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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false, and openWorldHint=false, so the safe-read profile is covered structurally. The description adds genuinely useful context beyond that: no authentication/sign-in is required, and it explicitly does not purchase, clarifying the side-effect-free nature of the call.

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 short sentences, front-loaded with the core behavior (what is returned) before the constraints. Every clause carries information — scope, price basis, auth status, and the no-purchase boundary — with zero filler.

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 4-parameter read-only search with no output schema and a well-covered input schema, the description is largely sufficient: it explains what is returned and the pricing basis. It stops short of describing result shape (e.g., pagination, per-number fields), but with no output schema declared that omission is minor.

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 75% (country, currency, number_type all documented in-schema), so the schema carries most parameter meaning and the baseline is 3. The description adds only that city is optional and that pricing is the rate-checker list price; it adds nothing on currency options or number_type semantics 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 clear verb+resource+scope: returns available phone numbers filtered by country and optional city, with the monthly rate-checker list price. This is specific, but it does not distinguish itself from nearby siblings like quote_number, list_did_numbers, or get_did_rate, so an agent must infer the difference.

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 description gives one useful boundary — 'Does not purchase a number' — and notes no sign-in, implying this is a browse/preview step. However, it never names the alternative tools for pricing (quote_number) or purchasing, nor states positively when this should be chosen over them, leaving selection partly to inference.

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