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

Annotations only state readOnlyHint and openWorldHint, but the description adds rich behavioral details: receive-only nature, fair-use limit of 25 SMS/day, live stock per duration with guidance to skip OUT OF STOCK, regional differences (US/UK real mobile, Canada VoIP), and prohibitions on banking/financial/crypto-exchange verification. This far exceeds expected disclosure.

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 sentences, each adding distinct value: the first defines the resource and key operational traits, the second describes the output fields and stock interpretation, the third adds geographic and use-case restrictions plus the purchase link. No redundancy or filler.

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?

With no input schema parameters and no output schema, this description alone must convey enough for correct usage. It covers what the tool returns, how to interpret stock, special cases (VoIP), restrictions, and how to proceed after listing. This is fully adequate for a simple listing operation.

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 tool has zero parameters, so the baseline is 4. The description correctly avoids inventing input details and instead focuses on output aspects (country, rentalId, duration tiers, prices, stock), which is useful for an agent setting expectations.

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 starts with the verb 'List' and names the resource 'long-term SMS RENTAL numbers', making its function unambiguous. It also distinguishes from siblings like list_esims or list_orders by focusing on rental numbers and specifically mentions the purchase counterpart (rent_sms_number).

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 clearly implies when to use it: whenever you need to list available long-term SMS rental numbers. The pointer to 'Purchase with rent_sms_number' indicates the typical follow-up action. However, it does not explicitly compare against alternatives like list_sms_orders, even though the self-descriptive name reduces ambiguity.

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