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

Annotations already cover safety (readOnly, idempotent, non-destructive, open-world), so the description's job is to add context — and it does: it discloses the external fetch, the two authoritative registries consulted (VIES, national register), the hashed-account IBAN matching, cost ('Free'), and the four possible outcomes. It stops short of explaining how 'review' differs operationally from 'verified'/'invalid_card', which is the one gap.

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 cost and usage trigger, then mechanism, then return values, then inputs — a sensible order. It is dense and slightly run-on in the mechanism sentence, but every clause (spec URL, VIES, register, hashed accounts, four outcomes) carries information an agent needs.

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 two-parameter, read-only, open-world tool with full schema coverage and no output schema, this is nearly complete: it says what is fetched, what is cross-checked, what the possible statuses are, and that it is free. What's missing is guidance on interpreting 'review' versus 'invalid_card' and what to do next in each case.

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, but the description adds real meaning: it labels iban as optional ('with iban says whether...') and explains it is matched against the supplier's own hashed accounts rather than merely checked for validity. domain's role as the card host is also made explicit.

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 — reads https://<domain>/.well-known/supplier.json, cross-checks VAT in VIES and company number in the register, and optionally matches an IBAN against the supplier's hashed accounts. This is clearly a composite card verification, distinguishable from single-concern siblings like iban_check, eu_vat_check, or domain_check.

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 explicit timing and a precondition: 'Use before onboarding or paying a supplier that has a website.' That condition implicitly excludes suppliers without a site, but it never names alternatives (e.g. eu_vat_check for suppliers lacking a card, or supplier_invoice_check for invoice-level checks), so routing to siblings is left to inference.

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