Look up a number's carrier (assigned block)

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

With no annotations, the description takes full responsibility for behavioral disclosure. It reveals that data is from public block allocation, not live, and coverage is limited to major metros with uncovered areas returning covered:false. It also notes that ported numbers may yield incorrect current carriers.

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 concise and front-loaded, with only four sentences that efficiently convey the action, data source, limitations, and disclaimers. No filler or redundant 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?

The description adequately covers input, output fields (carrier, line type, rate center, covered), and limitations in the absence of an output schema. It could mention the output format explicitly, but the provided details are sufficient for an agent to understand what to expect.

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 describes the single parameter 'number' with an example format, achieving 100% coverage. The tool description adds context about the number being US/Canada and the rate center lookup, but does not add significant meaning beyond the schema. Baseline 3 is appropriate.

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 states the tool's purpose: looking up carrier, line type, and rate center for US/Canada phone numbers based on assigned NPA-NXX blocks. It distinguishes itself from sibling tools like lookup_phone_number, check_number_reputation, etc., by specifying its unique functionality.

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 provides clear usage guidance: it's based on block-allocation data, not a live lookup, and warns about ported numbers and coverage limitations. It explicitly states what it is not for (caller identity, fraud, FCRA decisions), helping agents avoid misuse.

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