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

Annotations already declare readOnly/idempotent/non-destructive, and the description adds genuinely new behavioral facts the structured data cannot carry: the per-request credit cost (including on failure), the API-key prerequisite, and the crucial semantic limit that a pass does not confirm the account exists. That last point prevents an agent from over-interpreting a success.

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 action and its limitation, then the cost/auth constraints. Efficient, though the fragment 'IBAN Validation API.' is mild padding that repeats the product name already implied by the tool name.

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?

Covers purpose, cost, credentials, and the interpretation caveat for a single-parameter tool with no output schema. It does not hint at the shape or fields of the response, which is the one thing an agent must infer since no output schema exists.

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's format hint ('with or without spaces') lives in the schema. The description adds nothing about the iban argument, so the baseline 3 applies.

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?

States a specific verb and resource ('Check the structure and check digits of one IBAN') and immediately bounds the claim with 'It does not prove that the account exists.' This distinguishes it from every sibling, none of which touch IBANs.

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?

Gives clear operational context: one credit per request, a failed request still counts, and the IBAN Validation API key is required. It does not name an alternative tool, but no sibling offers IBAN validation, so there is nothing to route against.

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