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

Goes well beyond the readOnly/openWorld/idempotent annotations: it discloses that this is a live call against a third-party registry, that data may be partial ('when published'), and — critically for a paid tool — the exact cost model (0.003 USDC via x402 or 2 credits) and the free 5-calls-per-day allowance that requires no key. These are the operational facts an agent needs before spending money.

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?

Opens with the trigger condition, then layers data source, result content, scope, and cost in a tight sequence. Every clause carries operational information — no filler or repetition of the title.

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 correctly compensates by describing the return content (registered/valid flag plus trader name and address when published). Payment and free-tier mechanics are fully covered. Minor gap: it does not say what a not-found or malformed number yields, though 'registered and valid or not' implies a negative result is possible rather than an error.

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% and the schema already documents the single 'eori' parameter with an example (IE6388047V). The description adds only the coverage constraint (EU only, no GB), which is useful scope information but not additional parameter syntax or format guidance. 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 ('check an EU EORI customs number'), names the authoritative data source (European Commission's EORI service), and describes what comes back: registered/valid status plus trader name and address. The resource is clearly distinct from siblings like eu_vat_check or company_lookup, and regional scope plus the explicit 'GB numbers are not covered' exclusion removes ambiguity.

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?

"Use when you need to check an EU EORI customs number" gives a clear trigger, and the GB exclusion tells the agent when NOT to use it. However, it never names a sibling alternative (e.g. eu_vat_check for VAT numbers), so the agent must infer routing itself.

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