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

With no annotations, the description carries the full burden and does well: it declares offline/deterministic behavior, no network call, no third-party dependency, and the per-call price and payment rail (x402/USDC). It also previews the returned fields. It omits error/malformed-input behavior and any rate limits, keeping it from a 5.

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 sentences, front-loaded with the core operation and its checks, then behavior, then cost. The "no third party that can go down" phrasing is slightly promotional, but nothing is redundant or padded.

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 validator with no output schema and no annotations, the description covers inputs accepted, processing semantics, returned fields, and cost. Missing only failure-mode detail for invalid IBANs, which is a minor gap.

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 the schema already documents at 100% coverage that spaces and lowercase are accepted. The description adds no further syntax or format guidance beyond the schema, so the baseline 3 for high schema coverage 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 (validates) and resource (one IBAN) and enumerates exactly what is checked: country, length, ISO 7064 mod-97, SEPA membership, and account-number split. This distinguishes it from the other util_* validators (util_vat, util_phone) without needing their schemas.

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?

"Built for cleaning a list of accounts one at a time" gives clear usage context and implies the call granularity an agent should expect. It does not name when-not to use it or point at any alternative tool, so it stops short of a 5.

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