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

With no annotations, the description carries the full burden and delivers: it discloses the paid model ($0.01 USDC on Base via x402), the two-step payment handshake (call without x_payment to get the challenge, then retry), and the per-call domain cap. It also describes the return payload's shape, so an agent knows what it gets before paying.

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 purpose and triggers, then checks, then returns, then cost. Dense and mostly waste-free, though the payment mechanics are stated twice (once as pricing, once as the two-call procedure) and could be tightened.

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?

Although there is no output schema, the description enumerates the return values (0-100 score, letter grade, records found, prioritised issue list), the domain limit, and the full payment protocol. For a three-parameter, single-purpose tool this leaves nothing material an agent needs unstated.

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%, so the baseline is 3 and the schema already documents all three parameters including the x_payment flow. The description only restates the 4-domain cap and alludes to selectors via 'the common selectors' / 'sensible defaults', adding little syntax or format detail beyond the schema.

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 ('Grade a domain's email authentication from DNS') and enumerates the exact checks performed (MX, SPF incl. '+all', DMARC policy, DKIM selectors). This is clearly distinguishable from siblings like lookup_domain or the audit_* tools, which cover SEO, site health, and accessibility rather than email authentication.

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 concrete triggering scenarios ('when mail is landing in spam, or someone asks whether a domain is protected against spoofing'), which is well beyond generic context. It stops short of naming an explicit alternative or stating when not to use it, so it is strong-but-not-complete guidance.

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