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

With no annotations, the description carries the full disclosure burden. It explains the two-step behavior (RFC syntax check + live MX probe), specifies the exact output format, and discloses the per-call cost of $0.001 USDC on Base. It does not mention side effects (e.g., whether it writes anything), but as a validation check it is implicitly read-only; the cost is a notable side effect that is disclosed.

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 sentences with no filler. It front-loads the core purpose, then immediately gives the output format and cost—key factual details that an agent needs. Every sentence earns its place, and the structure is crisp.

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 tool with an output schema (which likely documents return fields), the description sufficiently covers the behavior (syntax + MX) and cost. It does not explicitly mention timeout, error conditions, or prerequisites like network access, but these are likely handled by the output schema or general runtime context. It is complete enough for an agent to call the tool 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% since the only parameter 'email' has a description ('Email address'). The tool description adds no extra information about the parameter—it does not provide format examples, normalization rules, or edge cases. Baseline 3 is appropriate because the schema already documents the single parameter adequately.

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 states a specific verb and resource: 'Email deliverability check' with details on RFC syntax and MX probing. It clearly distinguishes itself from sibling tools like dns_lookup or domain_whois, which are generic DNS or WHOIS lookups, not email-specific validation. No ambiguity about what it does.

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 for email deliverability validation, giving clear context. However, it does not explicitly mention alternatives or when not to use it. Since none of the sibling tools overlap in purpose, the agent can infer the correct scenario without explicit exclusion guidance.

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