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

Goes well beyond the readOnly/idempotent/openWorld annotations by disclosing the per-call price ($0.01 USDC via x402 on Solana), the live external dependency on VIES, and the fact that trader name/address are only returned where the member state discloses them. The GR→EL and XI→Northern Ireland normalization rules are behavioral details an agent cannot infer from structured fields.

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 dense sentences, front-loaded with the core purpose before edge-case format rules, pricing, and the alternate endpoint. Every clause (VIES, input forms, country mapping, price, HTTPS URL) carries information an agent would otherwise lack.

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?

With no output schema, the description still tells the agent what comes back (validity flag plus optional trader name/address) and flags the partial-disclosure caveat. Pricing, external dependency, and normalization rules cover the remaining gaps for a 2-parameter read-only lookup.

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 coverage is 100%, so the baseline is 3; the description still adds value by documenting the country-prefix mapping conventions (GR mapped to EL, XI meaning Northern Ireland) that the schema does not state. It reinforces the required prefix-with-optional-country pattern but adds little beyond that example.

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 ('Validate an EU VAT number') plus the authoritative source (European Commission VIES) and the return payload (valid/invalid, trader name and address). An agent can distinguish this from siblings like iban_validate or validate_batch without opening a schema.

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?

Explains accepted input shapes ('IE6388047V style input or ?country= + number') and offers a fallback access path over HTTPS, giving clear context for invocation. It stops short of explicitly naming when to prefer a sibling (e.g. batch validation) or when not to use it.

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