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

Annotations already declare idempotentHint=true and destructiveHint=false, so the safety profile is covered. The description adds valuable behavioral context: it checks the ISO 7064 checksum, returns the country and formatted IBAN, and costs 1 credit per call. The credit cost is a behavioral trait not present in annotations, which is useful for an agent deciding whether to call this tool. It doesn't describe error behavior for invalid IBANs, but the core behavioral traits are 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?

Three sentences with zero waste. The core action is front-loaded, the checksum standard and return values are stated in the second sentence, and the credit cost is a single final sentence. 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 validation tool with full schema coverage and idempotent annotations, the description is nearly complete. It covers what the tool does, the checksum standard, return values, and cost. The only minor gap is that it doesn't describe the response format for invalid IBANs (e.g., error vs. false return), but with no output schema, this is a small omission rather than a critical one.

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 description coverage is 100%, so the schema already documents the single 'iban' parameter with an example. The description adds the checksum standard (ISO 7064) and the return fields (country, formatted IBAN), which gives context about what the parameter is used for, but it doesn't add syntax or format details beyond the schema. Baseline 3 is appropriate since the schema does the heavy lifting.

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 an IBAN, checks the ISO 7064 checksum, and returns the country and formatted IBAN. This is a specific verb and resource, and it distinguishes itself from sibling validation tools (validate_email, validate_phone) by naming the IBAN-specific checksum standard.

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 when to use it (when you need to validate an IBAN) but does not explicitly state when not to use it or mention alternatives. Sibling tools like validate_email and validate_phone are obvious alternatives for other validation tasks, but the description doesn't explicitly route the agent away from them. The context is clear enough for a single-purpose tool, but explicit exclusions are missing.

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