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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the bar is lower, yet the description adds substantial context: live querying of the EC service, partial return data (name/address only when published), and a detailed cost model (0.003 USDC via x402, 2 credits per Factline key, 5 free paid-tool calls/day). This is exactly the payment/auth detail an agent needs before invoking.

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?

The trigger condition is front-loaded and the sentences are dense but purposeful; each covers a distinct concern (scope, coverage gap, pricing, free tier). It is a long single block, but no sentence is wasted.

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, and the description compensates by stating what comes back (registered/valid status plus trader name and address when published). With pricing, auth options, and geographic coverage all covered, an agent has everything needed to call correctly.

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 both parameters are documented (the eori example and the api_key provenance from get_trial_key), so the schema carries the load. The description reinforces the payment/key paths but adds no syntax or format detail beyond the schema, so 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') with scope qualifiers (all EU member states, GB excluded, live validation against the European Commission's EORI service). This clearly distinguishes it from near siblings like eu_vat_check or eu_company_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?

'Use when you need to check an EU EORI customs number' plus the explicit exclusion 'GB numbers are not covered' gives clear when-to-use and when-not context. It does not name an alternative tool for non-EU/GB checks, so it stops short of a 5.

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