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

Annotations already cover the safety profile (readOnly, idempotent, non-destructive, openWorld), and the description adds two genuinely new traits: it does not fetch the target page so it cannot be blocked/rate-limited on the site, and it requires a free API key. Those are useful operational facts the annotations cannot convey.

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?

Short paragraphs, lead sentence front-loads the payload, and each clause adds information. Slightly conversational phrasing and the standalone API-key line cost it a point but nothing is padding.

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 an output schema present, return values need no explanation, and auth is covered. However the 0%-coverage parameter set is left largely unexplained and no sibling differentiation is given, so an agent has real gaps before it can call this confidently.

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 description coverage is 0% with three parameters. The description only loosely gestures at `backlinks` ('who links to it') and never explains `limit` or the expected `url` format, so it fails to compensate for the near-total absence of parameter documentation.

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 resource and enumerates what is returned (age, registrar, hosting, backlinks), and cleanly frames it as off-page data versus a page scrape. It does not, however, distinguish itself from close siblings like domainReport, domainScreen, or domainSnapshots, so an agent cannot route among the domain-* tools from the description alone.

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?

Gives concrete trigger scenarios ('comparing competitors, judging a source, checking whether a domain was registered last week') and an explicit when-not via contrast with a page scrape. It never names an alternative sibling tool, which caps it below a 5.

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