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

Annotations provide readOnlyHint and openWorldHint, but the description adds substantial behavioral detail: payment requirement and exact payTo address, DNS-over-HTTPS mechanism, no SMTP, disposable-domain and MX-absent invalidity, and fail-closed error handling. This richly exceeds the annotation baseline.

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 one dense, front-loaded sentence with no filler. Every clause conveys essential operational or validation information.

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 validation tool with no output schema, the coverage is strong: behavior, failure mode, and payment requirement are all stated. The only gap is the absence of any hint about the return value or response format, which the lack of an output schema makes somewhat relevant.

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% for the single 'addr' parameter, and the description adds meaning by defining what makes an address invalid (disposable domain, missing MX). This directly informs which parameter values the tool will accept or reject.

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 communicates the tool's function: validating email syntax and MX records via DNS-over-HTTPS, with no SMTP. It is specific enough to distinguish from sibling tools like 'domain' or 'check', though it lacks an explicit verb like 'validate'.

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 implies usage context by specifying constraints: email only, no SMTP, paid $0.01 USDC, and fail-closed behavior. However, it does not explicitly state when to prefer this tool over alternatives or name any sibling tools for comparison.

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