check_email_domain

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

With no annotations, the description carries the full burden and does so well: keyless and free, no paid API, DNS-only, and the explicit limitation that mailbox existence is not probed. It also enumerates the grade meanings (B/D/F) so the agent can interpret results without a follow-up call.

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?

Front-loaded with purpose, then return shape, then grade legend, then usage trigger. Every sentence carries information; nothing is padding.

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?

No output schema exists, but the description spells out the returned object shape and the meaning of each grade, plus the key limitation. For a one-parameter read tool, an agent has everything needed to call and interpret it.

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 100% and already documents that a domain or email is accepted, so the baseline is 3. The description adds the behavioral detail that the domain part of an email is extracted, which is marginally useful beyond the schema text.

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?

Opens with a specific verb+resource: 'Grade the email deliverability posture of a domain from its public DNS.' The grading semantics and the keyless/DNS-only scope make it unmistakable against siblings like enrich_company or company_hiring_signal.

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 a concrete trigger ('Use before trusting a role inbox or scoring a lead') and implicitly rules out paid verification APIs. It does not name a specific alternative tool or state when-not to call it, which keeps it short of a 5.

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