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

With no annotations, the description carries the disclosure burden and does it well: it states the lookup is offline, that carrier/owner data is NOT returned, that malformed numbers are rejected before payment, and the exact price/mechanism ('$0.0021 via x402'). Missing only finer details like idempotency/charge semantics, which are in the schema instead.

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?

Four tightly packed clauses with zero filler, and the core capability is front-loaded before caveats, exclusions, and pricing. Every sentence earns its place.

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?

No output schema exists, so the description must communicate return values — and it does, listing the exact parsed fields. Combined with the pricing, rejection, and scope caveats, an agent has everything needed to decide and call correctly.

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 67% — region and request_id already carry their own descriptions, while 'number' has none. The description mentions region implicitly via output formats but adds no syntax or format guidance beyond what the schema provides, so it sits at the baseline.

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+resource ('Parse and validate one phone number') and enumerates the outputs (valid/possible, type, region, E.164, national/international formats). It also carves out scope by negation ('No owner or carrier lookup'), so an agent can distinguish it from a hypothetical carrier-lookup tool without opening the 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?

Gives clear operating context: offline, single-number, no owner/carrier lookup, and unparseable inputs are refused before payment. It does not name an explicit alternative sibling, but the exclusion of carrier/owner lookup effectively routes that need elsewhere.

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