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

With no annotations supplied, the description carries the full burden, and it delivers the two non-obvious traits: results come from offline formatting/number-range data (no network probe), and the call runs on the caller's OpenAny gateway credentials with charges and authorization determined by the gateway. It does not cover failure behavior (invalid number responses) or latency/caching, which keeps it below 5.

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?

Short and front-loaded: the core behavior and its limitation come first, with credential/billing context second. The billing sentence is slightly boilerplate but does carry real information (who is charged), so little is wasted; a tighter single paragraph would still convey everything.

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, no-output-schema tool, the description supplies what the response contains (number type, issuing network, formatted output), the scope limitation, and the auth/cost model — everything an agent needs to call it correctly and interpret the result. Remaining gaps (error semantics) are minor at this complexity level.

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% — input_format, output_format and telephone_number are each documented in the schema, including accepted ISO3/E164 forms. The description adds no parameter-level meaning beyond that, so the baseline of 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?

Uses a specific verb and resource ('Validates the number and returns number type, issuing network and formatted output') and immediately delimits scope against the HLR-lookup family with 'without a live HLR lookup' — important because the tool name itself contains 'hlrlookup' and would otherwise mislead. An agent can tell this is an offline validation/normalization call rather than a reachability check without reading 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?

'Formatting and number-range information only; no live network reachability' effectively states when this tool is appropriate and when it is not (any task needing live reachability). What's missing is an explicit pointer to the alternative sibling (e.g. a phone-lookup or fraud-lookup tool) that should be used instead when reachability matters, so routing is implied rather than prescribed.

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