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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, no open world), and the description adds meaningful behavior beyond that: it enumerates the return shape {ok, country?, normalized?, issues} and explicitly disclaims VIES lookup and active-registration checks, aligning with openWorldHint=false. This tells the agent exactly what the tool will and will not verify.

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 the core action, followed by usage and scope-limitation. Every clause earns its place: no restatement of the name, no filler.

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 read-only validator with no output schema, the description supplies the return shape, the usage context, and the scope limitation. An agent has everything needed to call it and to interpret its result without further inspection.

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 the single 'vat' parameter is documented with a format example ('DE123456789'), so the schema carries the load. The description's phrase 'country-specific patterns' implies prefix handling but adds no syntax or country-list detail beyond the schema. 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 verb and resource ('Check country-specific VAT number patterns') and scopes it to format validation rather than registration. The 'no VIES lookup' clause immediately distinguishes it from a real VAT verification service. Against a sibling set of is-*-safe format validators, the resource (VAT number) alone makes it identifiable.

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 concrete usage context ('Use to screen VAT fixtures before form submission') and an explicit boundary ('no VIES lookup or active-registration check'), which tells the agent when this tool is insufficient. It does not name an alternative tool for active-registration checks, so it stops short of a full when/when-not/alternative triad.

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