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

With no annotations, the description carries the burden and does well by disclosing rate limiting, no-key access, report URL generation, and commission behavior. It does not explicitly state read-only semantics or failure handling, but the 'check' verb and operational constraints provide a solid transparency baseline.

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?

The description is three sentences, each earning its place: core behavior, operational constraints, and optional parameter explanation. It is front-loaded with the primary purpose and contains no redundant or promotional filler.

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?

Despite lacking an output schema, the description explains what the tool checks, the grading scale, the shareable report URL, and rate-limit/auth constraints. It is complete for a simple check tool, though it could optionally clarify the exact return format or provide an example.

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?

The schema already covers both parameters (100% coverage), and the description adds meaning for 'ref' by explaining the referral/commission relationship. For 'domain', the description reinforces its meaning as the subject of the mail-deliverability check, exceeding the baseline schema-only case.

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?

The description states a specific action ('Check whether anyone can spoof mail from a domain') with detailed scope (SPF, DMARC, MX, DKIM) and output (A-F grade, shareable report URL). It clearly distinguishes itself from siblings like list_services and partner_program by describing a domain-specific diagnostic operation.

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 description provides useful context like 'Free, no key, rate-limited per caller' and explains the optional referral code, implying when to use it. However, it does not explicitly state when to use this tool over alternatives or mention any exclusions, so usage guidance remains implied rather than explicit.

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