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

With no annotations, the description carries the full burden and does well: it discloses the external dependency (official EC EOS service) and the conditional return behavior ('trader name/address only when the trader consented to publication'). It says nothing about rate limits, auth requirements, or failure modes for malformed numbers, so it is not complete.

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, front-loaded with purpose, then usage, then behavior – no filler. The final clause ('XI EORIs are') is truncated and ambiguous, which slightly undercuts the structure.

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, and the description does explain the return shape (valid/invalid plus conditional trader name/address), which is the key thing an agent needs. Missing error semantics for invalid formats or service outages, but nothing essential to making the call is absent.

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 0% and the single property is bare ('Eori' string), so the description must compensate. It supplies two concrete format examples and the GB/XI scope carve-out, which materially helps an agent construct a valid input, though it never states the expected format rules explicitly (length, country prefix pattern).

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 ('Validate an EU EORI number') with concrete format examples ('SE5560360793', 'DE1234567890123') and names the authoritative source (European Commission EOS). It does not explicitly distinguish itself from the sibling verify_eori_batch, though the singular/batch naming makes the split reasonably inferable.

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 concrete use contexts ('import/export, customs declarations and freight-forwarder onboarding') and useful exclusions ('UK (GB) EORIs are not in EOS (HMRC checks those)'). It stops short of naming alternatives such as verify_vat or verify_eori_batch for related-but-different checks.

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