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

Annotations already declare readOnly/idempotent/non-destructive, but the description adds the crucial operational facts those annotations cannot: the data source (Certificate Transparency back to 2013), that it reads precomputed data only, and that an unknown domain yields a cautious 'no history seen' rather than a live scan. That last point prevents an agent from misreading an empty result as a negative signal.

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?

Front-loaded with the use case and scope before the caveats, and each paragraph carries a distinct job (trigger, justification, inputs/outputs, exclusions). The long keyword list in the opening sentence is deliberately broad for retrieval but is denser than strictly necessary.

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?

An output schema exists, so return values did not need explaining, yet the description still summarizes the shape of the answer (first-seen date, certificate history, ABN match, lookalike domain, risk verdict) so the agent knows what to do with it. Combined with the source-limitation caveat, nothing needed for correct invocation is missing.

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?

The schema is a bare string with 0% description coverage, so the description must carry the load — and it does, stating it accepts a domain, URL, or email address and extracts the registrable domain. It stops short of giving concrete input examples or stating case/format normalization rules, which would earn a 5.

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 (Australian .au domain) and enumerates the exact TLDs covered. The intent is unambiguous without opening the schema, and the enumeration of user-facing intents (age, operator, ABN, scam risk) makes routing trivial.

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 explicit when-to-use (any .au domain/URL/email question), explicit when-NOT-to-use ('Do NOT use for non-.au domains or phone numbers'), and an explicit alternative to prefer over this one (web browsing/WHOIS) with the reason why browsing fails. This is a complete routing decision.

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