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

Annotations already declare readOnlyHint, idempotentHint, non-destructive and closed-world, so the safety profile is covered. The description adds real value by clarifying that output is "structurally valid" and "synthetic" — i.e., passes format checks but corresponds to no registered taxpayer. It does not say whether the generated value is random per call (which sits uneasily with idempotentHint) or what the return shape is.

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?

A single front-loaded sentence with the verb, the guarantee, and the usage restriction, and no filler. Nothing is repeated from the title or annotations.

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?

The tool is trivial and the annotations carry the safety profile, so most of what an agent needs is present. However, with no output schema and a format parameter whose two modes are unexplained, the agent cannot predict what a legacy_15 versus nik_16 result looks like or which is the default.

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 0% and the description never mentions the single parameter "format" at all. The enum values legacy_15 and nik_16 are opaque to an agent that does not know Indonesian NPWP formats, and no default is documented, so the description does nothing to compensate for the schema gap.

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?

"Generate structurally valid synthetic NPWP" names a specific verb and resource, and "for testing only" bounds the scope. It implicitly contrasts with the validate_npwp sibling, but never names it, so the differentiation is inferential rather than explicit.

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?

"For testing only" is a genuine when/when-not signal: the agent knows not to use this for real taxpayer data. It stops short of naming the alternative (validate_npwp) or stating prerequisites such as which format to prefer, so it is clear context without full routing guidance.

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