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

Annotations already declare readOnlyHint=true and destructiveHint=false, so the safety profile is covered. The description adds that the result is a set of durations measured in hours paired with prices, which is useful context, but says nothing about pricing currency, availability, or per-country variation.

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 short sentences, front-loaded with the scoping clause; nothing is padded. The opening noun fragment 'Rented SMS numbers.' is the only piece that does not clearly earn its place.

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?

An output schema exists, so return values need no explanation, and both required parameters are at least named in context. The remaining gap is provenance of the two IDs and price currency, which is minor for a read-only two-parameter lookup.

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 0%, so the two required parameters carry no documented meaning in the schema itself. The phrase 'for a service+country' maps them to service_id and country_id, but leaves format/ID-source questions open (service_id is a string, country_id an integer) and never points to sms_rental_list_services or sms_rental_list_countries for obtaining IDs.

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 gives a specific verb ('List') and resource ('rentable durations (interval in hours) and prices'), scoped by service+country, which is distinct from every sibling (list_numbers, list_services, list_extend_options). The leading fragment 'Rented SMS numbers.' is truncated and adds little, but the main clause 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?

Usage is only implied: an agent can infer this is a pre-purchase lookup of available rental durations and prices. It never says when to call it versus sms_rental_list_extend_options or sms_rental_buy_number, nor that it is a prerequisite step before buying.

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