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

Beyond the annotations (readOnlyHint, openWorldHint, destructiveHint=false, which already signal a safe read), the description discloses the key behavioral trait an agent cannot infer elsewhere: a $0.0025 USDC per-call charge paid in-band via x402 across four chains, auto-pay clients like @x402/mcp, and a free sample path. It also describes the return payload country-by-country, which matters because there is no output 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?

Front-loaded with capability, then the parameter, then payment mechanics, then a copy-pasteable sample. Dense but every element (validation scope, cost, x402 chain list, example input) earns its place; only the parenthetical repeats the schema slightly.

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 correctly enumerates the returned fields (bank code, branch, account, SEPA flag, plus bank name/BIC/city for DE/AT/CH/NL/BE/LU/LI). The payment requirement, the one parameter, and the sample input cover everything needed to invoke and interpret the call.

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% for the single parameter, so the schema already documents 'iban' including that spaces are allowed. The description repeats 'iban (required, spaces allowed)' and only marginally adds a concrete spaced example format. Baseline 3 applies when the schema does the heavy lifting.

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 precise verb+resource (validate an IBAN) and enumerates exactly what is validated (checksum, country length, structure), then lists the derived outputs (bank code, branch, account, SEPA status, and BIC/bank/city for specific countries). This clearly separates it from the many sibling *_validate tools (vat_validate, email_validate, phone_validate) by naming the exact artifact it operates on.

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 purpose (validate an IBAN before relying on it) and the example input shows how to test, but there is no explicit when-to-use / when-not-to-use and no alternative named among the sibling validators. The free-sample hint is the only practical guidance.

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