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

Annotations already mark the call as read-only, idempotent, and non-destructive, and the description adds substantial behavior beyond that: offline computation, normalized and print-formatted return forms, country/SEPA/bank-identifier output, rejection messages that name which check failed, pricing per call, and the auth/credit-token model. No annotation contradiction exists, and the description meaningfully enriches the behavioral contract.

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?

Though the description is longer than average, every sentence earns its place: purpose, validation logic, outputs, failure-mode reporting, limitation, required input, pricing, and authentication. It is front-loaded with the core purpose and then layers supporting details in logical order. There is no filler or repetition of the annotations.

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?

The description covers what the tool validates, what it returns, how failures are reported, its structural-only limitation, required input, cost, and auth requirements. With an output schema present, the summary of return values is sufficient. For a single-parameter read-only validator, this is fully complete and leaves no operational ambiguity.

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?

The schema already provides 100% coverage for the single required parameter 'iban', described as 'IBAN', so the description need not repeat it heavily. The description does confirm 'Required input: iban' and ties the parameter to the structural validation behavior, but it adds little format-level nuance beyond what the property name and type already imply. This is the baseline case where the schema carries the parameter semantics.

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 opens with a pointed question and then states the precise job: 'Validate an IBAN offline' covering country registry length, structure, and ISO 7064 mod-97 checksum. It clearly identifies the resource (IBAN) and the specific validation behavior, and it carves out what the tool is not: 'Structure only: a valid IBAN is not proof the account exists.' This is far beyond a vague restatement and fully distinguishes the tool's purpose.

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 description gives strong when-to-use context: an offline structural sanity check for an IBAN, with explicit required input and no wallet/API key requirement. It also gives a clear when-not-to-use boundary: structural validity is not account existence. It does not name a sibling alternative for deeper account validation, so it stops short of a 5, but the context is clear enough for an agent to select it appropriately.

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