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

With no annotations, the description carries the full behavioral burden and does so: it discloses the method (DNS-over-HTTPS), the safety boundary (no mailbox probing), the billing exception ('Not charged if the DNS lookup fails'), and the price ($0.0032 via x402). It also characterizes the return ('One verdict with reasons and sources'). Cost and failure-billing semantics are exactly the traits an agent needs before invoking a paid tool.

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?

Three dense sentences, front-loaded with what is checked before method, billing, and price. Every clause carries information an agent can act on; there is no filler.

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 and no annotations, the description still tells the agent what is checked, what comes back, what the method is, and what it costs. The only real gap is that `request_id` and the email length bounds are never acknowledged, so a caller could miss the idempotency mechanism entirely from the prose.

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 only 50%: the required `email` parameter has no description, while `request_id` documents its own idempotency semantics. The description says it checks 'an email address' but adds nothing about format constraints (minLength 3, maxLength 254) or about the idempotency key, so it does not compensate for the uncovered half.

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?

Names a specific verb (Check) and resource (email address) and then enumerates exactly what is verified: syntax, canonical form, MX, SPF, DMARC, disposable domain. No sibling tool in the list overlaps with email deliverability, so the agent can place it unambiguously.

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 scope line 'DNS-over-HTTPS only, no mailbox probing' tells the agent this is a passive, non-intrusive check and rules out a class of use (mailbox verification), which is genuine usage context. It does not, however, state when to prefer it over another approach or any prerequisites, so it falls short of a full when/when-not statement.

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