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

With zero annotations the description carries the burden and does it well: it discloses the paid gate, exact price and rails ($0.25 USDC on Base, or 250 credits via api_key), the free-preview fallback, and that records come ordered with a reason each. It stops short of stating whether anything is written to DNS or any rate limits.

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 front-loaded sentences, each earning its place: what it fixes, what it returns, how payment works. The parenthetical pricing detail is dense but necessary; nothing is redundant.

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?

No output schema exists, so the description correctly specifies the return shape and ordering plus the payment/free-advice semantics, leaving little ambiguity for a two-parameter read tool. Lacks only side-effect confirmation and any failure-mode guidance.

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 both parameters are already documented in the schema, including that api_key pays 250 credits instead of x402. The description largely restates that, adding no new format or constraint detail, so baseline 3 applies.

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?

Specific verb+resource: 'Fix a domain whose email goes to spam or fails SPF, DKIM or DMARC', with the return payload spelled out (host, type, value). It implicitly separates itself from analysis/inspection siblings ('fix' vs 'analyze'/'inspect'), but never names one outright.

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 trigger condition is explicit (spam placement or failing SPF/DKIM/DMARC), and the payment branches are given rules (no payment = free preview; no records needed = free advice). No exclusion against pick-one alternatives like analyze_email_authentication or flatten_spf.

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