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

Annotations already declare readOnly, idempotent and closed-world, so the safety profile is covered. The description adds substantive behavior beyond that: check-digit algorithm (ISO 7064 mod-97-10), zero left-padding to registered widths, return in electronic and print formats, 89-country SWIFT registry coverage, and the $0.05 x402/USDC on Base price. That is real operational context an agent needs.

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?

Four sentences, front-loaded with purpose, then input modes, then return behavior, then coverage and price. Every sentence carries information, though the price/payment sentence is dense and slightly interrupts the technical flow. No filler.

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 exists, so the description must cover returns – and it does, stating electronic and print formats plus the parsed bank/branch/account parts. Combined with the algorithm and coverage details, an agent has everything needed to call and interpret the tool.

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%, so the baseline is 3. The description goes beyond it by explaining the padding rule for digit fields and the constraint that named parts must span the whole BBAN (otherwise pass BBAN), which the schema does not state. It adds genuine semantic value over the field-level descriptions.

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?

Specific verb+resource: 'construct an IBAN from a country code and the domestic account parts, computing the ISO 7064 mod-97-10 check digits.' This clearly contrasts with the sibling iban_validate (generation vs validation). An agent can route between them without opening either schema.

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?

Gives conditional guidance: 'Takes a single BBAN string or named parts... Named parts work only where they span the whole BBAN; else pass the BBAN.' This tells the agent which input shape to pick. It stops short of explicitly naming iban_validate as the tool to use for the opposite operation, so there is no 'when-not' exclusion.

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