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

Goes well beyond the annotations: public DNS only, sends no mail, changes nothing, failed calls are free, and it explains the non-idempotent/costly nature explicitly ('each call is charged, so repeats are not free'). Pricing mechanics ($0.01/call via x402 USDC or card key, first 25 free) are disclosed up front, which is exactly the kind of context annotations cannot carry.

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 the one-line purpose, then scope, then use cases, then pricing — a sensible order. It is a single dense paragraph, and the pricing/licensing details occupy roughly a third of the text, which is justified by the cost model but makes it heavier than ideal.

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 still describes the return shape (0-100 score, grade, list of fixes, optional address sub-check) and covers cost, safety and prerequisites. An agent has everything needed to decide and call it correctly.

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 already 100%, so the baseline is 3; the description adds real value by saying what the optional email parameter actually validates (syntax, MX, disposable domain) rather than just 'an address to check'. The domain parameter's URL-tolerance is repeated from the schema rather than extended, keeping it below a 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 and resource ('check a domain's email setup') and then enumerates exactly what is inspected: MX, SPF with all-policy, DMARC with policy, DKIM at named selectors, MTA-STS, TLS-RPT, BIMI, plus a 0-100 score and grade. This clearly separates it from generic siblings like dns_lookup, domain_report or pack_domain_intel.

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 situations: 'Use before a campaign, when mail lands in spam, or to vet a sender domain,' plus a worked example (domain=github.com). It stops short of naming an alternative tool or an explicit when-not-to-use case, so it is clear context rather than a full routing rule.

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