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

Annotations already declare a safe, read-only, idempotent lookup, so the safety profile is covered. The description adds genuinely useful context above that bar: no reputation score is returned, and Spamhaus DBL is reported as 'not checked' rather than 'clean' when the paid key is absent. It does not, however, describe response shape or latency/caching behavior. A 3 reflects that the annotations carry most of the behavioral weight.

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?

Four compact sentences, each carrying a distinct fact: use case, signals checked, verdict taxonomy, and the two caveats about scores and Spamhaus. Front-loaded with the triggering scenario. The first sentence is slightly idiomatic but not wasteful.

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 no output schema available, the description correctly enumerates the possible verdicts and explains what the tool rules out, which is exactly what an agent needs to interpret results. The mutation-free, single-parameter nature makes remaining gaps minor.

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?

Only one parameter, fully documented in the schema (100% coverage with an example). The description adds no semantics beyond the schema, so the baseline of 3 for high-coverage schemas 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 and resource ('Checks the domain against SURBL, its authentication, its age and the address its website resolves to') and defines the output space (listed, broken, young, clean, unknown). It implicitly differentiates itself from single-check siblings like check_surbl by bundling multiple signals, but never names an alternative 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 opening scenario quote ('this domain used to land and now it does not') gives a clear triggering situation, and 'returns a verdict by elimination' clarifies the intended diagnostic use. No when-not guidance or named alternatives (e.g., check_surbl for a single-signal lookup) are provided.

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