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

Annotations already declare readOnly/idempotent/openWorld, and the description adds substantial information they cannot carry: the exact cost (0.001 USDC via x402 or 1 Factline credit) and the free-tier rule (first 5 paid calls per day, no payment or key). It also discloses a meaningful limitation, that holding is not verified.

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, front-loaded with the use case, then the scope caveat, then pricing. There is no filler; each sentence carries decision-relevant 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 one-parameter read-only tool, the description covers purpose, limits and payment model well, and effectively tells the agent what results are produced (checks plus Dutch bank). It stops short of describing the response shape, but nothing needed to decide or call the tool 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?

There is a single required parameter with 100% schema description coverage including a format example, so the schema does the heavy lifting. The description explains what the IBAN is validated against but adds no syntax or format guidance beyond the schema example.

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 and resource ('check an IBAN') and enumerates exactly what the check covers: country length, mod-97 checksum, SEPA membership, and Dutch bank identification. This is clearly distinguishable from siblings such as eu_vat_check, kyc_check or x402_payee_check.

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 a concrete trigger ('before a payout or invoice') and an explicit scope boundary ('Does not confirm the account holder'), which implicitly routes account-ownership checks elsewhere. It does not name a sibling alternative by name, so it stops short of a 5.

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