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

The annotations already provide readOnlyHint and destructiveHint=false, so the description does not need to restate safety. It adds useful behavioral context by qualifying the scope as account-owned numbers and clarifying their role as possible 'from' numbers, going beyond what the schema or annotations express.

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 a single sentence that leads with the verb and resource, uses a parenthetical to add important semantic context, and contains zero filler or repetition of the title.

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 zero parameters, a lightweight input schema, and no output schema, the description is complete enough for an agent to invoke the tool correctly. It states what is returned and what the result is for, so no critical context 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?

The tool has zero parameters, so the description is not burdened to explain parameter meaning. The baseline for 0-parameter tools is 4, and the description does not introduce any conflicting or unnecessary parameter context.

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 uses a clear verb and resource — 'List the phone numbers' — and adds ownership scope ('owned by this account') plus intended usage ('usable as from numbers'). This distinguishes it from sibling tools such as search_available_numbers and buy_phone_number.

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 makes the usage context clear: use this tool when you need this account's owned phone numbers for the 'from' field. It does not explicitly say when not to use it or compare against list_caller_ids, so it stops short of a 5.

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