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

Beyond the readOnly/idempotent/openWorld annotations, the description discloses the per-call price ($0.001 paid via x402 in USDC on Base), the return shape {type, valid, normalized, reason}, and the data-handling policy (input never echoed or stored, personal identifiers masked). Those are exactly the behavioral facts an agent cannot get from annotations or the schema.

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 tool identity is front-loaded as 'IBAN: ...', followed by the algorithm, pricing, return shape, and privacy note in compact semicolon-separated clauses. Dense but every clause carries information; no filler sentences.

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?

With no output schema, the description responsibly enumerates the return keys and covers cost and privacy, which is enough to invoke the tool correctly. It stops short of explaining the possible values of `type`/`reason` or any failure/error behavior, so it is not fully complete.

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 including an example, so the schema fully carries the parameter burden. The description adds no syntax, format, or normalization rules for the `value` 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?

The description states a specific verb and resource ('IBAN') plus the exact algorithm (country length + ISO 13616 mod-97 checksum), which is far more precise than a bare 'validate' tool. It does not explicitly distinguish itself from the validator siblings (validate_eu_vat, validate_pl_nip, etc.), relying on the resource name to do that work, so it falls short of a 5.

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 guidance on when to use this tool versus the sibling validators, nor any prerequisites or exclusions. The only implied usage is that an IBAN is the input, which an agent could infer from the name alone.

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