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

Annotations already declare readOnlyHint=true, idempotentHint=true, destructiveHint=false and openWorldHint=false, so the safety profile is fully covered by structured data. The description adds only the precondition that the caller must already possess an id; it says nothing about what happens on an unknown id (error vs empty) or about the projection behavior of `fields`.

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, purpose-first, with the usage trigger and the required argument immediately following. Nothing is redundant or padded.

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 single-record lookup with rich annotations and a declared output schema, the description supplies enough to call it correctly – purpose, trigger, and required argument. The only omission is any hint that a `fields` projection exists, which the schema covers anyway.

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 both `id` (integer, minimum 1) and `fields` (comma-separated keys) are already documented in the schema. The description only echoes `id` and adds an illustrative value (1861); `fields` is never mentioned, so no meaning is added beyond the schema baseline.

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?

Specific verb+resource: retrieves one IANA registrar keyed by id, with a concrete example value (1861). It implicitly contrasts with the plural sibling ud_list_iana_registrars via 'One ... by id', but never names or explicitly delineates itself from that list tool.

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 a clear trigger condition – 'Use it when someone holds an IANA registrar number and asks who it is' – which tells the agent this is an id-based lookup, not a search. No explicit exclusions or named alternative (e.g. use ud_list_iana_registrars when you don't have an id), so it stops short of full routing guidance.

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