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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint=false, so the safety profile is covered. The description adds genuinely new context beyond the annotations: no API key, nothing persisted, and a hard capability limit (it will never identify the account holder). What it omits is any sense of cost/rate limits or failure behavior for malformed input.

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, front-loaded with the free/no-key/no-storage fact and then the concrete checks. Nothing is padding, though the parenthetical "for the countries listed in the answer" is slightly awkward and refers to output that isn't otherwise described.

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 has to hint at the return shape, and it does reasonably: country-dependent length checking and the CH/LI QR-IBAN and reference-type verdicts are named. It stops short of describing the response structure or what a failing check returns, which would seal the gap.

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% and the single parameter is fully documented in the schema, including the "with or without spaces" input flexibility, so the description does not need to carry parameter burden. It adds no syntax or formatting detail beyond what the schema already states, which is the baseline-3 case.

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 states a specific verb and resource (check an IBAN) and then enumerates the exact checks performed: shape, length, ISO 13616 checksum, and CH/LI QR-IBAN and reference rules. That level of specificity makes it unmistakable next to siblings like uid_check or verify_licence.

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?

It gives useful framing ("Free, no key, nothing stored") and a scope exclusion ("Formal only: no bank or register is asked, so it never says whose account it is"), which tells the agent when this tool will not answer an account-ownership question. However, it never names or routes to an alternative, so the when-to-use guidance is implied rather than explicit.

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