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

Annotations declare a non-read-only, non-idempotent, open-world operation, and the description adds genuinely new context: the funding rail (USDC on Base via x402), that credits never expire, that they apply after the free monthly allowance, and that an organization key is required. It does not contradict the annotations ('Buy' is consistent with readOnlyHint=false) and usefully surfaces the auth prerequisite.

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 compact sentences with the core action front-loaded, followed by the operational details (expiration, allowance, auth). Every clause carries information; there is no filler or repetition.

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 purchase tool with no output schema, the description covers the payment method, authentication requirement, credit lifetime, and when credits are consumed. It could state what a successful purchase returns or debits, but the essential calling context is present.

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 100% and the single 'pack' enum is fully documented with prices (1k/10k/100k in USDC) in the schema itself. The description adds no parameter detail beyond what the schema provides, so the baseline of 3 applies.

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 ('Buy prepaid Privi Guard checks') plus the payment rail ('USDC on Base via x402'), so an agent immediately understands this is a credit-purchase operation distinct from guard_check or guard_balance. It does not explicitly name a sibling to avoid, keeping it short of a 5.

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?

The line 'used after the free monthly allowance' implies the condition under which purchase is needed, giving an implicit when-to-use. There is no explicit when-not guidance and no routing to alternatives such as guard_pricing or guard_balance, so usage remains inferred rather than stated.

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