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

Annotations already cover read-only/idempotent/closed-world, but the description adds genuinely useful context beyond them: the full set of returned fields, the unicode_host normalization stance (WHATWG/UTS 46 non-strict), and the payment model ($0.05 per successful call via x402 USDC on Base). It stops short of describing failure behavior or what happens on invalid input.

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?

Purpose is front-loaded, then return payload, then the unicode_host caveat and pricing. Dense and information-bearing with no filler sentences, appropriate for a parse-and-canonicalize tool.

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?

No output schema exists, so the description carries the return-shape burden — and it enumerates scheme, host, port, path, decoded query params, fragment, suffix, eTLD+1, subdomain, ICANN/private flag, and canonical form. An agent has everything needed to call and interpret results.

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 100%, so the schema already documents url and base, and the baseline is 3. The description's parameter-adjacent note about unicode_host normalization doesn't actually map to either declared parameter, so it adds little beyond the schema.

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 specific verbs and resource: parse, canonicalize, and extract the registrable domain (eTLD+1) via the Public Suffix List. An agent can separate it from punycode_convert or unicode_confusables_check without opening any 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?

Implies the use case with 'the part an agent needs to decide whether two URLs belong to the same owner,' which is real context. But it never states when to prefer this over adjacent siblings like punycode_convert or unicode_confusables_check, and gives no exclusions.

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