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

Annotations already cover safety (readOnly, idempotent, non-destructive, openWorld), so the description justifiably focuses on extra behavior: it discloses that each request spends 1 credit and that failed requests also count, plus the API key requirement. It does not describe rate limits or response shape, but the billing/auth disclosure is genuinely useful context beyond annotations.

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?

Front-loaded with the core action and its outputs, followed by two short clauses for billing and auth. Slightly dense but every sentence carries information; 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?

With no output schema, the description helpfully enumerates the enrichment fields an agent can expect, and it covers cost and credential prerequisites. What it lacks is any statement of when to prefer this over sibling tools, a minor gap for a clearly-scoped lookup.

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%, so the schema already documents both parameters including the country code rule and phone format. The description adds no syntax or format guidance beyond what the schema provides, making the baseline 3 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?

States specific verbs (validate and enrich) plus the resource (a phone number) and enumerates the returned facets: format, carrier, line type, location, line status, risk, and breaches. This clearly distinguishes it from siblings like validate_email, validate_iban, and lookup_ip, which target different resources.

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?

Usage is implied by the resource rather than stated: no explicit when-to-use, when-not, or named alternatives among the sibling validators/lookup tools. It does add useful prerequisites (Phone Intelligence plan and API key), which partially compensates.

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