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

Annotations already declare readOnlyHint=true and destructiveHint=false, so safety is covered. The description adds genuine behavioral context beyond that: the call is a prerequisite lookup for a later mutation and it returns price data, not just identifiers. It does not discuss caching or rate limits, so it stops short of a 5.

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 short sentences, with the action and the pre-call rule both front-loaded and nothing redundant. The opening fragment "Rented SMS numbers." is a truncated topic label that adds little and slightly interrupts the flow.

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 annotations cover the safety profile. The description supplies the remaining agent-critical information: what the tool yields, and the ordering relative to sms_rental_extend.

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 `rental_id` parameter is already documented as a UUID in the schema. The description adds only that the id must refer to an existing rental, which is marginal beyond the structured field. Baseline 3 applies when the schema carries the parameter burden.

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: list the durations an existing rental can be extended by and the cost of each. The phrase "extended by" separates it from the sibling sms_rental_list_intervals, which lists initial rental durations, and from sms_rental_extend, which performs the mutation.

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 sequencing rule: call this before sms_rental_extend to pick a valid `hours`. It names the alternative tool and the condition that selects it, so the agent does not have to infer the workflow.

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