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

With no annotations, the description carries the full behavioral burden. "List" implies a read-only operation, and "this account owns" usefully constrains scope to owned numbers, but there is no disclosure of pagination, result size limits, rate limits, or what fields each entry contains.

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 with zero filler, appropriate for a one-parameter list tool. It borders on under-specified rather than verbose, but nothing in it 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?

The tool is simple, so a short description is defensible, but with no annotations and no output schema the definition should say at least what a returned entry looks like or whether results are paginated. As written, an agent knows what it gets at a high level but not enough to consume the response confidently.

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?

There is only one parameter, api_key, and the schema documents it at 100% coverage including its origin (sign_up). The description adds nothing about the parameter, so this lands at the baseline for high coverage rather than above it.

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 pairs a specific verb ("List") with a specific resource ("phone numbers") and scopes it to what "this account owns," which separates it from a global or public number search. It doesn't name the sibling get_phone_number, but the list/get distinction is inferable from the verbs alone.

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?

There is no explicit when-to-use, when-not-to-use, or named alternative. An agent must infer on its own that get_phone_number is the right choice when details for a single number are wanted, and nothing indicates whether released or pending numbers appear in the list.

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