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

With no annotations, the description carries the full burden. It discloses a specific behavioral trait (checksum validation, not full IBAN format validation), which is helpful. But it does not describe return format, error behavior, or edge cases, limiting transparency.

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?

The description is a single, front-loaded sentence with no wasted words. It immediately conveys the core function, making it highly scannable and efficient.

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 simple one-parameter tool, the description is adequate but minimal. It lacks details about the return value (e.g., boolean vs. detailed report) and whether checksum validation is the only validation performed. With no annotations or output schema, this leaves some uncertainty, but the description is sufficient for basic invocation.

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 coverage is 100%, so the baseline is 3. The description adds meaning by specifying that validation is checksum-based, which goes beyond the schema's generic 'IBAN to validate' and clarifies exactly what is checked.

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 'Validate IBAN checksum' uses a specific verb and resource, clearly distinguishing this from sibling validation tools (validate_email, validate_domain, etc.) by naming the exact target (IBAN) and the specific check (checksum). It is unambiguous and concise.

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 is for validating an IBAN's checksum, which is sufficient for a simple single-purpose tool. However, it does not explicitly state when to use it versus other validators or mention any exclusions, leaving usage context to inference.

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