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

Annotations already declare readOnly, idempotent, non-destructive and openWorld, so the safety profile is covered. The description adds genuinely non-obvious behavior: the input is not stored, and there is a hard quota of 10 free calls/day followed by metered pricing via x402 (MCP params._meta["x402/payment"] or HTTP) or X-API-Key credits. It does not describe how a failed validation is signalled, but an output schema exists to carry that.

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 short sentences in a sensible order: what it does, what happens to the data, what it costs. The dense payment clause is compressed into one sentence and nothing is repeated from the schema or annotations.

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 low-complexity single-parameter validator with a full output schema and rich annotations, the definition covers purpose, data handling, and billing, which are the things an agent cannot infer elsewhere. Only the failure-mode signalling and input formatting tolerance are 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?

There is a single required parameter with 100% schema description coverage, so the schema does the heavy lifting and a 3 baseline applies. The description does not add anything the schema lacks, such as whether embedded spaces in the IBAN are tolerated, even though its own example is space-formatted.

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?

The description names a specific verb and resource ('Check an IBAN') and goes further to state exactly what is checked: country length and mod-97 check digits, plus the derived outputs (BBAN, print format). That is far more specific than a tautology. It does not explicitly contrast itself with near-neighbours like card_number_check or address_validate, so it stops short of a 5.

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 resource: use this when you need to validate an IBAN. There are no stated exclusions or pointers to alternative validators for other identifier types, and no prerequisites such as 'input may contain spaces'. The payment/credits sentence is a cost precondition rather than a when-to-use rule.

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