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

Annotations only declare readOnlyHint and openWorldHint; the description adds genuinely new behavioral context — that a rented number is held for the entire period and receives every inbound SMS, contrasted against single-code OTP orders. That semantics-of-what-you-get detail is exactly what annotations can't convey. It doesn't describe the shape of the returned listings, but no output schema exists to lean on.

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 scope, then the rental-vs-OTP distinction, then the omission behavior. No filler and nothing buried.

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?

One optional enum parameter, full schema coverage, read-only annotations, and no output schema — a low-complexity tool. The description covers the action, the time-range scope, the product distinction, and the omitted-parameter mode, which is everything needed to invoke it correctly.

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 100% and the enum values are fully documented, so the baseline is 3. The description earns an extra point by explaining the otherwise undocumented semantic of omitting `country` — that it switches the response to a list of rentable countries, a mode change the schema does not express.

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 ('List the rental periods and prices') and bounds the scope with '(1 day to 12 months)'. It explicitly contrasts rentals with OTP orders, which disambiguates it from create_otp_order, but it never addresses the overlapping siblings get_pricing and get_inventory, so an agent can't fully rule those out from the description alone.

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?

'Omit `country` to list the countries numbers can be rented in' gives concrete calling guidance for the optional-parameter path, and the 'unlike an OTP order' clause tells the agent which scenario this tool serves. It stops short of explicitly saying 'call this before rent_number' or naming any case where another tool is preferred.

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