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

The annotations already establish readOnly, idempotent, non-destructive, closed-world behavior, so the safety profile is covered. The description still adds real value beyond them by disclosing that inactive numbers pending identity registration are included in the result, which is a behavioral nuance an agent would not otherwise know.

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?

A single front-loaded sentence that states the resource first and then the one qualifying detail. No filler, no restatement of the title or name.

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 no-parameter, no-output-schema list tool, the description covers what is returned and one important edge case (inactive numbers). It could say a bit more about the returned shape (e.g. what identifies each number), but nothing essential to calling it 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 takes zero parameters, so there is nothing to disambiguate; per the rubric this is the baseline 4. No schema or description detail is missing on the parameter side.

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 ('Returns this account's phone numbers') and scopes it to the caller's own account, which implicitly separates it from search_numbers (catalog search) and quote_number. It stops short of explicitly naming those siblings, so the distinction must be inferred.

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?

Usage is implied by the 'this account's phone numbers' scoping, but there is no explicit when-to-use guidance and no named alternative for the closely related search_numbers or list_identities tools. The agent has to infer that this is the inventory-listing tool.

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