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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds genuinely useful behavior context beyond that: validation is local/syntactic only, with no VIES or tax-agency lookup, so the agent knows a pass does not imply a live registered VAT number.

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 short sentences, no filler, with the core constraint (format-only) front-loaded before the usage and exclusion guidance. Every sentence carries distinct information.

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, closed-world, read-only validator with no output schema, the description covers everything an agent needs: what is checked, what is deliberately not checked, and where to go for the fuller check.

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 parameter with 100% schema description coverage ('Country-prefixed VAT number string such as DE123456789'), so the schema already documents the format and example. The description reinforces 'country-specific format' but adds no syntax or normalization detail beyond the schema.

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/screen) and resource (EU VAT ID, country-specific syntax) and explicitly bounds the scope to format rather than registration status. An agent can distinguish it from siblings like is-iban-safe or is-us-tin-safe by the named jurisdiction and check type.

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 an explicit when-to-use ('screen a test VAT ID for country-specific syntax') and an explicit when-not with the alternative ('to confirm tax registration or active status, query an official VAT register separately'). The routing decision is fully specified.

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