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

Beyond the annotations it discloses that the call spends wallet funds, queues an irreversible external transaction, and must not be retried on an ambiguous result – the last point is genuinely additive given idempotentHint=false. It also implies the authentication context, which no annotation covers.

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 dense sentences, front-loaded with the trigger, then the precondition, then the risk warning. No filler and nothing buried.

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?

Return values are covered by the output schema, and the risk profile is well handled, but for a fund-spending purchase tool the required 'reference' parameter is left completely unexplained – an agent has no basis for constructing or reusing it. Adding one clause on reference semantics would make this complete.

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 only 50%, so the description needs to compensate, and it does not: 'amount' and 'reference' are never explained, despite 'reference' being required and the operation being non-idempotent (where a caller would expect guidance on uniqueness or reuse). 'Available product and denomination' only loosely hints at product_id and voucher_type, which the schema already documents.

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 and resource ('buy a confirmed VAS voucher') and scopes it to an authenticated SIMcloud user, which separates it from the read-side siblings like get_vas_order and list_vas_products. It stops short of explicitly contrasting with those siblings by name, but the intent is unambiguous.

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 a concrete precondition (call list_vas_products first and use an available product and denomination) and an explicit when-not-to-act rule ('do not automatically retry an ambiguous result'). It does not say when to prefer this over the other purchase siblings (airtime/data/electricity), but the product naming makes those distinctions mostly self-evident.

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