Validate an IBAN (mod-97) + extract bank code (paid $0.001)

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

With no annotations, the description carries full behavioral disclosure. It reveals the paid nature, the challenge flow, pure-offline behavior, and exactly what data is returned. This gives the agent a strong understanding of side effects and prerequisites.

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 dense sentences with no filler. Front-loaded with payment warning, then clear purpose, then payment behavior. Every word earns its place.

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 paid tool with no output schema, the description is remarkably complete: it explains purpose, inputs, outputs, payment/challenge flow, and offline nature. Enough for an agent to select and invoke correctly without additional context.

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 covers 100% of parameters with descriptions, so baseline is 3. The description adds context about the IBAN parameter (e.g., bank identifier extraction) and the x_payment parameter via the challenge flow, but does not significantly enhance parameter-level understanding beyond the schema.

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 uses a specific verb ('Validate') plus resource ('an IBAN') and enumerates concrete outputs: ISO 13616 length, mod-97 checksum validation, country, formatting, and bank identifier. This clearly distinguishes it from sibling tools like validate_vat or validate_email.

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 clearly describes the context (pure-offline, paid, returns x402 challenge without payment), but does not explicitly name alternative tools or state when not to use it. The sibling names make the intended use obvious, yet explicit exclusions are missing.

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