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

Annotations cover the safety profile (readOnly, idempotent, openWorld), and the description adds substantial context beyond them: deterministic checksum-only scope, the critical limitation that a valid checksum does NOT establish ownership/beneficiary identity/absence of fraud, and the offline-verifiable signed receipt return. This is exactly the disclosure a fraud-sensitive validator needs.

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?

Front-loads the definition (validates, standard, algorithm) before the scope caveat and the anti-pattern warning. Dense but every clause carries weight; slightly long but none of it is filler, so just shy of a 5.

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 validator with annotations covering safety and an output schema covering the receipt, the description supplies the remaining essentials: determinism, checksum-only scope, the fraud-misattribution caveat, and the return artifact. Nothing an agent needs to call it correctly is missing.

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?

Only one parameter with 100% schema description coverage ('IBAN to validate; spaces allowed'), so the schema already carries the semantics. The description adds no format or syntax detail beyond the schema, making 3 the correct baseline.

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 verb (validates) plus resource (international bank accounts / IBANs), names the exact standard (ISO 13616) and algorithm (MOD-97-10), and clearly scopes the operation to structure and checksum only. An agent can distinguish this from siblings like swift_bic_check or batch_validate without opening the schema.

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?

Gives clear context ('before payout') and an explicit negative directive against validating with LLM regexes, which tells the agent when not to hand-roll the check. However it does not route to sibling alternatives (e.g., batch_validate for bulk, or receipt_verify for the returned receipt), so routing guidance is partial.

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