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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered. The description adds genuine context beyond that: it discloses that the call performs operator detection and returns recipient value, validity and USD price, which tells the agent what the response contains.

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?

Two compact sentences, front-loaded with the core action and followed by the return contents and the input constraint. No filler; it could be marginally tighter but nothing wastes space.

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 one-parameter read-only lookup with no output schema, the description covers enough: it names the return fields and the input format requirement. Missing only pagination/availability caveats, which are minor for this shape.

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% and the single 'phone' parameter is already documented as a full international number. The description reinforces the country-code requirement and adds the concrete example +911234567890, which is mild added value over the schema but not substantial.

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 specific verbs and resources: detects the operator of a mobile number and returns live top-up options. The 'top-up' framing separates it from the sibling get_esim_plans, but the description never explicitly contrasts the two product lines, so an agent must infer the distinction.

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?

Implies when it applies (you have a mobile number and want top-up options) and gives a hard prerequisite — the number must include the country code. It offers no explicit when-not-to-use or routing against get_esim_plans / create_checkout_link, so guidance stays implied.

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