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

Annotations only cover readOnlyHint and openWorldHint, so the description adds meaningful context: deterministic/non-AI behavior, the organizational-domain fallback when no record exists, and the x402 pricing model with five free calls per day per client. The cost and rate-limit disclosure is exactly the kind of operational detail annotations cannot carry, though failure modes (no record found, malformed record) are not described.

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?

Two sentences, front-loaded with the core action and scope, followed by the commercial terms. The enumerated field list is dense but earns its space as return-value disclosure. 'Deterministic, no AI' is mildly promotional but does convey a real behavioral property.

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?

There is no output schema, so the description correctly compensates by listing the record components returned plus the RFC 7489 section governing external report authorization. Combined with the fallback and pricing disclosures, an agent has enough to call this correctly, though error/empty-result behavior is unspecified.

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 with full schema description coverage, so the baseline is 3. The description's mention of falling back to the organizational domain slightly clarifies how the input domain is resolved, but adds no format, normalization, or validation details beyond the schema's example.

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 verb pair (finds and validates) and resource (a domain's DMARC record), then enumerates the exact fields extracted (policy, subdomain policy, pct, alignment modes, rua/ruf, external report authorization). This makes it clearly distinguishable in substance from spf_lookup_count and email_auth_report, though it never names or contrasts those siblings explicitly.

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 implies usage via the organizational-domain fallback and the pricing/free-tier note, which tells the agent this is a paid, per-call lookup. However, it gives no explicit when-to-use guidance, no prerequisites, and no steering toward or away from the sibling tools, leaving the selection decision largely to inference.

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