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

Annotations cover safety (readOnly, idempotent, non-destructive), and the description adds real behavioral context beyond them: it runs offline/structurally, is priced at $0.002 per x402 call on Base, supports a prepaid x-credit-token header, offers 1 free call/day, and needs no API key. Cost, auth path, and rate limit are exactly the traits annotations cannot express.

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 a question that names the resource, then scope, then limitations, then pricing — a sensible order with no wasted preamble. The pricing/auth sentence is long but each clause carries distinct information, so it earns its space.

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?

An output schema exists so return values need no explanation, and annotations cover the safety profile. The description supplies the remaining essentials: the structural-only limitation, the single required input, and the payment/rate-limit model, which is enough for an agent to decide and invoke correctly.

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 (creditorId) with 100% schema description coverage, so the schema already carries the load; 'Required input: creditorId' merely restates the required field. The description adds a little meaning by listing the identifier components validated, but no format, length, or example syntax is given, 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 ('Validate a SEPA Creditor Identifier offline') and enumerates exactly what is checked: country, mod-97 check digits, business code, national id. It also draws a boundary against a different kind of check ('not a scheme-manager registration proof'), so an agent can distinguish it from iban_check/bic_check/vat_check siblings.

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 a clear when-to-use ('Catches mistyped creditor ids before a mandate setup') and an explicit when-not ('Structure only — not a scheme-manager registration proof'). It stops short of naming the sibling alternative (e.g. iban_check) for callers who actually need a registration or IBAN check.

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