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

Annotations already declare readOnly, idempotent, openWorld and non-destructive, so the safety profile is covered. The description adds the useful geographic constraint (US/Canada only) but says nothing about behavior on invalid/unparseable numbers, rate limits, or whether a miss returns an error or an empty result.

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?

One short sentence, front-loaded with the verb and the returned fields, with zero filler. Nothing could be removed without losing information.

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 simple one-parameter read tool with rich annotations and no output schema, the description names the return fields (line type, carrier) and the region, which covers most of what an agent needs. It stops short of describing miss/error behavior, the only real remaining gap.

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% for the single parameter, so the schema already documents 'phone' as an E.164 string; baseline is 3. The description contributes one extra semantic constraint — the number must be US/Canada — which slightly narrows the seemingly global E.164 format in the schema.

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 (look up) and specific returned data (line type, carrier) scoped to US/Canada numbers, so the resource is unambiguous. It does not differentiate itself from siblings such as get_number or search_available_numbers, which keeps it from a 5.

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 phrase 'for any US/Canada number' implies the applicable context and geographic scope, but there is no explicit when-to-use, when-not-to-use, or pointer to an alternative (e.g. get_number for owned-number metadata). Usage is inferable rather than stated.

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