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

The description explicitly states the side effects: 'charges account and purchases number.' This goes beyond the annotations (destructiveHint: true, idempotentHint: false) by clarifying the financial impact and the definitive nature of the action personal. There is no contradiction with annotations.

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 extremely concise yet effective. The main purpose is stated in the first line, followed by a clear callout of side effects and confirmation requirement. Every sentence earns its place, with no fluff or redundancy.

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 the tool's complexity (billing impact) and that it has an output schema (implicitly, since has_output_schema is true), the description covers all critical contextual aspects: it states the action, the billing impact, the need for confirmation, and the side effects. An agent has enough information to decide and execute 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?

The schema description coverage is 100%, so both parameters are well-documented in the schema. The description adds no extra detail about parameters beyond what's in the schema, but it does emphasize the confirmation requirement, which is a crucial semantic hint. For coverage this high, baseline 3 is appropriate.

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 states the action (purchase) and the resource (Fax.Plus number by item ID). It also distinctively highlights billing impact and explicit confirmation, setting it apart from siblings like revoke-number or update-number, which are about other operations.

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 explicitly notes that this operation has billing impact and requires explicit confirmation, which clearly guides when to use it (when the user intends to purchase a number and has confirmed). It also implies that it should be used only after user consent, distinguishing it from read-only or management tools.

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