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

Annotations already declare readOnlyHint, idempotentHint, closed-world and non-destructive, so the safety profile is covered. The description adds genuinely useful behavior beyond that: it lists the actual checks performed (Luhn check digit, date of birth, gender, citizenship) and states the key limitation that these are offline checks that do not confirm registration. It stops short of describing what happens on an invalid input or the shape of the result.

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, front-loaded with the core operation and the enumerated coverage, then a closing limitation clause. No filler, no repetition of 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?

A two-parameter, read-only, closed-world tool with no output schema; the description covers scope, validation rules per type, and the offline caveat. Only a brief note on the return value (valid/invalid plus derived attributes) would make it fully self-contained.

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 100%, so baseline is 3. The description goes further by clarifying what each enum value validates (VAT is 10 digits starting with 4; ID covers Luhn, DOB, gender, citizenship), which gives the agent real meaning for the 'type' parameter choices rather than just restating the enum.

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?

The description names a specific verb+resource ('Format and check-digit checks for South African numbers') and enumerates exactly which number families are covered (ID, income tax, VAT, PAYE/SDL/UIF, CIPC). This cleanly separates it from the sibling tools, which are all calculators rather than validators.

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 implied by the enumerated number types and the 'default auto-detect' schema note, so an agent can infer this is the pre-check tool. However, there is no explicit when-to-use routing (e.g. 'run this before sa_vat_calculate or sa_vat_classify') and no stated when-not-to-use conditions beyond the offline limitation.

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