Rent a real test phone number

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

With no annotations provided, the description carries the full burden of disclosing side effects. It explicitly states 'Debits the account balance' and 'Returns a rentalId', which are important behavioral traits. It does not mention potential failures like insufficient balance or cancellation, but the key side effects are covered.

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 three sentences, all substantive and free of fluff. It front-loads the core purpose, then adds the critical side effect, and finally gives actionable follow-up guidance.

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?

Given there is no output schema and no annotations, the description provides enough context: it explains what the tool does, what it returns (rentalId), and how that return value is used with a sibling tool. It could mention error cases or cost details, but for a two-parameter tool, it is reasonably 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 parameters are already well-documented. The description adds little beyond the schema, aside from contextualizing serviceSlug as a 'given service' and period implicitly affecting the debit. This meets the baseline but does not exceed it.

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 clearly identifies the action ('Rent'), the resource ('a real UK phone number'), and the purpose ('receive an SMS/OTP verification code'). It is easily distinguished from sibling tools like create_test_email or wait_for_test_sms by focusing on the phone number rental and service-specific context.

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 conveys when to use the tool: when you need a phone number to receive an SMS/OTP for a specific service. It also tells the user to use wait_for_test_sms with the returned rentalId, providing a clear next step. However, it does not explicitly exclude alternatives or compare with other tools, so it falls short of a 5.

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