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

Annotations already declare readOnlyHint, idempotentHint, and destructiveHint false, so the read-only nature is covered. The description adds value by disclosing that the tool runs three lookups together, returns a verdict plus raw records, and produces specific fields (can_receive_mail, spoofable, findings). This enriches the agent's understanding beyond the annotations without contradicting them.

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?

The description is tightly structured: a one-line outcome summary, a sentence on mechanics and output, and two sentences on usage context. Every sentence adds information; there is no filler or repetition of schema or annotation content.

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 single-parameter, read-only, output-schema-bearing tool, the description covers what it does, how it behaves, when to use it, and how it relates to sibling tools. The output schema handles return-value details, so nothing needed for correct invocation 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 coverage is 100%, and the schema's parameter description already explains that a domain is required and that a full email address is accepted with the domain extracted. The tool description adds no additional parameter-specific meaning, so the baseline of 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?

The description opens with a specific, outcome-focused statement: 'Full email posture for a domain: can it receive mail, and can it be spoofed?' It then names the exact operations (MX, SPF, DMARC lookups) and explicitly contrasts itself with the sibling single-record tools, so an agent can distinguish it immediately.

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?

The description gives explicit guidance: 'Use this as the default check before emailing an unfamiliar domain, when judging whether inbound mail claiming to be from a domain could be forged, or when auditing your own domains.' It also states when not to use it: 'Prefer this over the single-record tools unless you already know which record you need.' This leaves no ambiguity about selection.

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