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

With no annotations, the description carries the full burden and does so well: it discloses the hard constraint ('never contacts mail servers, so it cannot confirm that a mailbox exists'), the side-effect profile ('no side effects'), the batch ceiling (25 addresses), and the payment mechanics and price ($0.0015 per call, USDC on Base via x402). This is exactly the behavioral 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?

Front-loaded with the core purpose, then the check list, the limitation, and pricing in descending order of importance. Dense but every clause is informative; the long enumerated check list is the only mild cost to readability.

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?

No annotations and no output schema, so the description must cover purpose, behavior, cost, and result shape — and it does, including what each verdict means and the no-network limitation. Nothing an agent needs to call this correctly is missing.

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%, so the baseline is 3, but the description adds real meaning beyond the schema: it explains the 25-address batching limit and enumerates the verdict values (valid, risky, invalid) that the output can take, which the schema does not describe. The payment_signature behavior is only in the schema, not the description, keeping this just short of 5.

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 ('check whether an email address looks real and deliverable') plus the exact scope of what is examined (RFC syntax, MX/A records, disposable list, role account, SPF/DMARC) and the verdict taxonomy. An agent knows precisely what the tool returns and what it does not.

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?

Gives clear context ('before sign-up, outreach or invoicing') and an explicit selection condition ('use when you need a quick check with no side effects'), plus the key limitation that it cannot confirm mailbox existence. No alternatives are named, but there are no sibling tools to route against, so this is close to complete.

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