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

Annotations cover the safety profile (readOnlyHint=true, destructiveHint=false, openWorldHint=true), so the bar is lower; the description still adds real value by disclosing that validation is an offline algorithmic check with no account lookup or network dependency, and by listing what it checks and extracts. It does not describe the response shape for invalid input, but the behavioral disclosure is well beyond the annotations.

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 opening three fragments ('Validate and parse an IBAN. Validate IBAN. Validate any IBAN...') are near-duplicate restatements that consume the front-loaded position without adding information. The remaining content is dense and useful, but two of the first sentences do not earn their place.

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 tool with no output schema, the description covers what is verified, what is returned (bank code, branch code, account number, SEPA flag), and the offline nature of the check. The only gap is behavior on invalid input, which is minor given the tool's simplicity.

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?

There is a single parameter with 100% schema description coverage, which includes the format hint ('spaces allowed') and an example, so the baseline is 3. The description only loosely reinforces this by mentioning 'formatted and electronic forms' and adds no syntax beyond the schema.

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 (validate/parse) and resource (IBAN), and enumerates exactly what is validated: checksum, length, country format, national check digits. No sibling tool in the list is close enough to require explicit differentiation, but the first three fragments ('Validate and parse an IBAN. Validate IBAN. Validate any IBAN...') restate the same idea without adding precision.

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?

There is no explicit when-to-use vs. when-not guidance, but the closing note 'Offline algorithm, no account lookups' implicitly scopes the tool: use it for structural validity, not for confirming that an account exists. With no competing sibling to route against, this is adequate but inferred rather than stated.

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