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

With no annotations, the description carries the full behavioral burden. It does disclose useful behavior: two validation modes (offline syntax/checksum vs live registration check) and that rates are returned. However, it omits failure semantics (does an invalid number error or return a flag?), the fact that live checks depend on an external service subject to availability/rate limits, and any permission requirements.

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?

One front-loaded sentence that leads with the core action and appends the optional live mode as a trailing clause. No filler, no restatement of the 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?

The tool is simple (1 required param, no nesting) and the description does partially compensate for the absent output schema by naming what is returned (standard and reduced VAT rates). Still, with no output schema and no annotations, an agent lacks detail on the response shape, invalid-number behavior, and live-mode failure modes.

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 only two parameters exist, so the schema already documents both vatNumber (format, prefix example) and live (VIES/HMRC verification). The description restates the live semantics but adds no format, casing, or country-prefix detail beyond the schema. 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/UK VAT number) plus the mechanism (VIES/HMRC syntax and checksum rules) and the payload (standard and reduced VAT rates). This is clearly distinct from the sibling validators (validate_iban, validate_lei, validate_iata_awb, etc.) without needing to name them.

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 description signals the base use case and explains the conditional path: 'optionally verify live registration status against VIES or HMRC,' which tells the agent when the live flag matters. It stops short of explicit exclusions or fallback guidance (e.g., what to do when VIES is unavailable), so no 5.

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