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

readOnlyHint=true already declares this as a safe read, so the description's remaining job is light; it does add the useful domain context that these contacts serve as registrants and names the underlying endpoint. It discloses nothing about pagination behavior or result ordering beyond what the parameters imply.

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?

Two tight sentences with the purpose and the domain-specific role front-loaded, and the endpoint tacked on without padding. Nothing is wasted.

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?

For a simple read-only list with fully documented parameters and an output-schema-free response, little is missing. An agent would still not know the shape or volume of results, but that gap is minor at this complexity level.

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 page, per_page, and sort are fully documented in the schema already. The description adds no syntax or format detail for any of them, making the 3 baseline correct.

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 ('List') and resource ('contacts') and clarifies the domain-specific role of contacts as registrants, which distinguishes it from the sibling list_* tools. It stops short of naming any alternative or scope constraint, but the resource itself is unambiguous.

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?

The parenthetical '(used as domain registrants)' hints at why an agent would call this, i.e. to obtain a registrant contact for domain registration, but there is no explicit when-to-use statement, no prerequisite, and no mention of alternatives.

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