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

No annotations are provided, so the description carries full behavioral disclosure. It transparently lists the operations performed (syntax, disposable-domain, MX via DNS-over-HTTPS), the return type (verdict), cost (2 credits), and authentication requirement (API key). It does not explicitly state it is non-mutating, but 'validate' and 'returns a verdict' imply a read-only operation.

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 two concise sentences, front-loaded with the core action and resource, then adding specific checks and cost/auth requirements. Every sentence adds value; there is no padding or redundancy.

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 simple one-parameter tool with no output schema and no annotations, the description covers the core operation, specific checks, return value, cost, and authentication. It does not describe the verdict format or edge cases, but these are not critical for a basic validation tool and the description is sufficiently complete to guide correct usage.

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 input schema has one parameter 'email' with no description (0% coverage). The description provides the semantic meaning by stating 'Validate an email address' and explaining what checks are performed on it. This compensates for the lack of schema-level parameter documentation, though it does not go into additional detail about formatting.

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 uses the specific verb 'Validate' with the resource 'email address' and clearly enumerates the validation types: syntax, disposable-domain check, and MX record via DNS-over-HTTPS. It also states the return value (deliverability verdict). This clearly distinguishes it from sibling tools like validate_iban or validate_vatid.

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 provides clear context for when to use the tool: validating an email address with specific checks. It does not explicitly mention alternatives or when not to use it, but the sibling tools handle other identifiers (IBAN, VAT ID), so the intent is unambiguous. It also notes that it costs 2 credits and requires an API key, which is usage-relevant.

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