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

Annotations already declare readOnlyHint=true and openWorldHint=true, so safety is covered. The description adds genuinely useful context the annotations cannot convey: a cost of 1 email credit per successful check (noting that failures are free) and the single-address throughput limit. It omits any statement of accuracy or latency, which keeps it below a 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?

Two sentences, front-loaded with what the tool does and what it checks, then cost and usage in the second. Every clause carries information an agent needs; nothing is restated from the name or schema.

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 return-value semantics, and it partially does so by naming the four check dimensions a caller can expect results on. It does not describe the result shape or status vocabulary (e.g., deliverable vs. risky vs. undeliverable), which is the one gap for an agent that must interpret the response.

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?

There is only one parameter and schema description coverage is 100%, so the schema fully documents 'email'. The description adds no format, normalization, or edge-case guidance beyond what the schema already says, which is the baseline case for a fully covered simple parameter.

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 ('Verify') and resource ('an email address') and then enumerates exactly what verification covers: syntax, MX records, SMTP deliverability, and disposable-address detection. No sibling tool performs email validation (validate_phone is a distinct resource), so the scope is unambiguous to an agent scanning the tool list.

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 concrete usage contexts ('validate signup emails or clean mailing lists') and an important operating constraint ('one address at a time'), which steers an agent away from attempting bulk use. It stops short of naming an alternative or an explicit when-not-to-use condition, so it is clear context rather than full routing guidance.

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