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

With no annotations, the description carries the full burden, and it does disclose that the rental is paid from the balance, which is a meaningful cost/auth signal. It omits whether the order is reversible, what happens at term expiry, and the consequences of autoRenew (recurring balance deduction), which matter for a paid mutation.

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?

Two tightly packed sentences with no filler; the core action and its cost source lead, and the mode semantics follow. Every clause carries information.

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?

For a 6-parameter paid mutation with no annotations and no output schema, the description covers type semantics and payment source but leaves out lifecycle behavior (expiry, renewal, cancellation path via cancel_virtual_number) and what is returned. Adequate but with clear gaps.

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%, so baseline is 3, but the description goes beyond the terse schema text by explaining the semantic difference between the two enum values and where the "service" id must come from. It adds real meaning rather than restating field descriptions.

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+resource ("Rent a virtual number") and adds a scope detail ("from the balance"). It does not, however, differentiate itself from the sibling order_number, which appears to be a near-identical action an agent must choose between.

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 gives conditional guidance on the two modes (unlimited vs service) and routes to get_number_pricing to obtain a service id, which is genuine usage steering. It says nothing about when to prefer this tool over order_number, nor about prerequisites (balance sufficiency, country availability).

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