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

With no annotations at all, the description carries the full behavioral burden and does well: it discloses the price, the deterministic rule set applied, the coverage boundary of 50 countries, and the shape of the result (pass/fail with all checks and issues). It leaves out auth/rate-limit considerations and whether a failing IBAN yields billable charges, which would have completed the picture for a paid tool.

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?

Three tight sentences with zero filler: price first, then the exact validation rules, then the return shape and scope. Every clause carries information an agent needs and the most decisive constraint (paid) is front-loaded.

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 validation tool with no output schema, the description covers purpose, rules, cost, scope, and a rough return shape, which is close to sufficient. Since no output schema exists, naming the result fields beyond "checks and issues" would have removed the last ambiguity, but the gap is small.

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 description coverage is 100% and the single parameter already documents format and whitespace tolerance ("spaces allowed") with an example. The description adds no further meaning about the iban argument, so the baseline of 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?

"Validate an IBAN" gives a specific verb and resource, and the description enumerates exactly what is checked (ISO 13616 structure, country registration, length, MOD 97-10 checksum), so an agent knows this is IBAN-specific rather than a generic checksum utility. It stops short of explicitly distinguishing itself from the sibling mod97_check, which an agent might otherwise confuse with the checksum portion described here.

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 cost marker "Paid ($0.05)" and the "Covers 50 countries" limit give practical context for deciding whether to call it, and the country limit implies a when-not-to-use boundary. However, no alternative tool is named for out-of-scope IBANs or for raw MOD-97 checks, so routing guidance is implied rather than stated.

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