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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety is covered. The description adds genuinely useful context beyond them: the per-call cost ($0.003 USDC via x402) and the shape of what comes back for a VAT check. It does not say what happens when a number is invalid vs. the register being unreachable, which is why it is not a 5.

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?

Three sentences, zero filler, front-loaded with the core capability before the use case and the price. The cost note in particular is placed where an agent will read it before deciding to call.

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?

No output schema exists, yet the description discloses the key return fields for the VAT path, which compensates well. The remaining gap is the zero-required-parameter design: nothing tells the agent whether a call with only iban, or with no arguments at all, is valid or useful.

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 only 33% (just the vat example), so the description has to carry more weight. It does map each of the three fields to an operation (VAT register lookup, IBAN checksum, BIC format validation), which is real added meaning, but it never addresses that all three are optional, how they combine, or what format is expected for iban/bic.

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?

Names a specific verb+resource ('EU VAT number check against the official VIES register') and spells out exactly what it validates and returns (validity, company name, address), then extends to IBAN checksum and BIC/SWIFT format validation. An agent can distinguish this from siblings like uk_kyb, uk_company or company_registry 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 a concrete usage context: 'Check a supplier before paying an invoice.' That is a clear when-to-use signal tied to a real workflow. However, it offers no exclusions or named alternatives (e.g. when to use company_registry or sanctions instead), so it falls short of the 5-level bar.

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