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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), so the description is free to add the behavioral facts that matter: it is a pure/offline computation and it does not perform account-activity verification. One mild tension exists with openWorldHint=true, since 'pure compute' implies a closed, deterministic lookup, though the description's clarification (structural check only) makes the intent unambiguous rather than contradictory.

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?

Two sentences, zero padding. The mechanism (ISO 13616 MOD-97 + country length) is front-loaded, and the limiting condition (not account existence) follows immediately.

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 one-parameter, low-complexity tool this covers scope, method, and the key limitation. The remaining gap is the return shape: with no output schema present, an agent does not know whether a pass yields a bare boolean, a reason string, or parsed IBAN components (country code, check digits).

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 100% for the single parameter, including the 'with or without spaces' note and the 15–42 length bounds. The description adds nothing further about input format, so the baseline 3 is correct when the schema carries the parameter burden.

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 (an IBAN's structure), and goes further by naming the exact standard and mechanism (ISO 13616 MOD-97 check digits + country length). The resource name alone separates it from the sibling vies_validate, which handles VAT numbers rather than bank accounts.

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?

There is no explicit 'use this when' clause, but the second sentence draws a hard boundary: it confirms well-formedness, NOT that the account exists or is active. That exclusion is the most important routing signal for a validation tool, since an agent must know not to treat a pass as proof of a live account. It stops short of naming a preferred alternative for account-existence checks.

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