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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, closed-world), and the description reinforces this with 'bundled country registry, with no bank or network lookup.' It goes further than the annotations by disclosing what the check does NOT establish (ownership, bank existence, transferability), which is genuinely useful behavioral context.

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 tight sentences: what is checked, when to use it, when not to. Front-loaded and free of filler, with every clause carrying load-bearing information.

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?

For a single-parameter predicate tool with no output schema and full annotation coverage, the description supplies everything needed: input expectations, scope of validation, and explicit exclusions. Nothing material is missing.

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?

With a single parameter at 100% schema description coverage, the schema already documents format and the 'do not submit real private bank account details' warning. The description's 'fabricated or public IBAN' phrasing echoes rather than extends this, so the baseline 3 is appropriate.

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?

The description names the exact resource and checks performed (format, country length, checksum against a bundled registry), which an agent can distinguish from a naive regex validator. It stops short of explicitly contrasting with the sibling validators (is-card-safe, is-vat-safe), but the family pattern makes scope clear.

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 to check a fabricated or public IBAN's country length and check digits' names the intended input, while 'Do not use to confirm account ownership, bank existence or actual transferability' gives explicit exclusions. Both when-to-use and when-not-to-use are stated directly.

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