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

Annotations already declare readOnlyHint=true, destructiveHint=false and openWorldHint=true, so the safety profile is covered. The description adds meaningful context beyond that: the specific verification stages and the credit cost. It does not, however, address auth (the api_key parameter) or rate/error behavior.

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 tight sentences, no filler. The core capability is front-loaded and the cost caveat is appended where it belongs.

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 read-only verification tool with no output schema, the agent is told what is checked and the credit cost, which is adequate on the safety axis. But the return shape (e.g., deliverability verdict fields) and the undocumented api_key leave gaps an agent would want before calling.

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% — neither 'email' nor 'api_key' is documented in the schema. The description implicitly covers the email input ('one email address') but says nothing about the api_key parameter, its default, or when it is needed, so it fails to compensate for the coverage gap.

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 and resource ('Verify one email address') and enumerates the exact checks performed: syntax, disposable-provider check, and MX lookup. The word 'one' implicitly contrasts with the plural sibling verify_emails (bulk), letting an agent distinguish them without opening a schema.

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?

Usage is implied by 'one email address' versus the bulk sibling, and the cost note ('Costs 1 credit') is a useful selection signal. However, no alternative tool is explicitly named and there is no statement of when NOT to use this (e.g., for batches) — the agent must infer the routing.

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