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

Annotations already declare readOnly/idempotent/closed-world, so the description focuses on what annotations cannot convey: it discloses a per-call price and payment rail ($0.002 USDC, x402, Base mainnet) and explicitly limits scope to offline format validation with no registry status check. It does not describe the negative-result shape (what an invalid number returns), which is the main remaining gap.

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?

Front-loaded with the core action and format, followed by scope limit, use cases, and pricing in a tight sentence chain. Dense but every clause carries information; the pricing clause could be considered boilerplate but is operationally relevant.

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 single-parameter, non-destructive validation tool with no output schema, the description covers input format, decoded outputs, scope limitation, and cost — enough to call it correctly. Only the shape of a failed validation is unstated.

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?

Only one parameter and schema coverage is 100%, so the baseline is 3. The description restates the expected format YYYYY/NNNNNN/NN, which is a modest enrichment over the schema's single example value, but adds no further semantics such as tolerance for spaces or slashes.

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 and resource ('validates a CIPC registration number') plus the exact format pattern (YYYY/NNNNNN/NN) and what it returns (registration year and entity type). The resource is clearly distinct from the sibling validators za_validate_id_number and za_validate_tax_number, so an agent can route correctly without opening the 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?

Gives concrete usage contexts (supplier onboarding, invoices, tender paperwork) and an explicit exclusion: 'Format check only, not a CIPC status lookup.' It does not name an alternative tool for status lookups, so it stops short of full when/when-not routing, but the boundary of applicability is clear.

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