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

With no annotations supplied, the description carries the full burden and does so well: it discloses the output taxonomy (valid/risky/invalid with 0-100 score and reasons), the limitation that no SMTP probe occurs, and the payment/auth requirement ('$0.002 USDC on Base via x402'). Cost and payment rail disclosure is exactly the kind of operational context an agent needs before invoking.

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 decision question is front-loaded, followed by a compact enumeration of checks, then return shape, then a limitation and pricing note. No sentence is filler; each clause adds a distinct fact.

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?

There is no output schema and no annotations, so the description must supply both behavior and return semantics — it does, naming the three verdict classes, the 0-100 score, and the inclusion of reasons. Combined with the stated non-SMTP limitation and cost, an agent has everything needed to decide and call 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?

There is a single required parameter with 100% schema description coverage ('Email address (jane@example.com)'), so the schema already fully specifies it. The description adds no format or syntax guidance beyond the schema, which is acceptable given the one-parameter, fully-documented surface.

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 opens with a concrete decision question ('Is this email address worth sending to?') and then enumerates exactly what is checked: syntax, MX/implicit MX/null MX records, provider, disposable domains, role accounts, free providers, and typo detection. This clearly separates it from siblings like email_find (discovery) versus email_validate (quality assessment).

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 framing 'worth sending to' makes the use case (pre-send deliverability screening) obvious, and the explicit 'No SMTP probe' tells the agent what this tool will not verify, which implicitly routes mailbox-existence needs elsewhere. It stops short of naming an alternative sibling or stating exclusions positively (e.g., don't use for finding addresses).

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