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 one genuinely useful behavioral fact — the result set grows over the rental lifetime, motivating polling — but says nothing about rate limits, whether polling costs credits, or how pagination behaves across polls.

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, front-loaded with the core action and followed by the polling advice. The leading fragment 'Rented SMS numbers.' is close to redundant with the rest and is the only wording that doesn't 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?

For a simple read tool with an output schema that already describes return values, the description covers the essentials: what is listed, the scoping to a rental, and the temporal/polling nature of results. The main omission is how to use last_id to fetch only new messages across polls.

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 67%: rental_id and last_id are documented in the schema, while per_page is bare. The description does not mention any parameter, pagination cursor behavior, or result limits, so it adds no meaning beyond the schema — baseline 3 applies since the schema does most of the work.

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 uses a specific verb and resource: 'List the SMS messages received on a rented number.' The phrase 'rented number' implicitly separates it from the activation siblings (sms_activation_get_code), but it never names the alternative explicitly, so sibling differentiation is left partly to inference.

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?

It gives real usage guidance: 'Poll periodically; new messages arrive over the lifetime of the rental,' which tells the agent this is a repeated-read tool rather than a one-shot fetch. It stops short of saying when to prefer it over the activation counterpart or when not to poll, so no exclusions are provided.

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