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

Annotations already declare readOnly/idempotent/openWorld, and the description goes further with genuinely non-obvious behavioral facts: it reads public DNS only, never connects to mail servers, and therefore cannot test SMTP TLS, certificates, or mailbox existence. It also commits to a latency bound (under 2 s), which helps an agent decide between one bulk call and many.

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 the core action, then scope, then routing guidance, then limits — a logical order with no filler sentences. The check enumeration is dense but every clause maps to a real output field, so only its length keeps it from a 5.

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?

An output schema exists, so return-shape explanation is not required; the description instead supplies what the schema cannot — the meaning of the grade levels (strong/fair/weak/no_mail/none) and the tool's hard limitations. Nothing an agent needs to invoke or interpret it correctly is missing.

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 'domain' parameter is already well documented in the schema (registered domain or subdomain, not an email address or mail server host). The description adds no additional input 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 (grade) and resource (one domain's email authentication), then enumerates the exact checks covered (SPF limit/all qualifier, DMARC inheritance/pct, MTA-STS, TLS-RPT, BIMI, DANE, null MX). It explicitly distinguishes itself from the sibling lookup_target, so an agent can route correctly without opening either 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?

Gives explicit trigger scenarios ('spoofing, deliverability or a DMARC rollout') and a direct comparison to the alternative: lookup_target gives only the overall grade, while this tool returns per-check status and a one-line finding. The when-to-use-this-vs-sibling decision is fully resolved.

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