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

With no annotations, the description carries the full load and does so well: it discloses upfront balance charging, automatic refund on provider failure, the prerequisite workflow, and that the response id is the orderId consumed by check_sms/get_order/cancel_order. It stops short of permissions or failure-mode detail, but the financial behavior is clearly surfaced.

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?

Four sentences, each front-loaded with a distinct concern (purpose, cost/refund behavior, prerequisite, downstream id, alternative). Nothing is redundant against the schema or annotations.

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 purchase tool with no output schema, the description covers purpose, prerequisites, cost behavior, the returned id and its consumers, and the sibling alternative. An agent has everything needed to invoke it correctly.

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%, so the baseline is 3, but the description adds cross-tool meaning: provider and operator must come from get_pricing, and operator is optional ('any operator when omitted'). That is genuine value beyond the schema's own field descriptions.

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 one-time SMS activation: a number that receives the verification code for one service') and explicitly contrasts itself with the sibling order_virtual_number. An agent can distinguish it from order_virtual_number, get_virtual_number, and renew_virtual_number without opening any 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 an explicit prerequisite ('Call get_pricing first and pass the provider and operator it returns') and an explicit when-not with the alternative named ('For a number kept for days or months, use order_virtual_number instead'). No inference required.

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