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

Annotations only declare readOnlyHint and openWorldHint, but the description goes much further: it lists the exact verdict values, the full set of signals returned (MX/null MX, SPF, DMARC, RDAP age, website load), and critically discloses a limitation (no mail-server contact, so mailbox existence cannot be confirmed). This is exactly the behavioral context annotations cannot carry.

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 purpose is front-loaded as a question, followed by use cases, return signals, and the key limitation. It is dense but every sentence carries information, with no boilerplate padding.

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?

With no output schema, the description fully covers return values (verdict plus each evaluated signal) and the important negative capability. An agent has everything needed to call it and interpret results correctly.

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?

There is a single parameter with 100% schema coverage, and the schema already documents that it accepts an email or a bare domain. The description restates this duality but adds no syntax or format detail beyond the schema, so 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?

The description opens with a specific, unambiguous purpose (determining whether an email address or domain is real/legitimate) and enumerates what it evaluates. It is easily distinguished from siblings like check_ai_crawler_access, package_check, or vendor_trust_profile.

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?

It names four concrete contexts (lead research, signup/fraud screening, outreach list cleaning, vendor checks), which clearly signals when to use it. However, it never names an alternative tool or an explicit when-not-to-use condition, so it stops short of full routing guidance.

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