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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, openWorld), yet the description adds non-obvious operational detail the annotations do not carry: the tool returns a paid x402 request URL (GET /v1/email/verify at $0.01 per call) that must be called with an x402 or MPP client, and that bad input is refused before any charge. Pricing, payment protocol, and the no-charge-on-invalid-input guarantee are exactly the kind of context an agent needs.

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 trigger condition is front-loaded and every clause carries information, but the middle sentence packs payment URL, price, return payload, and client requirement into one dense run-on. Slightly tighter segmentation would improve scanning without losing content.

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?

Despite having no output schema, the description tells the agent what it gets back (the exact x402 request, response schema, and an example) and how to obtain the actual data. For a single-parameter, annotation-covered read tool, nothing needed to call it 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% and the single 'email' parameter is documented in the schema, so the baseline is 3. The description adds meaningful constraint beyond that: the check is IDN-aware and scoped to one mailbox (not a list), which clarifies the accepted input shape. The gain is modest rather than rich, so 4 rather than 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+resource ('check an email address') and enumerates exactly what is checked: IDN-aware syntax, MX with A/AAAA fallback, null MX, disposable and role addresses. It also explicitly names what it is not ('Checks one mailbox; for a list use check_email_batch; for a whole domain use domain_report'), so an agent can separate it from siblings without opening any schema.

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 an explicit triggering context ('before sending, signing up a user or adding a lead') and routes to alternatives for both adjacent cases: check_email_batch for lists and domain_report for whole-domain DNS/SPF/DMARC. When-to-use and when-to-use-something-else are both stated.

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