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

Annotations already declare readOnly, idempotent, non-destructive, open-world behavior, so the safety profile is covered. The description adds genuinely useful behavioral context beyond that: checking is scoped by target type, and domain blocklists differ from IP-based mail-server checks.

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, no filler, with the core purpose front-loaded and the input rule immediately after. Every clause carries operational weight.

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 full annotation coverage, this is sufficient to call correctly. The only mild gap is the absence of any note on the shape of the result (listed/not listed, which blocklists), which matters slightly since no output schema exists.

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 100% schema coverage and a single required parameter, the baseline is 3. The description goes further by explaining the semantic distinction between supplying an IPv4/IPv6 address versus a domain name and what each is checked against, adding meaning the schema itself does not convey.

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 and resource: checking whether an IP or domain appears on public DNS blocklists (DNSBLs). It also scopes the operation ('A domain is checked on domain blocklists only'), which lets an agent distinguish it from the DNS-record siblings like check_spf or lookup_mx without opening a 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?

It gives clear conditional guidance on input selection: use a domain for domain blocklists, use the IP address to check a mail server. It does not explicitly name alternative tools or state when this check is unnecessary, so it stops short of a 5, but the context is unambiguous.

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