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

With no annotations, the description carries the full burden and does well: it discloses cost ('Free'), the exact query scope, the deliberate exclusions (subscription-gated lists), and that domain inputs trigger MX and A resolution. It does not mention rate limits, error behavior, or response shape beyond 'the result says which lists were checked'.

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 tight sentences with no filler; the cost signal and the scope/exclusion information are front-loaded. The leading 'Free.' fragment is slightly abrupt but earns its place in a paid-API context (cf. get_x402_payment_endpoints).

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 lookup with no output schema, the description covers what it checks, what it skips, and why. Only the shape of the result (per-list verdicts vs. a summary flag) is left underspecified.

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 coverage is 100% so the single parameter is documented, but the description adds real meaning beyond 'IPv4 address or domain': it explains that a domain's MX and A records are resolved and each checked against the lists.

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 check (blocklist listing status) against a named resource (SpamCop and PSBL) for named input types (IPv4 or a domain's MX/A addresses). This is clearly distinct from siblings like inspect_domain_dns or check_tls_certificate, though no sibling is named 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?

Usage is implied by the purpose, and the description usefully states which lists are out of scope (Spamhaus, Barracuda require subscriptions), which helps an agent decide whether this tool suffices. However, no alternative tool is named and there is no explicit 'use this when...' framing.

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