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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false. The description adds valuable behavior: returns JSON with valid boolean, countryCode, etc.; on malformed input returns valid:false with reason rather than throwing; and states no storage. No contradiction with annotations.

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 well-structured in three paragraphs, front-loading core purpose. Each sentence adds value, though a bit verbose. No extraneous information.

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?

Given the single parameter, no output schema, and rich annotations, the description fully covers what the tool does, what it checks, return fields, error handling, and sibling differentiation. It is complete and self-contained.

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% for the single parameter 'iban' with a description. The tool description adds practical details: case-insensitivity, tolerance of spaces, and examples (e.g., 'DE89 3704 0044 0532 0130 00'). This adds value beyond the schema's basic description.

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 clearly states the tool validates a single IBAN against ISO 13616 structure per country. It uses a specific verb (validate) and resource (IBAN), and explicitly distinguishes from siblings by recommending validate_bulk_ibans for many IBANs and extract_ibans_from_text for extracting from text.

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 explicitly says when to use ('when you have one account number to verify') and when not ('for many IBANs prefer validate_bulk_ibans; to pull IBANs out of prose use extract_ibans_from_text first'). It also notes that validation runs in memory and is discarded, providing clear context.

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