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

With no annotations, the description carries the full burden and does substantial work: it discloses the scoping limit (no SMTP probing), the quota model (20 free checks/day per network without a key, 1 check with a key), and the shape of the response including suggested_action and did_you_mean. It does not mention latency, caching, or whether addresses are logged/stored, which are the main remaining behavioral unknowns.

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 tightly packed sentences: what is checked first, the exclusion second, the output contract and cost model third. Every clause carries information an agent would otherwise have to guess, and nothing is repeated from the 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?

There is no output schema, and the description compensates by naming the key result fields (suggested_action, did_you_mean, verdict, reason) and the decision outcomes. Combined with quota and no-SMTP disclosure, an agent can call this correctly; only minor gaps like error behavior on malformed input remain.

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 coverage is 0% and there is one required string parameter, 'email'. The description refers to 'an email address' but adds no format, normalization, or validation expectations beyond what the parameter name already implies. The single-parameter surface keeps this adequate, but the description does not compensate for the missing schema description.

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 (Check) and resource (an email address) and enumerates the exact checks performed: syntax, MX/A via DNS-over-HTTPS, disposable domain, role address, free provider, and typo suggestion. It is unambiguous about what the tool does, but it never mentions the sibling validate_emails, so an agent gets no explicit signal that this is the single-address variant rather than the bulk one.

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 'an email address' (singular) and by the explicit exclusion 'No SMTP probing', which tells the agent what this tool will not do. However, there is no statement of when to prefer validate_emails for multiple addresses, no prerequisites, and no guidance on handling results beyond the listed suggested_action values.

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