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

Annotations only declare it is a safe read; the description adds real behavior context beyond that: paid per call, without payment it returns isError with a fresh x402 PaymentRequired invoiceId, and how to retry via _meta["x402/payment"] or the payment_signature argument. It does not describe the success return shape, so it is not a 5.

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?

The content is dense and mostly earns its place, but the payment-pricing sentence is front-loaded ahead of the actual purpose, so the reader hits pricing before learning what the tool does. Some repetition of the payment mechanics ('paid per call', 'unpaid calls cost nothing') could be trimmed.

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 no output schema, the description should carry the return contract, and it only documents the error path (isError plus PaymentRequired object) rather than what a successful format/checksum result looks like. The payment flow is complete, which keeps it at a minimum-viable 3.

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 50% and the iban parameter has no schema description at all, so the description carries weight by explaining what the tool reads from it (length and mod-97 checksum). It also adds meaning to payment_signature beyond the schema: that it may be omitted on the first call and that it substitutes for the _meta header.

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 states a specific verb and resource: 'IBAN length and mod-97 checksum reading', and explicitly bounds the scope with 'format and checksum reading, not proof an account exists'. It does not name the obvious sibling bic_format, so it falls short of full sibling differentiation, but the purpose is unambiguous.

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 says exactly when to use it ('solves one question: IBAN length and mod-97 checksum reading') and when not to expect value from it (it is not account-existence proof). It also gives the operational flow: call unpaid, receive PaymentRequired, retry with signed payload. No alternative tool is named, which keeps it from a 5.

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