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

Goes well beyond the annotations by justifying the readOnlyHint=false flag ('Reads or computes only; not marked read-only because each call is charged') and by disclosing cost ($0.003/call), payment paths (x402 USDC or card key), the first-25-free allowance, and that failed calls are free. This is exactly the kind of billing/failure context an agent needs before invoking a paid tool.

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 validation behavior, and the usage/pricing detail follows in a compact form. The pricing clause is dense but relevant; a couple of sentences pack multiple facts, keeping it readable overall.

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?

With one required parameter, no output schema, and pricing sensitivity, the description covers what is validated, an input example, when to use it, the output formatting, and the billing/failure rules. Nothing an agent needs to call it correctly 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?

Schema coverage is 100% and the schema already states spaces are allowed, so the baseline is 3; the description adds a concrete example value ('iban=SE45 5000 0000 0583 9825 7466') that demonstrates the accepted spaced format, a modest but real increment.

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/validate) plus the resource (IBAN) and enumerates exactly what is verified: format, country-specific length, and mod-97 check digits. It also discloses the return transformation (grouped in fours), so it is distinguishable from other validators like validate_isbn or 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 names the situation to use it: 'before saving bank details or paying out.' That is clear context, though it gives no when-not guidance or explicit pointer to the sibling validators.

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