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

The description reveals that the tool interacts with external systems by quoting a 'live registrar price' and returning a 'Stripe Checkout URL'. This provides useful behavior context, but the annotation explicitly says openWorldHint: false, which indicates that the tool is presumed closed‑world. Because the description directly contradicts this annotation, the behavioral score drops to 1 per the evaluation rules.

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?

The description is compact and front-loaded: the first sentence delivers the core action and payment-flow note, and the following sentences handle exceptions and prerequisites. Each sentence earns its place, though the phrase 'failed-precondition answer' is slightly awkward. Overall, it is well-structured and not bloated.

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?

Given that an output schema exists, the description does not need to spell out return values. It covers the prerequisites and major constraints, but it does not address the optionality of the `domain` parameter—in the schema `domain` is not required, yet the description implies a domain must be involved. That missing nuance is a gap for an agent's correct invocation, though most of the operational context is present.

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?

With 50% schema description coverage, the description does not meaningfully clarify parameters. It only says 'domain purchase' and 'for the company,' which does not explain the domain parameter's format or optionality (it is not in the required list), nor does it detail what companyId represents beyond 'UUID value wrapper.' The description adds little over the bare parameter names and 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?

The description clearly states the action ('Starts'), the resource ('PAID domain purchase'), and the specific outcome (returns a Stripe Checkout URL). It distinguishes this tool from the free-domain flow and, implicitly, from transfer-related tools like formation_create_domain_transfer_checkout by calling it a domain purchase. An agent can accurately select this tool over siblings without opening the 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?

It gives an explicit when‑not condition: companies eligible for their included free domain are refused and their first domain uses the free flow. It also states a prerequisite (passed identity verification/KYC) and instructs the agent to walk the user through verification if that prerequisite fails. It does not explicitly name alternative sibling tool IDs, but the guidance is clear enough to route to the correct flow.

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