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

Annotations already declare readOnlyHint, idempotentHint and closed-world behavior, so the bar is lowered. The description adds pricing ($0.05 per successful call via x402 on Base) – a paid-call constraint not in the annotations – plus the 'no existence check' limitation, both useful beyond the structured fields.

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 tightly packed sentences: what is checked, what is returned, what it does not cover – with the payment note last. No filler, front-loaded with the verification mechanics.

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 validator with no output schema, the description fully covers the check performed, the 89-country scope, the return fields (verdict, reason, electronic/print formats, decomposed codes) and the pricing/caveat. Nothing an agent needs to decide or invoke 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 description coverage is 100%: the single `iban` parameter already documents space stripping, case handling and the 64-char cap. The description adds nothing format-wise beyond the schema, so baseline 3 applies.

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 precise verb+resource ('validate an IBAN's length... check digits') and enumerates exactly what is checked (country length, character classes, ISO 7064 mod-97-10). It is clearly distinguishable from the sibling iban_generate, which produces rather than validates an IBAN.

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 scope is well framed: 'Structure and checksum only, it does not confirm the account exists' explicitly tells the agent when this tool is insufficient. It does not name iban_generate as the alternative, but validation vs generation is unambiguous from the name pair.

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