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 substantial context beyond them: $1 USDC per call via x402 with the payment key location, the fact that invalid input / unknown domain / DNS failure is not charged, that nothing is stored, and the 10-lookup SPF merge limit with DMARC staging behavior. This is exactly the extra behavioral detail the 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?

A single dense paragraph, but the purpose and deliverables are front-loaded and most clauses carry real information (providers detected, SPF merge, DMARC staging, pricing). It is on the long side, and the provider enumeration and payment mechanics could be tightened without loss.

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 carries the return-value burden and does so: it specifies the ordered record set, the per-host click paths, and error/charging semantics. An agent has everything needed to call it and interpret the result.

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% and there is only one parameter, so the schema already documents that an email address or URL on the domain also works. The description adds no syntax or format detail beyond that, so the baseline 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 ('get an email security fix plan for one domain') and enumerates the concrete deliverables: SPF/DKIM/DMARC records with action, name, value and rationale, plus DNS-host click paths. This is clearly distinguishable from the sibling validators (check_email_address, validate_email), which inspect addresses rather than produce a remediation plan.

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 explicit triggering contexts: 'when mail lands in spam, before a DMARC rollout, or for a client.' It also carves out a scope condition ('domains that send no mail get a lock-down set'). It does not name a sibling alternative or state when-not to use it, so it stops 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.