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

Annotations already flag destructiveHint=true and openWorldHint=true, but the description adds the crucial business consequence those hints cannot express: it debits the real wallet balance and requires user confirmation. It also discloses what the call returns (the number and rental end time), which no annotation or output schema provides.

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?

Four short sentences, front-loaded with the action and its required inputs, then the side effect, then the warning. Every sentence carries information the agent needs and nothing is repeated.

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 output schema, the description still tells the agent what comes back (number and rental end), what the call costs, and how to read messages afterwards. Combined with the fully documented schema and destructive annotation, nothing needed to invoke it correctly is missing.

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% and both enums are fully enumerated with human-readable expansions, so the description's mention of 'country' and 'period' adds nothing new. Its availability caveat ('not every period is on sale in every country') is already stated in the period schema description, so the baseline 3 applies.

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 ('Rent a phone number') plus the scoping dimensions (country, period). It is clearly distinguishable from siblings like get_rental_offers (browse) and read_rental_sms (consume), and an agent can identify it without opening the schema.

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 prerequisite ('get the periods and prices from get_rental_offers first') and an explicit confirmation step before spending. It also points to the follow-up tool ('Read its SMS with read_rental_sms'), so both the entry and exit paths of the workflow are named.

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