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

With no annotations, the description still discloses three non-obvious traits: the tool is free (no auth/cost), it implements Inland Revenue's written steps rather than IRD's published Example 3, and it deliberately reports 150000026 as invalid. That last point is genuinely useful behavioral context an agent could not infer from the schema, though input-format expectations and error behavior remain unstated.

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 leads with the action and scope, and the parenthetical carries real information rather than filler. It is slightly dense and the trailing caveat is a touch cryptic, but 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?

An output schema exists, so return values need not be explained, and the algorithm source is given. However, the undefined 'jurisdiction' parameter, unspecified input format, and the ambiguous phrasing 'is reported invalid' (by whom?) leave gaps for a two-parameter validation 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 description coverage is 0%, so the description must carry parameter meaning and it largely does not. It implies what 'number' is but never states format or length constraints, and the 'jurisdiction' parameter (default 'NZ') is not mentioned at all, leaving a caller unaware it can be set.

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 (check/validate) and resource (an NZ IRD number's check digit), and the 'NZ IRD' scope cleanly separates it from the sibling validators validate_abn (Australian) and validate_nz_bank (NZ bank accounts). An agent can route to it without opening any schema.

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?

Usage is only implied by the name and the 'NZ IRD' qualifier; there is no explicit when-to-use, when-not-to-use, or named-alternative guidance. The sibling set makes the routing fairly obvious, but nothing in the text tells the agent when this tool is the wrong choice.

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