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

Annotations already declare readOnly, idempotent, non-destructive, and closed-world, and the description adds genuinely new behavioral facts: it runs offline, stores nothing, and costs $0.003 USDC per call via x402 on eip155:8453 — a paid-endpoint fact an agent must know before invoking. It also discloses return content (normalized/print-formatted IBAN, BBAN, and DE/GB bank or sort codes) that no output schema provides.

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 front-loaded sentences, each carrying distinct information: what is checked, what comes back, and operating constraints including price. Slightly dense, but nothing is redundant and the most decision-relevant content (validation scope) leads.

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 takes on the burden of describing return values and does so concretely (normalized IBAN, BBAN, DE/GB bank codes), plus cost and privacy behavior. For a one-parameter validation tool this covers everything needed to call it correctly, with only input-format tolerance left implicit.

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 'iban' parameter, so the schema only supplies type and maxLength 64. The description hints that input may be unnormalized (it returns a 'normalized and print-formatted' IBAN) but never states whether spaces, hyphens, or lowercase are accepted — the most common real-world failure mode for this call.

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 ('Validate an IBAN') and immediately specifies the checks performed: country-specific length and ISO 13616 mod-97 checksum, across 80+ countries. The resource (IBAN) alone separates it from email_validate and json_validate in the sibling list; no schema inspection is needed to know what it does.

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 context for use (validating International Bank Account Numbers for 80+ countries) and implies the boundary is IBAN-specific, with no sibling doing that job. It does not include explicit when-not guidance or name alternatives, but the domain is narrow enough that none is needed.

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