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

Adds substantive behavior beyond the annotations: it costs 1 account credit per successful check, it does not prove mailbox ownership or guarantee delivery, and it does not send email. The credit-cost disclosure is especially valuable because it explains why readOnlyHint is false, something the annotations alone leave ambiguous.

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?

Three sentences, each earning its place: capability, limitations, side effects/cost. The core action is front-loaded and there is no filler or restatement of the title.

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 one required parameter, no output schema and openWorldHint=true, the description covers capability, cost, and limitations well enough to call correctly. It stops short of hinting at the result shape (e.g. flags versus a single verdict), which is the only remaining gap given no output schema exists.

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?

Only one parameter, and schema coverage is 100% with its own description ('Email address to check'). The description adds meaning about what gets done with the address but no format or constraint detail beyond the schema's maxLength/minLength, 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 and resource ('Check an email address') and enumerates exactly what is inspected: syntax, MX records, disposable-domain and role-address indicators. A sibling list of dns_check, ip_lookup and phone_lookup makes the resource boundary obvious, so an agent can route without opening the 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?

The phrase 'supplied by the user' implies the usage context (validating a caller-provided address), but there is no explicit when-to-use, when-not, or contrast with dns_check, which could also resolve MX records. Usage is inferable but not spelled out.

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