Validate email deliverability

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

Beyond the readOnlyHint annotation, the description discloses critical billing behavior: every address is billed regardless of result, with specific cost and how to find actual charges. It also notes that the verdict itself is the deliverable. This adds significant context beyond structured annotations.

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 every sentence earns its place: it starts with the purpose, immediately warns about billing, provides pricing specifics and how to read costs, and includes an exclusion. It's well-structured and front-loaded with the most critical 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 billable operation, the description covers the action, cost implications, billing exceptions, and how to retrieve real charges. With an output schema also present, the agent has everything needed to call and interpret results correctly without missing details.

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 describes both parameters with 100% coverage ('Email addresses to validate. Each one is billed.' and 'Request timeout in milliseconds.'). The description adds pricing context but does not elaborate on the meaning of individual parameters beyond what the schema provides, so it meets the baseline.

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 action ('Check deliverability of emails you already have') and the resource ('emails you already have'), distinguishing it from generate_email which creates emails. It's specific and unambiguous, with no tautology.

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?

Explicitly instructs when NOT to use the tool: 'Never validate an address that generate_email just returned as valid; it is already verified.' It also references get_balance for pricing, guiding the agent on alternative actions. This is clear usage differentiation.

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