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

With no annotations, the description carries the full burden and does so: it discloses the return format, the Irish-IBAN default-pass quirk, that no bank transfer occurs, and the exact cost/rate-limit model (1 free attempt per UTC day per IP, shared REST/MCP, failures count, then 0.005 USDC with explicit approval). These are precisely the side-effect and billing facts an agent needs before calling.

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 key differentiator ("Cheapest call") and the core action, then packs return format, exclusions, alternatives, side effects, and pricing into compact sentences. Every sentence supplies information the agent would otherwise lack, with no filler.

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?

Given no annotations and no output schema, the description supplies everything needed: what is validated, what is returned, what is explicitly out of scope, which sibling to use instead, and the cost/rate-limit behavior. No material gap remains for a single-parameter validation tool.

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?

Only one parameter and schema coverage is 100%, so the schema already documents normalization of spaces/lowercase and the checksum-only scope. The description reinforces the same scope but adds no new syntax or format detail beyond the schema, 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 ("Validates an IBAN") plus the exact checks performed (ISO 13616 mod-97 checksum, length, country) and the return shape (PASSED/FAILED with reason code). It also names what it is not (ownership, Confirmation of Payee, supplier identity), so an agent can separate it from every sibling without opening a schema.

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?

Explicitly gives the when-to-use ("before you pay it"), the when-not-to-use ("not account ownership, bank Confirmation of Payee or supplier identity"), and routes each adjacent need to the right alternative: check_vat, check_company, decide_supplier_payment. Nothing is left to inference.

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