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

Annotations already cover the safety profile (idempotent, non-destructive, openWorld), so the bar is lower, and the description adds genuinely new context: the 1-credit cost and the country-code fallback requirement. It also enumerates the returned fields, which matters because there is no output schema. It does not explain caching behavior despite exposing bypass_cache.

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 tightly packed sentences with no filler; the output list and the country-code constraint are both front-loaded and immediately actionable. The output enumeration is slightly long but justified given no output schema exists.

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 stateless validation call with no output schema and no nested objects, the description covers inputs, output fields and cost adequately. The only meaningful omissions are the bypass_cache parameter and any note on latency or result caching behavior.

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 0%, so the description must carry the parameter burden. It explains `number` (the required param) and `default_country` with an example, adding real semantics beyond the bare schema, but `bypass_cache` is never mentioned, leaving one of three parameters undocumented.

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?

Specific verb (validate/check) plus resource (phone number) with an enumerated list of what is checked: validity, E.164, line type, region, time zone, carrier, VoIP, risk score, recommendation. It is clear what the tool does, but it never distinguishes itself from overlapping siblings like reverse_phone or carrier_lookup, so an agent cannot tell from the description alone which one to pick.

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 input prerequisite (numbers without '+' need default_country) but no when-to-use guidance and no routing to alternatives such as reverse_phone or carrier_lookup, which cover adjacent phone data. With three sibling tools in the same domain, the absence of any selection criteria is a real gap.

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