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 well: it discloses that data is offline (not a live carrier lookup), names a concrete accuracy limitation (US/Canada VoIP undetected), and states a rate limit (free 30 checks/day). It does not mention authentication or what happens when the quota is exhausted, which are the remaining behavioral gaps.

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?

Three tight sentences with the capability and coverage front-loaded, followed by the accuracy caveat and the quota. Every clause carries information; nothing is repeated from the name or padded.

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 2-parameter tool with no output schema and no annotations, the description supplies the return-value inventory, the data-source caveat, and the rate limit, which is nearly everything an agent needs. The one real omission is semantics for 'default_country', which leaves the caller guessing how national-format input is resolved.

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% for 2 parameters, so the description must compensate and only partly does. It references country and national format, hinting that a country context is needed for parsing, but never explains the 'default_country' parameter (its US default or when to override it) and gives no format guidance for the 'phone' argument itself.

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 and resource ('Check one phone number') and immediately scopes it with coverage (245 countries/territories, offline data). It also enumerates the returned fields (E.164/national format, country, region, time zones, line type, risk_score, accept/review/reject), so an agent can tell it apart from the sibling verify_email without opening either schema.

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 'one phone number' framing implies single-record validation rather than batch, and the 'No live carrier lookup; US/Canada VoIP is not detected' sentence tells the agent when this tool's answer is not authoritative. It lacks an explicit statement of when to prefer a sibling or a live-lookup alternative, so it stops short of full routing guidance.

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