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

Annotations already carry the safety profile (readOnly, idempotent, openWorld, non-destructive), so the description only needs to add context — and it does: it names the upstream EOS service, states the batch ceiling, and discloses that name/address are returned only where the operator consented to publication. 'A service that is down costs nothing' hints at failure/cost behavior, though the wording is oblique.

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 the core question and compact at three short sentences. The closing line about downtime is a slightly cryptic use of space but does convey cost information, so nothing is outright 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?

With no output schema, the description usefully previews what comes back (name and address, conditional on publication consent) and covers the batch limit and service origin. The scalar-vs-array invocation choice is the only notable gap for an agent to resolve from the schema.

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?

At 50% schema description coverage, the single/scalar 'eori' parameter is undocumented in both places, and the description only reinforces the array cap ('up to 10 numbers per call') that the schema already states for 'eoris'. The one-or-many shape must be inferred from the anyOf in the schema rather than being explained in prose.

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?

Opens with a specific verb+resource framing ('Is this EORI number valid for EU customs?') and names the backing service (European Commission's EOS). It is distinguishable from vat_check and lei_lookup by domain, though it never explicitly contrasts itself with those siblings.

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?

The rhetorical question implies the use case (validating an EORI for EU customs) and the batch hint ('up to 10 numbers per call') tells the agent how to use it. There is no explicit when-to-use/when-not framing versus vat_check or company_lookup, leaving the agent to infer the routing.

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