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

Goes beyond annotations by disclosing billing ($0.01/call via x402 USDC or card key), the free tier (first 25 free, failed calls free), and explaining why idempotentHint=false — repeats are charged. Annotations already cover read-only and non-destructive, but the cost model is genuinely new and important context.

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 checks; example and pricing follow logically. Every sentence is informative, though the pricing sentence is dense and could be trimmed slightly.

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?

No output schema, but the description explains the return (IBAN formatted in groups of four) and hints failures are handled (failed calls free). Adequate for a single-param validator, though success/failure response shape isn't fully spelled 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?

Schema coverage is 100% and already states spaces are allowed, so the schema carries most weight. The description reinforces this with a concrete spaced-IBAN example (SE45 5000 ...), giving a usable format sample beyond the schema text.

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 (check) and the exact checks performed — format, country-specific length, mod-97 check digits — plus the formatting output. Clearly distinct from siblings like validate_isbn and validate_luhn.

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?

Explicitly says when to use it: "Use before saving bank details or paying out." There's no sibling alternative for IBAN validation, so no exclusions are needed, but the timing guidance is concrete.

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