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

Annotations already declare readOnlyHint/idempotentHint/non-destructive/openWorld=false, so the safety profile is covered. The description adds genuine value beyond that: it is offline, uses a specific checksum algorithm plus country length rules, and explicitly limits the guarantee to well-formedness. Auth and rate-limit concerns are irrelevant here, so this is nearly complete.

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 tightly written sentences, front-loaded with the core verb and scope. Every clause earns its place, including the disambiguating 'not that the account exists'.

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, read-only validation tool with full annotation coverage and 100% schema description coverage, the description supplies everything an agent needs: method, offline nature, and the limit of what a pass means.

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?

Schema description coverage is 100% and the single 'iban' parameter already documents that spaces are ignored. The description adds no format or normalization 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') and pins down the mechanism (ISO 7064 mod-97 checksum, country-specific length). The closing clause carves out the boundary between format validation and account-existence checks, so an agent knows exactly what this tool does and does not confirm.

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?

The description gives clear context for when the tool applies ('well-formed' checks, 'offline') and implicitly excludes account-verification use cases ('not that the account exists'). It stops short of naming any sibling (e.g., vat_validate) or explicitly stating a when-not condition, so it is clear but not fully routeable.

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