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

Annotations already declare readOnlyHint, idempotentHint and openWorldHint, so the safe-read profile is covered structurally. The description adds that a 'signed XDR-1 receipt' is produced, which is useful beyond the annotations, but it does not say how an invalid PIN is reported (error vs. boolean result) or what signing entails.

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?

Three compact sentences with no filler, leading with the validation action and format before the business context. Slightly dense but every clause carries information.

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 one-parameter validator with an output schema, the description covers format, business purpose and return artifact. The remaining gap is failure-mode behavior, which the output schema may partially address.

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 single parameter is documented in both schema and description. The description nonetheless adds semantic detail beyond the schema's '11-character KRA PIN' by decomposing the structure into A/P prefix, 9 digits and checksum character.

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 ('Validate Kenyan Revenue Authority Tax PIN structure') and spells out the exact format (A/P + 9 digits + checksum). This clearly separates it from sibling format validators like ein_format_check or company_number_format.

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?

Explicitly names the triggering contexts: 'Required for M-Pesa B2B disbursement and Kenyan remote agency invoicing.' That is real when-to-use guidance, though it gives no when-not conditions or pointers to alternatives such as batch_validate for bulk PIN checks.

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