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

Annotations already cover the safety profile (readOnly, idempotent, closed-world), and the description adds genuinely new context: the paid x402/Base mainnet cost of $0.001 USDC per call and the specific classification vocabulary returned. It omits what happens on an unparseable or non-SA number, which leaves a real gap for a tool marketed as validation.

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 core capability is front-loaded in the first sentence, with use cases and pricing following. Dense and largely waste-free, though the pricing clause is bolted on rather than integrated.

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 carries the burden of describing returns and mostly does: it lists both output formats and the classification categories including area lookup for major cities. The missing piece is error/invalid-input behavior, which matters for a validation tool.

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 coverage is 100% and the single 'number' parameter already carries an example. The description's phrase 'any common format' reinforces tolerance of varied input syntax but adds no format rules beyond what the schema shows, so baseline 3 applies.

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: formatting (to E.164 +27 and local) plus classification (mobile, landline with city area, toll-free, VoIP, premium-rate). This clearly distinguishes it from the sibling validators, which target ID numbers, tax numbers, company numbers and domains rather than phone numbers.

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?

Names concrete contexts where the tool is the right pick: cleaning contact lists, SMS/WhatsApp messaging, form validation. It does not name an alternative tool or state exclusions, so it falls short of the explicit when/when-not routing a 5 requires.

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