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

Annotations already declare readOnlyHint, destructiveHint=false, and openWorldHint, so the safety profile is covered; the description goes further by disclosing the return payload (validity, country, failure reasons) and the tier-gated fields (bank/branch/account, correct check digits, SEPA flag). It also states an explicit cost of 2 credits, which is real behavioral context an agent cannot get from annotations. It stops short of describing failure/error behavior for malformed input.

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?

Structure is front-loaded: checks first, then outputs, then tier-gating, then cost. The only waste is the redundant 'IBAN Validator: IBAN Validator checks...' opener that restates the title, but the remainder is dense and every clause carries information.

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 validator with an output schema, an agent has enough to call it correctly and interpret results: inputs, checks performed, returned fields, conditional returns by plan, and price. The only gap is what happens on structurally malformed input (error vs. an invalid verdict), which matters given the 'format' framing.

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?

With a single parameter at 100% schema description coverage, the schema already explains that spaces, dashes and lower case are accepted, so the description need not repeat it. The description adds no normalization or format-syntax detail beyond the schema, making the baseline 3 correct here.

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 names a specific verb and resource and enumerates the exact checks performed: mod-97 checksum, country-specific length and format, and national check digits where defined. This unambiguously separates it from adjacent validators in the sibling list such as BIMIRecordValidator, RoutingNumberLookup, or CardValidator, which all validate entirely different identifiers.

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?

Usage is implied by the nature of the tool — you call it when you have an IBAN that needs validating — but there is no explicit when-to-use framing, no statement of when it is unnecessary, and no routing to alternative tools for adjacent tasks like SWIFTCodeLookup or RoutingNumberLookup. The paid-tier note hints at capability boundaries but is a billing statement, not usage guidance.

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