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

Annotations already cover read-only, idempotent, closed-world behavior, and the description adds what really matters here: format-only validation with no registry query, no data stored, and a per-call price of $0.002 USDC via x402 on Base mainnet. The cost/payment requirement is behavior an agent must know before calling, and it is disclosed plainly.

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 what is validated, followed by limit, use cases, and price. Every sentence carries distinct information and nothing is repeated from the schema.

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, read-only validation tool this is nearly complete: input shape, validation depth, limits, use cases, and cost are all covered. Without an output schema, the exact response shape (e.g., validity flag plus detected type) is only partially implied by 'says which type it is.'

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% and the schema already says spaces, dashes and slashes are ignored. The description adds the 10-digit length constraint and clarifies that either a VAT or income tax number is accepted and the type is identified in the response, which is useful meaning beyond the schema.

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?

Specific verb+resource: validates a South African 10-digit SARS VAT or income tax reference number and reports which type it is. The scope is clearly distinct from sibling validators like za_validate_id_number and za_validate_company_number, and the check method (format plus modulus-10 check digit) is named.

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 (invoices, supplier onboarding, form validation) and an explicit exclusion: this is a format check, not a SARS registration lookup. It does not name an alternative tool for a real registry lookup, but the when-not guidance is clear.

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