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

Annotations cover the safety profile (destructiveHint=false, openWorldHint=true), and the description adds genuinely non-obvious behavior: the x402 payment of $0.001 USDC on Base, charged 'per successful call,' which explains the non-readOnly/non-idempotent annotations (a transfer occurs). It does not say what is returned on an invalid IBAN (error vs. structured failure), which is the remaining gap.

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?

Two sentences, front-loaded with the action and validation steps, then usage guidance, then the pricing constraint last. No redundant or filler text; every clause carries information an agent needs before calling.

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 one-parameter validation tool with no output schema, the description covers purpose, when-to-use, the returned format, and the cost model. The only missing piece is failure behavior (what an invalid IBAN yields), which is a minor omission against an otherwise complete definition.

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?

Single parameter with 100% schema description coverage ('The IBAN, spaces allowed'), so the schema already carries the parameter contract and the baseline is 3. The description's mention of country-specific length and mod-97 describes the checks performed, not additional input semantics, so it does not raise the score.

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 ('Check') plus the exact checks performed (format, country-specific length, mod-97 check digits) and the return shape (formatted in groups of four). An agent can distinguish this from sibling validators like validate_luhn or validate_isbn purely from the description, with no schema reading required.

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 explicit placement guidance: 'Use before saving bank details or paying out,' which tells the agent exactly when to reach for it. No explicit exclusion or named alternative is given, but the sibling set contains no competing IBAN tool, so there is little ambiguity to resolve.

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