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

Annotations already declare readOnlyHint, idempotentHint, and openWorldHint, so the description's added value is auth/side-effect context: 'Works on any public domain — no login required' and the enumerated return keys. It does not mention rate limits or how partial/missing records are reported, keeping it below a 5.

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 tight sentences: capability and output first, then when-to-call triggers, then the access precondition. 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?

There is no output schema, but the description enumerates the returned fields (domain, grade, provider, spf, dkim, dmarc, findings, migration), and it covers the sole parameter's constraint and access model. An agent has everything needed to call and interpret results.

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?

Only one parameter with 100% schema description coverage, and the schema already says not to include protocol or path. The description's 'any public domain — no login required' adds marginal context but no new syntax or format guidance beyond the schema.

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 and resource ('Check a domain's email authentication posture') plus the exact protocols covered (SPF, DKIM, DMARC) and the output form (A-F grade). This clearly separates it from siblings like detect_email_provider, check_blacklist, and get_dns_records.

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 concrete invocation triggers: deliverability questions, DMARC policy, SPF records, DKIM setup, spam complaints. It does not explicitly exclude or name alternatives (e.g. 'use get_dns_records for raw records only'), so it stops 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.