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

Annotations only say readOnlyHint=false and destructiveHint=false. The description adds what actually matters: this is an upsert, not a pure create — existing data is overwritten, so the caller must resend the full field set rather than a delta. It also explicitly states no charge occurs, which is essential context given the surrounding payment-related siblings.

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 definition leads with what the resource is, then the trigger, then the mutation semantics, in priority order. Every sentence carries non-redundant information: the definition, the error trigger, the requiredness rule, the no-charge assurance, and the upsert warning.

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 needn't be described, and annotations cover the safety profile; the description still adds the trigger, the no-charge fact, and the overwrite behavior. The only small gap is that it doesn't say what happens on validation failure or partial/invalid input, which is minor for this tool.

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 coverage is 89%, so the baseline is 3, and the description earns an extra point by clarifying requiredness beyond the schema listing (all required except state and organization, 'state only where applicable') and by reinforcing the full-set resubmission rule that governs every parameter on update. It doesn't restate the zip/phone/country formats, which the schema already handles.

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 gives a specific verb and resource ('Create or update the authenticated user's registrant (WHOIS) contact') and explains exactly what that resource is: the name/address/phone/email a registry requires for registration or marketplace transfer. No sibling tool touches WHOIS contacts, so the boundary against purchase_domain/register_domain is unambiguous from the text 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?

It names a concrete trigger ('Call this when a tool reports error registrant_contact_required') and ties the tool to the registration/transfer prerequisite that motivates it. It doesn't say when not to call it or discuss any alternative path, but the trigger condition is explicit enough for an agent to act on.

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