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

Goes well beyond the annotations: it discloses the DNS-over-HTTPS mechanism, the null-MX/A-fallback semantics, the explicit non-behavior (no SMTP contact) and its consequence, plus the full payment protocol — $0.001 USDC on Base, x402, the PaymentRequired object on an unpaid call, and the retry via params._meta['x402/payment']. That is exactly the operationally critical context an agent would otherwise hit blind.

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 decision question, then the check list, then the limitation, then the payment mechanics — a sensible priority order. The x402 payment sentence is dense but load-bearing since a first call will fail without it; only minor tightening is possible.

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 the description compensates by naming the components of the response (per-check results plus a verdict). Adequate for correct invocation; only the verdict's possible values are left unspecified.

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?

A single required parameter with 100% schema description coverage, so the baseline is 3. The description adds no format or normalization guidance for the email value (e.g. whether plus-tags or role accounts are expected input) beyond restating that it takes an address.

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 ('syntax check, then the domain's mail route via DNS-over-HTTPS') and enumerates exactly what is inspected (MX, null MX, SPF, DMARC, role accounts, plus-tags, verdict). An agent can tell this is an email-deliverability check, though it never distinguishes itself from the sibling domain_check, which overlaps on the DNS portion.

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?

'Is this address worth sending to?' frames the decision context well and the explicit limitation ('does not contact the mail server, so it cannot confirm that the mailbox itself exists') sets a clear boundary on when its answer is sufficient. No sibling is named as an alternative for the cases it can't cover, which keeps it short of a 5.

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