Rent a phone number

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

With no annotations provided, the description carries the behavioral transparency burden. It discloses the monetary side effect ('Debits the account balance'), the return value ('rentalId'), and the nature of the number ('real UK phone number'). It does not cover failure modes or prerequisites, but the disclosed side effect is crucial for a rental tool.

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?

The description is two sentences: the first states the purpose and capability, the second states the cost side effect and how to proceed with the result. Every word earns its place, with no fluff or unnecessary repetition.

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?

The description covers the essential aspects for a mutating tool with no output schema: what it does, what it costs, what it returns, and how to use the result. Minor omissions include error handling and explicit requirement to call list_services first, but the schema covers the serviceSlug source, so the description remains largely complete.

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 100%, so the schema already documents both parameters with descriptions and enum values. The tool description adds minimal parameter context beyond restating 'given service' and mentioning the return value, which does not exceed the baseline 3.

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 states a specific verb ('Rent') and a clear resource (a real UK phone number for a service) with the intended purpose (receiving an SMS/OTP verification code). It clearly differentiates from sibling tools: list_services lists services, get_rental/wait_for_sms check rental status or wait for SMS.

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?

The description implies usage when you need a UK number for OTP verification and explicitly mentions the follow-up tools get_rental or wait_for_sms, creating a clear workflow. It does not explicitly state exclusions or contrast with alternatives, but the context is clear.

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