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

The description discloses useful behavior: it runs offline and makes no per-lookup network call. Since there are no annotations, it carries the full burden, but it does not explain output shape, error behavior, or whether any state is changed.

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 dense sentence with the main purpose front-loaded and the key detail about no network call placed at the end. There is no wasted prose, though 'possibility checks' remains somewhat underspecified.

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?

With no output schema, no annotations, and an opaque nested input, the description must supply enough context to invoke the tool correctly. It covers purpose and offline behavior but omits the actual input contract and return format, making it incomplete for reliable use.

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 only provides an opaque nested input object with 0% description coverage. The description implies that a phone number is involved but does not specify the required input fields, value format, or how multiple numbers are represented, leaving the agent to guess the call contract.

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 names the resource (phone numbers) and the specific operations: E.164/national/international formatting, line type, and possibility checks. It does not use a strong imperative verb, but the domain and scope are unmistakable.

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?

It implies when to use the tool: offline or at the edge, when a per-lookup network call is undesirable. However, it does not explicitly state when not to use it or name any alternative tool.

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