only tool with an output schema, the description covers what is checked, what is explicitly not checked, the supported country scope, and the cost model. Little is missing, though it does not say what happens on an unsupported country prefix or what the failure result looks like.

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?

There is a single required parameter and schema description coverage is 100%, so the schema already documents the 'VAT number with country prefix' requirement; the baseline here is 3. The description adds only the accepted country-prefix set (GB/XI and EU), which is a marginal supplement rather than new syntax or format detail.

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 and the exact verification it performs (country format plus check digit) and scopes it to UK (GB/XI) and EU VAT numbers, which separates it cleanly from vat__rates and vat__calculate. It stops short of a leading verb, but the scope statement is precise enough that an agent can identify the tool without reading the 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?

The sentence 'Does not confirm registration' is an explicit when-not boundary that prevents the most likely misuse of a VAT-number tool. It does not, however, name the sibling tools or state when to prefer this over any of them, so the routing guidance is implied rather than explicit.

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