Validate an email: syntax + MX + disposable + role (paid $0.002)

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

With no annotations, the description carries full responsibility. It discloses the payment requirement, x402 challenge flow, statelessness, non-PII handling, no email storage, and no SMTP probing. This is comprehensive and gives the agent accurate expectations.

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 but well-structured. It front-loads cost and purpose, lists checks, then covers statelessness and payment flow. Every sentence adds unique information, with no fluff.

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 tool with no output schema and no annotations, the description is remarkably complete. It explains the challenge-response payment flow, the types of results returned, and the stateless/non-PII guarantee, giving the agent enough context to select and invoke 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?

Schema coverage is 100% with basic descriptions for both parameters. The tool description adds meaningful context about x_payment: that omitting it returns a 402 challenge and supplying it settles the call. This goes beyond the schema's definition.

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 the verb 'validate' and the resource 'email address', then enumerates the specific checks (syntax, MX, disposable, role). This distinguishes it from sibling validation tools (VAT, IBAN, VIN, geo) by domain and functionality.

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 intended use is implied: validating email addresses. It explicitly notes the tool does NOT probe SMTP mailboxes, providing a useful exclusion. However, it does not name alternative tools for mailbox checks or discuss when to choose this over other validators, so it falls short of 5.

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