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

Annotations only declare the safety profile (readOnly, idempotent, openWorld, non-destructive). The description adds substantial behavioral context beyond that: the exact finding taxonomy, that unverifiable domains are reported as 'unverified' and never 'available', the registry_confirmed gating rule, and the explicit constraint that findings are risk analysis rather than instructions because no SPF fix record exists. This is unusually rich disclosure for a read tool.

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 usage trigger and alternative, and the finding taxonomy is compact. It is long, but most of the length is load-bearing safety guidance; there is mild redundancy where the registry_confirmed: false caution is restated, which keeps it from a 5.

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?

For a single-parameter read-only audit with an output schema, the description supplies everything an agent needs: trigger conditions, finding semantics, interpretation caveats, and the sibling alternative. Return shape is covered by the output schema, so nothing essential is missing.

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?

With 1 parameter at 100% schema description coverage, the schema already documents the domain argument fully (bare names, subdomains, punycode normalization). The description adds no further parameter-level meaning, so the baseline 3 applies.

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 (audit) and resource (a domain's SPF supply chain / includes), enumerates exactly what it walks and returns, and explicitly contrasts itself with count_spf_lookups. An agent can distinguish it from the sibling lookup/count tools without opening the schema.

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?

Opens with an explicit trigger ('Use this when the user asks who can send email as their domain through SPF includes, or wants an SPF supply-chain or third-party sender audit') and names the alternative with its selecting condition ('Use count_spf_lookups instead when the question is only the 10-lookup limit'). When-to-use and when-not-to-use are both present.

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