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

Annotations already cover readOnlyHint, idempotency, openWorldHint and non-destructiveness, so the safety profile is settled. The description adds genuinely non-obvious behavior beyond that: a metered free tier of 10 calls/day, then $0.002/call, with the x402 payment path (params._meta["x402/payment"] or HTTP) or X-API-Key credits. That payment precondition is critical to correct invocation and is not in any structured field. Minor gap: no mention of recursion depth limits or failure behavior on malformed SPF.

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 technical capability is correctly front-loaded in the first sentence, which is dense and earns its place. The second half is dominated by pricing and payment-mechanism detail, which is necessary but overlong, and the trailing '[served by tools.moneytools.dev]' is extraneous metadata. The result reads as a capability sentence bolted to a billing notice.

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 an output schema present, return values need not be explained, and annotations plus one fully-covered parameter keep the structured burden low. The description covers what is checked, the limits that define the verdict (RFC 10-lookup cap), and the cost/payment gate. The only real omission is when to choose this over a plain DNS or email validator.

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?

There is a single parameter, and schema description coverage is 100%, though the schema's own description is only the bare word 'domain'. The description implies the domain is the SPF/DMARC subject but adds no format guidance (apex vs subdomain, whether URLs are accepted, punycode). With the schema carrying the field definition, 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?

The description names specific operations with concrete outputs: recursively expanding SPF includes, counting DNS lookups against the RFC 10-lookup limit, and flagging '+all'/'?all', duplicate records, and weak DMARC policy. This is far more specific than the name alone and clearly separates it from a generic dns_lookup. It stops short of explicitly naming a sibling or stating what it is not, so it lands at 4 rather than 5.

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?

There is no guidance on when to reach for this tool versus dns_lookup, email_check, or other siblings, and no stated prerequisites (e.g., needing the domain's authoritative DNS reachable). The only context is implicit in the name and check list. A caller must infer the use case from the description of the checks themselves.

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