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

Adds substantial context beyond the readOnly/idempotent annotations: offline operation, that rejections name WHICH check failed, the shape of returned fields (normalised and print forms, country, SEPA membership, bank identifier), plus auth/pricing mechanics (x402 on Base, prepaid x-credit-token header, 1 free call/day, no API key). This is exactly the behavioural detail annotations cannot carry.

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 purpose and the most decision-relevant constraint before the payment mechanics. It is somewhat long, and 'No API key required' overlaps with the preceding payment sentences, but nearly every clause contributes to correct invocation.

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 validator with an output schema, the description covers everything an agent needs: what is checked, the critical semantic caveat that structure does not prove existence, failure reporting granularity, and the payment/auth model. Nothing material is left out.

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?

There is a single parameter and schema coverage is 100%, so the schema already documents it. The description only restates 'Required input: iban' with no added format or syntax guidance; baseline 3 is appropriate when the schema does the heavy lifting.

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) with the exact mechanism — country registry length, structure, and ISO 7064 mod-97 checksum. This is clearly distinguishable from neighbours like bic_check or sepa_creditor, and the opening framing question makes the intent unmistakable.

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?

Gives strong context for when to reach for it (structural possibility of a bank account number, catching human/spreadsheet errors) and an explicit scope boundary — 'Structure only: a valid IBAN is not proof the account exists.' It does not name an alternative sibling for related tasks (e.g. bic_check, sepa_creditor), so it stops short of full routing guidance.

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