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

With no annotations provided, the description carries the full burden of disclosing behavioral traits, and it excels: it states the paid cost ($0.002), the x402 protocol, statelessness, non-PII (address never stored), and no SMTP probing. It also outlines exactly what checks are performed, leaving no ambiguity about side effects or safety. This is exemplary transparency for a tool with zero annotation support.

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 a single, dense paragraph with no filler. The paid nature and purpose are front-loaded, followed by the specific validation features and key behavioral disclaimers. Every sentence contributes unique information, making it appropriately concise and well-structured.

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 paid API with an x402 payment flow, the description thoroughly explains invocation, expected behaviors, and data handling. It does not describe the exact return value structure, but the listed validation categories imply the result shape. The absence of error-case details is a minor gap, but overall the description is complete enough for an agent to use the tool 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?

The schema already provides 100% coverage with basic descriptions for both parameters, so the baseline is 3. The tool description adds meaningful context beyond the schema by explaining the x402 challenge-response flow and how the email parameter is used across multiple validation checks. This enhances usability without being redundant, earning a 4.

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 clearly states it validates an email address with specific checks: syntax, MX-record, disposable domain, and role flag. The verb 'validate' plus the resource 'email' unambiguously distinguishes it from sibling validate_iban and validate_vat tools. The enumerated checks go beyond a generic validation tool, making the purpose specific and actionable.

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 provides clear context including the payment mechanism (omit x_payment for a challenge, include to settle), which is essential for correct invocation. It does not explicitly name alternatives, but the email-specific purpose and the sibling tool names (validate_iban, validate_vat) make it obvious when to use this tool. No exclusions or alternative conditions are stated, but the usage context is effectively communicated.

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