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

Annotations already cover read-only/idempotent/closed-world, and the description adds genuinely non-obvious context: the check-digit layers performed, the unauthenticated batch limit of 10 (vs. maxItems 100 in the schema), and the semantic ceiling of the result. It stops short of describing the response shape.

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 dense sentences: the validation scope is front-loaded, then the quota and the semantic caveat. No filler and nothing redundant with title or schema.

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?

With no output schema, the description should say what comes back — per-IBAN validity flags, error strings, or a boolean — and it does not. It is otherwise complete on inputs and semantics for a one-parameter validator.

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 ('IBANs, spaces allowed'), so the schema already carries the burden; baseline 3 applies. The description does add the effective per-call batch limit of 10 without a key, which materially affects how the agent sizes the array.

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?

Specific verb+resource with an enumeration of exactly what is checked (country format, length, MOD-97, national check digit). The closing line implicitly separates it from bank_test_ibans by stating it does not prove the account exists, though no sibling is named outright.

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 rather than stated: the 'does not prove the account exists' caveat hints that bank_test_ibans is the alternative for existence checks, but the description never explicitly routes the agent there or names when not to use this tool.

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