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

With no annotations supplied, the description carries the full burden and does disclose two important traits the schema cannot express: a per-call price (€0.0015 per number) and the auth mechanism (Bearer token in Authorization charged per call). It stops short of describing behavior on invalid input (error vs. validity=false) or any rate limits.

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?

Compact and information-dense, but structured as a telegraphic noun list led by output fields rather than a purpose statement, which pushes the most decision-relevant information (what the tool does, when to use it) to second place. Every fragment carries content, so nothing is padding, but the ordering works against front-loading.

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?

An output schema exists, so the enumerated return fields are a bonus rather than a necessity; combined with pricing, auth, and both parameters, an agent has enough to invoke the tool correctly. The remaining gap is behavioral — no statement of error handling or of how partial data is returned for unparseable numbers.

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 0%, so the description must compensate, and it does: 'phone' accepts 'any format', and 'country' is an ISO-2 default region with a concrete example (UA) for local numbers. It does not state that both parameters are optional or what happens when phone is omitted, which the schema leaves equally silent.

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 name plus the enumerated outputs (validity, E.164/national formats, country, line type, carrier, location, time zones) make it clear this is a phone-number validation/enrichment tool, distinguishable from email_verify, ip_geo and vin_decode. What's missing is an explicit verb phrase such as 'Validates and returns metadata for a phone number' — the description starts with a bare output list.

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?

There is a single implicit usage hint — 'country: ISO-2 default region for local numbers' — which tells the agent when the country parameter matters. But nothing states when to call this instead of email_verify, or what to do for numbers already in E.164 form, and no exclusions or preconditions are given.

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