Email Deliverability

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

Annotations already declare readOnlyHint, idempotentHint, destructiveHint=false and openWorldHint, so the safety profile is covered. The description adds real behavioral value by enumerating the response content (MX presence, SPF, DMARC, provider classification, overall verdict), though it says nothing about failure modes, caching, or rate limits.

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 compact sentences: the capability first, the returned signals second, the use case last. No filler, and the most decision-relevant information is front-loaded.

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 one-parameter read-only tool with an output schema already defined, the description supplies everything an agent needs: what it checks, what it returns in summary, and when to reach for it. Return-value detail can safely live in the output schema.

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 the single parameter already documents that it accepts a domain or an email address. The description's "(or the domain of an email address)" restates what the schema says rather than adding new semantics, 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 ("Assess whether... can receive mail and how strong its sender authentication is") against a specific resource (domain or email domain), and enumerates the exact checks it performs. This is enough to distinguish it from generic siblings like dns_lookup or domain_intel, which return raw records rather than a deliverability verdict.

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?

"Use to qualify leads and flag throwaway signups" gives a clear intended context for invocation. It stops short of naming when-not-to-use or pointing at an alternative sibling (e.g. dns_lookup for raw records), so it misses the top tier.

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