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

Annotations already declare readOnlyHint=true, idempotentHint=true, openWorldHint=true, and destructiveHint=false, so the safety profile is covered. The description adds valuable return-value detail (line type, carrier, region, canonical formats) that annotations do not provide, though it does not mention rate limits or credit consumption.

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?

Front-loads the purpose and return values, then input-format guidance, with no wasted preamble. The inline example largely duplicates the schema examples, which slightly reduces conciseness but does aid quick comprehension.

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 carries the burden of explaining return values, and it does so by naming the exact fields returned. Input formatting is covered, and annotations cover the safety profile, so an agent has everything needed to invoke the tool 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 description coverage is 100%, so all three parameters are already documented in the input schema. The description reinforces E.164 formatting and default_country usage but does not add semantics beyond what the schema already says, making the baseline 3 appropriate.

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 precise verb (Validate) and resource (phone number) and enumerates exactly what is returned: validity, line type, carrier, region, country, and canonical formats. This clearly distinguishes it from the sibling veriphone_credits tool, which manages account credits rather than validation.

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 practical input-format guidance: prefer E.164, and supply default_country for local numbers without a leading '+'. The context for use is clear, but it does not explicitly name alternatives or state when not to use the tool, which keeps it at a 4 rather than a 5.

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