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

With no annotations, the description carries full responsibility and does exceptionally well. It discloses the live MX lookup over DNS-over-HTTPS with Cloudflare/Google fallback, detection of NXDOMAIN and null-MX (RFC 7505), 1-hour caching, pricing, and verdict types. This goes far beyond what structured fields provide.

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 dense yet efficiently structured, starting with the primary purpose and then layering technical details, query format, output verdicts, cache, and pricing. Each sentence adds valuable information with no filler.

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?

Given the absence of an output schema, the description explicitly lists the verdict values (deliverable | risky | undeliverable), mentions the return of prioritized MX hosts, and covers caching and pricing. For a single-parameter tool, this is a self-contained and complete specification.

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?

The schema only defines 'email' as a required string with no description (coverage 0%). The description compensates by showing the query format (?email=someone@example.com) and indicating the expected input is an email address through the RFC 5322 syntax check. It does not exhaustively detail edge cases, but for a single simple parameter it is sufficient.

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 states it performs email verification for lead qualification and list hygiene, using a specific verb ('verification') and resource ('email'). It also outlines the process (RFC 5322 syntax check, MX lookup) and clearly distinguishes from sibling validation tools that target other entities like IBAN or VAT.

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 clear use cases (lead qualification, list hygiene) and a query format, but it does not explicitly mention when not to use it or name alternative tools. This is a minor gap given the obvious email-specific scope.

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