buy_number

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

Annotations only supply openWorldHint=true, so the description must carry the rest, and it does: it discloses that this is a paid purchase that charges the account balance and that confirm must be true to complete it. It stops short of stating reversibility or what happens to a number once purchased, but the financial-impact disclosure is the key behavior an agent needs.

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 sentences, front-loaded with the action and cost, then the two notable parameters, then the confirm gate. Slightly repetitive with the schema (which already documents confirm and area_code), but no sentence is wasted.

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 100% schema coverage, minimal annotations, and no output schema, the description covers the essentials: what it does, that it costs money, the confirm gate, and the two behaviorally interesting parameters. It omits what a successful result returns and whether country defaults matter, which are minor 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 the baseline is 3, but the description adds interpretive value: area_code maps to a region with a concrete example ('415' for San Francisco) and agent_id is described as attaching the number on purchase. That clarifies the effect of agent_id beyond the schema's terse wording.

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 ('Purchase a new phone number'), and the word 'new' implicitly separates it from attach_number, which attaches an existing number. It never names that sibling explicitly, so it falls short of the 5-tier bar for sibling differentiation.

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 one concrete prerequisite ('Requires confirm=true to proceed') and notes the cost, which is genuine usage guidance. However, it never says when to prefer this over attach_number or how it relates to list_numbers for selecting a number, leaving the choice to inference.

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