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

With no annotations, the description carries the full burden, and it does disclose a real behavioral trait: the x402 M2M cost of 0.002 USDC on Base, which an agent must know before invoking. It also enumerates the response fields. It stops short of stating idempotency/rate limits, but validation is inherently a side-effect-free read, so the gap is small.

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 capability, then cleanly segmented into Trigger/Input/Output/Cost. Every sentence carries information, though the label-heavy template reads slightly boilerplate.

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 one-parameter tool with no output schema, the description correctly supplies the return contract (validity, country code, carrier, line type) and the payment cost. Nothing essential to correct invocation is missing.

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% and the single 'phone' parameter is already documented with an example ('+14155552671'), so baseline is 3. The description reinforces the international/E.164 intent but adds no format constraints or validation rules beyond the schema.

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 set ('validates international phone numbers', 'carrier lookup', 'formats to E.164'), naming exactly what the tool returns. No sibling covers phone validation, so the tool is unmistakably distinct from the scraping/enrichment tools in the sibling 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?

The explicit 'Trigger: Call this tool when...' line gives clear deployment context (validating, checking validity, formatting, carrier/VoIP detection). No alternatives or exclusions are named, but no sibling is competing for this task, so the omission is minor.

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