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

Annotations already declare readOnlyHint and openWorldHint, yet the description adds real information beyond them: the credit price per successful call and the guarantee that schema-invalid input is rejected before billing. It does not say what happens on a valid-but-failing lookup (NXDOMAIN, resolver error) or whether those are billed, 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 tight sentences: what is fetched, what is returned, what it costs. The return shape and the pricing caveat are both front-loaded with no filler.

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, the description usefully spells out the response object shape, and it covers billing and rejection behavior for a single-parameter read tool. The only real gap is failure-mode behavior for lookups that resolve to nothing.

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?

There is a single parameter with full schema description coverage, including the private-suffix and IP-literal refusal rules, so the schema carries the semantics. The description only reinforces 'for a domain' without adding format or normalization guidance of its own – baseline 3.

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?

Names the exact resource (A, AAAA, MX, NS, TXT DNS records for a domain) and the transport mechanism (DNS-over-HTTPS), so the agent knows precisely what comes back and how it is fetched. It is clearly distinguishable from near-neighbors like rdap-lookup or mail-auth.

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 only implied by the subject matter; there is no explicit when-to-use statement and no routing to alternatives such as rdap-lookup (registration data) or mail-auth (SPF/DKIM/DMARC). An agent can infer the fit, but must reason about boundaries itself.

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