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

With no annotations, the description carries the full behavioral burden, and it does disclose the validation pipeline and the output behavior ('pretty-printed grouping returned when valid') plus a non-obvious cost/payment constraint ($0.01/call, USDC, eip155:8453). It omits what happens on invalid input or the error shape, keeping it short of 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?

Front-loaded with the core operation and appropriately brief, but the second sentence ('Use this when an agent needs to structure, length and mod-97 checksum validation') largely duplicates the first and could be replaced with genuine routing guidance.

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 single-parameter validator with no output schema or annotations, the description covers the checks, the successful return format, and cost. The main gap is failure behavior and whether the check-digit result is reported for an invalid IBAN.

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 single parameter already documents that spaces are ignored, so the schema does the heavy lifting. The description adds no additional parameter syntax, length limits, or example values beyond the structured field.

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 (validate) and resource (IBAN) and enumerates the exact checks performed: country code, country-specific length, and ISO 7064 MOD 97-10 check digits. This clearly separates it from the nearby validators like crypto_address_validate and geo_time_eu_vat_validate.

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 sentence 'Use this when an agent needs to structure, length and mod-97 checksum validation' mostly restates the first sentence rather than naming alternatives or when-not-to-use conditions. Usage is implied by the resource but no sibling routing is offered.

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