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

With no annotations, the description carries full burden for behavioral disclosure. It does not mention network calls for VIES validation, return formats, side effects, or limitations. The phrase 'VIES validation' is unexplained, leaving the agent unaware of what the tool actually does beyond a vague summary.

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?

The description is extremely short but is a fragment without a main verb, not a well-structured sentence. It under-specifies rather than being efficiently concise, omitting essential information about functionality and usage.

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?

Given the tool has 3 parameters and no output schema, the description is wholly insufficient. It does not indicate what the tool returns, how VIES validation is performed, or how errors are handled. The agent cannot determine whether to use this tool or how to interpret results from this description alone.

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?

The description adds no information about the parameters 'amount', 'country', or 'direction'. Schema coverage is 67% (amount and country have descriptions, direction does not), but the description does not compensate for the missing direction semantics or clarify how the add/extract modes work. It repeats no schema detail and adds negligible value.

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 'VAT rates by country plus VIES validation' is a noun phrase without an explicit verb. It does not clearly state that the tool calculates VAT (e.g., adds or extracts) on amounts; it could be interpreted as a lookup. It vaguely distinguishes from siblings but fails to convey the core calculator behavior.

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?

No guidance is provided on when to use this tool versus the many sibling calculators (e.g., canadian-sales-tax-calculator, us-sales-tax-calculator). It does not mention use cases, prerequisites, or alternative tools.

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