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

Annotations already declare readOnly/idempotent/non-destructive/openWorld, so the safety profile is covered; the description goes further by disclosing cost ($0.01 USDC via x402), the billing rule that errors are not charged, and the negative behavior that no SMTP probe is performed. Those are material traits an agent cannot derive from annotations or schema.

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?

One dense sentence that front-loads the action and scope, followed by a short constraint and payment clause. No filler, no restatement of the tool name, and every clause carries distinct information.

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?

For a one-parameter read tool with no output schema, the description covers what is checked, the verdict enum, the absence of SMTP probing, and the cost model. It stops short of describing what accompanies the verdict (e.g. which signals triggered a risky rating), which is the only remaining gap.

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 parameter with 100% schema description coverage, so the schema already carries the parameter definition and the baseline is 3. The description adds no format, normalization, or edge-case guidance for the email argument beyond what the schema states.

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 (check) and resource (one email address) and enumerates exactly what is evaluated: syntax, MX / null MX, disposable domain, role account, free provider. It also names the output verdict space (ok / risky / undeliverable), so an agent knows precisely what this returns without opening anything else.

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?

"Check one email address" scopes it to single-address validation, which implicitly distinguishes it from the domain-level sibling check_domain_email_security. "No SMTP probe" clarifies a boundary of the check. However, it never explicitly names an alternative tool or states when-not to use this one, so routing is inferred rather than stated.

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