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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive), and the description adds context they cannot: pricing at 0.002 USDC via x402 or 1 Factline credit, plus the first 5 paid calls per day being free with no payment or key. It also defines the output categories (deliverable, risky, undeliverable), which matters for a paid, open-world lookup.

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?

Two sentences, both front-loaded: the use case and check inventory come first, payment mechanics second. Every clause carries information an agent needs — no filler and no repetition of the title.

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, the description supplies the return shape by naming the three verdicts, and it covers the payment/auth paths an agent must handle. Nothing required to invoke the tool 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 description coverage is 100% — both the email format example and the api_key sourcing from get_trial_key are documented in the schema itself. The description adds no parameter-level detail beyond that, so the baseline of 3 is appropriate.

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 (verify an email address) and enumerates the exact checks performed: syntax, live MX routing, 9,000+ disposable domains, role addresses, free providers, typo suggestions and SPF/DMARC. The sibling phone_verify covers a different resource, so the distinction is implicit but unambiguous.

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?

Clearly names the trigger context — before signup, outreach or onboarding — so an agent knows when to reach for it. It does not name exclusions or alternatives (e.g. when a syntax-only check would suffice), 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.