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

With no annotations, the description carries the burden and does it well: it discloses the validation stages, that a failure reason is returned (likely typo vs wrong length), and the important coverage limit of ~80 countries offline. It does not say what happens for an unsupported country or whether formatting/spaces are tolerated, which is the remaining gap.

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 short sentences: what it checks, how it behaves and its coverage limit, and when to use it. Front-loaded with the action and zero filler.

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?

An output schema exists, so return-shape explanation is unnecessary, and the description covers the checks, failure reasons and geographic scope. What remains unaddressed is handling of unsupported/unrecognized countries and input formatting, which an agent may need before calling.

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 description coverage is 0% for the single required `iban` parameter, so the description must compensate. It implies the input is the IBAN string to check but adds no format expectations (spaces, lowercase, country prefix) or error behavior for malformed input, leaving the parameter minimally understood.

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 (validate) and resource (IBAN) and then enumerates exactly what is checked: country, length, ISO 13616 checksum, bank code and BBAN. No sibling tool in the list does IBAN validation, so this is unambiguous at a glance.

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 before sending a bank transfer or saving payout details" gives a concrete trigger for when to call it. It stops short of naming alternatives or stating when not to bother (e.g. non-IBAN account numbers, which other tools don't cover either), which keeps it below a 5.

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