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

Annotations already cover the safety profile (readOnlyHint=true, openWorldHint=true, destructiveHint=false), so the description earns credit for adding real context: which blacklists are consulted, that no DNS resolution happens, and that results are reported per list. It omits operational behavior an agent might care about, such as rate limits on DNSBL queries or how unlisted/timeout states are reported, which keeps it short of a 5.

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?

Three sentences, front-loaded with the domain case before the IP case, and each sentence carries distinct information. The only soft spot is 'and others', which is vague but does not cost the agent anything actionable.

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 one-parameter read-only lookup with no output schema, the description covers input routing, list coverage, and the shape of the return ('per-list listing status'). What is left implicit is the per-list result fields and how an inconclusive or timeout response appears, which is a minor gap rather than a blocking one.

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% and the schema already explains that a domain goes to domain blacklists and an IPv4 to IP blacklists, so the baseline is 3. The description adds meaning the schema does not: the named list providers, the count of IP lists (24), and the explicit statement that domains are not resolved to IPs.

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 (check) and resource (domain/IP against blacklists), and enumerates the actual list providers (Spamhaus DBL, SURBL, URIBL, ZEN, Barracuda, SpamCop). None of the sibling tools (whois_lookup, check_bimi, check_spoofability, etc.) do blacklist lookups, so the agent can route here unambiguously.

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 input type drives usage: pass a domain to hit domain blacklists, an IPv4 address to hit the 24 IP blacklists. The clause 'Checks the exact target given: domains are not resolved to IPs' is a genuine usage caveat that stops an agent from expecting IP-list coverage for a domain. No explicit exclusions or named alternatives, but no sibling overlaps enough to need them.

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