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

Annotations already cover readOnly, idempotent, openWorld and non-destructive, so the bar is lower. The description adds real value beyond them: the data source chain (TLD registry through IANA bootstrap), the negative result shape ('registered: false'), and the cost behavior for TLDs lacking RDAP.

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 short sentences, front-loaded with the intent question and then the mechanism and returned fields. A few fragments ('Or registered: false', 'costs nothing') are telegraphic but each still carries information.

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 enumerates the returned fields and the failure/negative case, and notes the no-cost case for unsupported TLDs. Complete enough for an agent to call it correctly, though pagination/error semantics are unstated.

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% and the single parameter's normalization behavior (URL/subdomain reduced to the registered name) is already documented in the schema. The description adds nothing further about the parameter, so the baseline 3 applies.

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 clear resource (domain registration data) and the mechanism (RDAP from the TLD's own registry via IANA bootstrap), plus the fields returned. It does not explicitly differentiate from siblings, but no sibling tool covers domain registration lookups, so confusion risk is low.

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 implied rather than stated: the framing question 'Who registered this domain and when?' suggests the lookup intent, and 'A TLD without RDAP costs nothing' hints that calling for any TLD is safe/free. There is no explicit when-to-use, when-not, or alternative named.

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