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

With no annotations provided, the description carries the full burden of behavioral disclosure, and it does so excellently. It reveals that the operator is a guess based on number range, that MVNOs resolve to host networks, that confirmation is required before purchase, and that a wrong-network voucher cannot be redeemed or refunded. It also notes the tool is free.

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?

Every sentence carries essential information: the action, the free aspect, the detection caveat, the MVNO behavior, the required confirmation workflow, and the failure consequence. The description is longer than average but not bloated, and the key caveat is emphasized with clear 'DETECTED' and a direct warning.

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 lookup tool with no output schema, the description covers the core return behavior (country, operator, offers) and even names other_brands as a correction field. It could be more complete by describing the exact output shape or error handling for invalid numbers, but what is present is enough for a capable agent to invoke it correctly and responsibly.

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?

The input schema already documents the single phone parameter with format E.164, giving 100% schema description coverage. The description does not add significant parameter-level detail beyond the schema, which is acceptable but not contributive, so the baseline of 3 applies.

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 opens with a specific action and resource: 'Identify the country and mobile operator of a phone number and list the top-up offers available for it.' This clearly distinguishes the tool from generic list/quote/buy siblings by adding phone-based lookup behavior, and it is not a tautology of the tool name.

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 gives clear operational guidance: read the detected brand back to the human, confirm it before buying, and correct it using other_brands. It also explains the risk of buying for the wrong network. It does not explicitly name alternative sibling tools, so it stops short of full when-to-use vs. alternative coverage.

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