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

Annotations already declare readOnly, idempotent, openWorld, and non-destructive, so the safety bar is low. The description adds genuinely new behavioral context by disclosing cost ('Spends 1 credit'), which an agent needs to decide whether to call this versus a bulk path, though it says nothing about rate limits or auth.

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?

A single front-loaded sentence covers purpose and output, followed by a three-word cost disclosure. No filler, no restatement of the title, and the most decision-relevant facts come first.

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 an output schema present, the description needn't explain return values, and annotations cover the safety profile; the credit cost is the key missing-in-annotations detail and it is present. The only gap is the lack of routing guidance against bulk_verify.

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% with only two well-documented parameters, so the schema carries the parameter semantics. The description adds no format guidance (e.g. accepted national formats) beyond what E.164/national and the ISO country example already provide.

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?

The description names a specific verb and resource ('Validate a phone number') and enumerates what the validation covers: real/active status, line type, carrier, DNC status, and risk level. It does not differentiate itself from the sibling bulk_verify, so it falls short of a 5.

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?

There is no explicit when-to-use guidance, no prerequisites, and no mention of alternatives such as bulk_verify for multiple numbers. The only implicit signal is the singular 'a phone number', which an agent must infer means one-at-a-time use.

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