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

Annotations already declare the operation read-only, idempotent, and non-destructive. The description adds meaningful behavioral context beyond those annotations by specifying the offline nature, the exact checksum algorithm, and the boundary of what 'validates' means here. This helps the agent set correct expectations about the tool's results.

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 short sentences deliver the key facts with no filler. The primary action and offline scope appear first, followed by the validation criteria and the important caveat about account existence. Every sentence earns its place.

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, the description is complete: it states the algorithm, the offline nature, and the limitation regarding account existence. The absence of an output schema is not a gap because the behavior of a boolean-style validation result is sufficiently implied by 'validates' and 'confirms'.

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 input schema already documents the single parameter with 100% coverage, including that spaces are ignored. The description does not add further parameter-level meaning beyond the overall validation behavior, so a baseline score of 3 is appropriate.

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 starts with a specific verb and resource ('Validates an IBAN'), then defines the exact validation performed: ISO 7064 mod-97 checksum and country-specific length. It also explicitly distinguishes what the tool does not do ('not that the account exists'), making its purpose unambiguous relative to potential verification tools.

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 clearly establishes when to use this tool: for offline, format-level IBAN validation. The phrase 'Confirms the number is well-formed, not that the account exists' provides an important limitation, effectively telling an agent not to use this tool for account-existence checks, though it does not name an alternative tool for that purpose.

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