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

The description goes beyond the annotations (readOnlyHint, openWorldHint, destructiveHint) by disclosing a crucial behavioral trait: when mailbox-level existence cannot be established, the field is null rather than a guess. This is a valuable reliability statement. It also discloses the exact cost and payment method ($0.02/call, x402 USDC on base), which is practical contextual information. No contradictions with 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 compact: one leading sentence defining the core function, one sentence on the null-behavior policy, and one short pricing note. Every sentence provides distinct, useful information. It is front-loaded and wastes no words.

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 tool with two parameters and no output schema, the description covers the essential behavior, the honesty policy around null results, and the cost. It lacks explicit mention of output structure or error handling, but the null-field hint gives some indication of the return format. Given the tool's simplicity, this is a high level of completeness, but not a perfect 5.

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 description coverage is 100%; both parameters have descriptions. The description supplements the 'emails' parameter by explaining what verification means (syntax, domain, MX), giving semantic depth beyond the generic 'Email addresses to verify.' The 'maxConcurrency' parameter is fully described in the schema and needs no further elaboration. Overall, the description adds meaningful semantic context for the primary parameter.

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 explicitly starts with 'Checks email addresses' and enumerates the specific checks: syntax, domain, and mail exchangers (MX records). This is a specific verb+resource combination that clearly distinguishes it from the sibling tools, all of which are company-related rather than email-focused. The null-behavior statement further clarifies the scope.

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 the tool should be used for email verification by stating exactly what it does. There are no close sibling alternatives, so exclusions are unnecessary. However, it does not explicitly state 'use this when you need to verify email addresses' or provide when-not-to-use guidance. Given the context and lack of competing tools, the usage context is clear, earning a 4.

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