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

With no annotations provided, the description carries the full burden of behavioral disclosure and does so well: it states the tool performs only a checksum validation, explicitly says it does not prove account existence, and discloses the $0.01 USDC Base x402 payment requirement. This is meaningful beyond a bare 'checks IBAN' statement, though it omits details about return values or error behavior.

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, each earning its place: the first states the operation and standard, the second states a critical limitation, and the third states the cost. The description is front-loaded with the main purpose and contains no filler or redundant wording.

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 simple tool with no parameters, no output schema, and no nested objects, the description covers the essential operational facts: what it checks, what it does not check, and the payment required. The lack of specification about the exact response shape or how the IBAN is supplied is a minor gap given the tool's simplicity.

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?

The tool has zero formal parameters, so the input schema already covers the parameter space completely (100% coverage). The baseline for zero-parameter tools is 4, and the description appropriately does not invent parameter details. All an agent needs to know about parameters is that there are none defined, which the schema already conveys.

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 identifies the tool as an IBAN ISO 13616 checksum validator, which is a specific operation on a specific resource. It clearly conveys the core purpose even without an explicit verb, and the resource differentiates it from sibling check_siret. However, it does not explicitly name or call out the sibling, so it falls just short of full differentiation.

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?

The description implies when to use the tool via the warning that it does not prove account existence, serving as a when-not-to-use signal. It also mentions the cost, which hints at conditions for use. However, it does not explicitly state when to choose this tool over alternatives or reference sibling tools, leaving the guidance largely implicit.

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