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 structurally. The description adds the authentication requirement (api_key needed, issued in the admin console) and the returned state machine, which is modest added value over the annotations.

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 sentences with the purpose front-loaded and the auth prerequisite tucked into a trailing parenthesis. Nothing is padded, though the api_key note could arguably sit in the schema only.

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 read-only getter with no output schema, the description usefully carries the return contract (enum states plus failure reason) and the auth requirement, and annotations cover the safety profile. The main residual gap is the undocumented avatar_id and lack of guidance versus list_avatars.

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 50%: api_key has a format description but avatar_id has none. The description partially compensates by flagging api_key as required and stating where it is issued, but avatar_id remains undocumented in both places. Adequate but not rich.

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 (返します) and resource (アバターの状態) and even enumerates the possible return values (preparing / ready / failed) plus the failure reason, which is unusually specific. It does not explicitly distinguish itself from list_avatars, but the singular scope and status-enum detail make the purpose 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?

There is no stated when-to-use or when-not-to-use guidance, and no alternative tool is named. The phrase 自分で作ったアバター implies a post-creation polling context, but that is left for the agent to infer rather than stated.

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