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 useful context beyond that: it is free, needs no key, and returns validity plus registered trader name and address. It omits operational caveats like VIES availability/rate limits, keeping it out of the top score.

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 with the core action and source front-loaded, followed by the usage boundary. Every clause carries information; nothing is redundant.

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 values need not be detailed, and the description still sketches them (validity, name, address). Combined with annotations and a fully documented two-parameter schema, an agent has enough to invoke it correctly; only external-service caveats (outages, rate limits) are 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 description coverage is 100% and both parameters carry clear examples (e.g. '6388047V', 'IE'), so the schema does the heavy lifting. The description adds no parameter-level detail such as format constraints or case sensitivity, so the baseline 3 applies.

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 (validate) and resource (EU VAT number) and names the authoritative source (European Commission VIES service). It is clearly distinguishable from company-register siblings like gleif_lei or companies_house_company.

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 a clear use case ('verify an EU business VAT identity') plus an explicit exclusion ('not a full company register'), which helps route the agent away from it for broader company data. It does not, however, name the specific sibling to use instead (e.g., gleif_lei), so it stops short of full alternative routing.

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