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

Adds substantial behavior beyond the readOnly/openWorld/idempotent annotations: keyless access to the official VIES service, that an outage is NEVER reported as invalid, that some member states disclose nothing and the response says so rather than returning an empty string, and the Ed25519-signed receipt auditable offline against a published key. These are exactly the failure-mode and trust details an agent cannot infer from structure.

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?

Front-loaded with the purpose in the first clause, then layered with behavior, scope, and the receipt mechanism. Most sentences earn their place, though the receipt sentence is long and somewhat elaborate relative to a two-parameter read tool.

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?

There is no output schema, so the description carries the return-value burden and does so explicitly: valid, invalid, or unknown, plus member state, register request date, and trader identity where disclosed. Nothing needed to call or interpret the result is missing.

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%, so both parameters (vat_number, country_code) are already documented with format and examples, including the GB exclusion and XI inclusion. The description reinforces the country scope but adds no syntax or format detail beyond the schema, so the baseline 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?

States a specific verb and resource ('An EU VAT number check — VAT validity, and the VAT registration behind it') and names the upstream source (VIES). An agent can immediately distinguish this from check_entity, check_sanctions, or check_document without opening any 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?

Gives clear scope boundaries and exclusions: GB is out of scope post-Brexit while XI is covered, and it explicitly disclaims 'not tax advice, not a credit check'. That narrows usage well, but it never names the sibling tools an agent should prefer for adjacent tasks (e.g. entity or sanctions checks), so the routing guidance is implicit rather than explicit.

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