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

Annotations already declare readOnlyHint and openWorldHint, so the safety profile is covered structurally. The description adds genuinely new operational context: a billing cost of 1 phone-validation credit per successful call, and that enrichment is returned as part of the operation. It omits failure modes and what happens on an invalid number.

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?

Two sentences, front-loaded with the verb and returned fields, then cost, then usage condition. Every clause carries information and nothing is redundant.

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?

With no output schema, the description usefully enumerates the enriched fields an agent can expect, and it discloses the cost, which is material for a metered tool. It stops short of covering error behavior or output structure details, but is adequate for correct invocation.

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 100%, so both parameters (phone_number, country_code) are fully documented in the schema with format and E.164 guidance. The description adds no parameter-level detail beyond that, so the baseline 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?

States a specific verb (validate) plus the resource (phone number) and enumerates the enrichment payload (carrier, line type, country, location, timezone, formatting). An agent can distinguish this from check_spam_score or caller_id_lookup on the stated purpose alone.

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?

"Use when you need to know whether a number is real, who operates it, or where it is registered" gives clear triggering conditions. It does not explicitly exclude or route to siblings such as check_spam_score, check_whatsapp, or caller_id_lookup, so the guidance is clear but not exhaustive.

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