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

Goes well beyond the annotations by disclosing the payment model (pay per address in stablecoins), the refund rule for addresses with no verdict, the supported chains, and the important interpretive rule that catch-all and strict servers are never reported as valid. These are non-obvious behaviors an agent could not infer from readOnlyHint/destructiveHint.

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 core capability is front-loaded and the remaining sentences are dense with operational facts (returned fields, risk semantics, payment, refunds, chains). Only the 'for AI agents' phrase is filler; otherwise no wasted text despite the length.

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?

With no output schema, the description carries the return-value burden itself by listing verdict fields (status, score, domain/mail-server existence, disposable, role-based, free-provider, catch-all, typo suggestion). Combined with payment, refund and chain details, an agent can call this correctly without further information.

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% and the single parameter already documents the 1–20 range, duplicate handling and per-address pricing. The description restates the 20-address cap and pricing but adds no syntax or format detail, so the baseline 3 applies.

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?

States a specific verb and resource ('Email verification API ... send up to 20 email addresses and get a verdict for each') and enumerates the returned fields, so an agent knows exactly what it gets back. Nothing in the sibling list does address verification, so the tool is unambiguous.

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 context is clear: batch-verify up to 20 addresses before using them. It also states the network constraint (Base or Polygon, not Solana) which is a real precondition. It never explicitly says when not to use it or names an alternative, but no sibling overlaps, so the gap is minor.

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