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

Annotations already declare readOnly, idempotent, non-destructive and closed-world, but the description still adds real value: it specifies the return payload (country, check digits, BBAN, 4-character grouping), the failure shape (valid:false with a reason), and that nothing is stored. These are the details an agent needs that annotations cannot convey.

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 short sentences, front-loaded with the action and mechanism, then return values, then failure mode, then the no-storage guarantee. Every sentence contributes and nothing is padded.

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 tool with no output schema, the description covers what is checked, what comes back on success and failure, and the side-effect profile. The only meaningful gap is input normalization rules (spacing, casing, max length of 64), which the schema declares numerically but the description does not explain.

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 0% for the single required 'iban' parameter, so the description must carry the burden. It identifies the input as an IBAN but does not say whether spaces, hyphens or lowercase are tolerated, or whether the country code must be included — format details that matter for correct invocation.

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?

Names a specific verb and resource (validate an IBAN) and explains the two checks performed: country-specific length from the IBAN registry and mod 97-10 check digits. This clearly distinguishes it from siblings like validate_bic or validate_pan, which validate 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 name and by 'Validate an IBAN', but there is no explicit when-to-use guidance, no pointer to alternatives such as validate_bic for the corresponding bank code, and no stated exclusions or input format requirements.

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