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 new behavioral context beyond them: the lookup is against a live external system (implying network dependency and possible latency/outage), it enriches with known company profiles, and it carries a per-call cost of $0.005 after 5 free calls per day.

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 compact sentences with zero filler: the core operation and its authoritative source come first, and the pricing caveat is correctly subordinated in parentheses. Every clause earns its place.

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 (one param), an output schema exists so return values need no explanation, and annotations cover side effects. The only remaining gap is failure semantics — whether a non-registered VAT number yields a false result or an error — which the description leaves unstated.

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?

With 100% schema description coverage and a single required parameter, the schema already documents the vatNumber format including the 2-letter country prefix. The description adds no parameter-level meaning beyond that, so the baseline of 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?

The description names a specific verb ('Validates') and resource ('European Union VAT identification numbers') and specifies the authoritative source (the live official EU VIES system). It does not explicitly differentiate itself from siblings like check_vendor or batch_resolve, so it stops short of a 5.

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?

Usage is implied by the purpose: an agent can infer this is the tool for single VAT number verification. However, there is no explicit when-to-use guidance, no prerequisites, and no routing to batch_resolve for bulk checks or check_vendor for vendor-level verification, which would be the obvious alternatives to name.

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