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

With no annotations, the description carries the load and does well: it discloses that checks are live DNS lookups (external network dependency), that no SMTP probe is performed, and that output includes a deliverability verdict plus a 0-100 score. It omits latency/error behavior and does not say what happens if both optional parameters are omitted.

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?

A single dense sentence, front-loaded with the action and scope, followed by the enumerated checks and a trailing caveat. Every clause carries distinct information and nothing is repeated from the schema or 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?

For a two-parameter, no-annotation tool with no output schema, the description covers input modes, the check list, the returned verdict and score, and the SMTP limitation. The one real gap is the zero-required-parameter schema: nothing tells the agent that a call with neither field is meaningless.

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 100% and both parameters are already documented (email address, or a domain such as stripe.com). The description corroborates the either/or intent by saying "email address or domain," but adds no format details, no precedence rule when both are supplied, and no note that at least one of the two zero-required parameters must be provided.

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?

Names a specific verb (Validate) and resource (an email address or domain) and enumerates the exact checks performed: syntax, MX/null-MX, SPF, DMARC, disposal/free/role flags, verdict, score. An agent knows precisely what this tool returns without opening the schema, and the sibling list (company_profile, site_audit, sitemap_urls) contains nothing that could be confused with it.

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?

"Validate an email address or domain via live DNS" establishes the call context, and "No SMTP probe" sets an explicit boundary on what the verdict does and does not prove (no mailbox-existence check). There is no named alternative tool to route to and no exclusion beyond the SMTP note, so it stops short of a full when/when-not statement.

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