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

With no annotations available, the description carries the full burden and provides ample behavioral detail: it discloses the $0.20 cost, states that unparseable numbers are not charged, explains the DNC registry limitation, and instructs where to get an API key. This is a genuinely transparent description for a paid verification tool.

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 front-loaded with purpose, then delivers limitations, costing, non-charge behavior, and auth guidance in a compact sequence. Every sentence carries useful information and there is no filler.

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 single-parameter tool with no output schema, the description covers what the tool returns, its restrictions, its billing semantics, and where to get credentials. Nothing essential is missing for an agent to decide whether to call it and how to call it correctly.

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 description coverage is 100%: the only parameter, 'phone', is already fully documented with format examples. The description's phrase 'single US phone number' reinforces the schema but adds no significant new parameter-level meaning, so the baseline score 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 opens with a specific verb and resource: 'Verify a single US phone number' and immediately enumerates the produced data (line type, carrier name, activity score, TCPA litigator status). This clearly distinguishes it from the sibling get_credit_balance, which has no relation to phone verification.

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 gives strong context for when to use the tool by covering price, credit-balance implications, and a factual exclusion ('Does not check Do Not Call registries'). It does not explicitly say 'use get_credit_balance to check your balance first,' but mentioning the NumberBroom credit balance inherently points toward the sibling tool.

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