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

With no annotations, the description carries full behavioral disclosure. It explicitly states the validation mechanism, country coverage, and that it makes no external call. It does not describe the return shape, but the read-only, local nature is disclosed.

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: the first delivers the core function and algorithm, the second adds a useful performance/architecture trait. No filler, front-loaded.

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, no-output-schema, no-annotation validation tool, the description covers function, mechanism, coverage, and latency. The only notable gap is whether the return is a plain boolean or a more detailed object, but that omission is minor given the simplicity.

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?

The schema coverage is 100% and the 'iban' parameter is already described as 'IBAN, spaces allowed'. The description adds no new parameter-level meaning beyond the overall validation scope, so the baseline of 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, distinct action: 'Validate an IBAN' with explicit technical scope (ISO 13616 structure + mod-97 checksum) and coverage ('70+ countries'). Clearly distinguishable from siblings like eu_vat_validate.

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 implies the tool's use case—when an IBAN needs offline validation—but does not explicitly compare it to alternatives or state when not to use it. The 'Instant, no external call' trait gives context, but no direct exclusions or guidance.

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