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

With no annotations, the description adds useful behavioral context: prices, credit consumption per letter/parcel, and payment via balance or x402. However, for a financial mutation tool it does not disclose whether purchases are final, how credits are applied, or what happens on failure.

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 tightly packed sentences front-load the core action and pricing, then add credit usage and payment context. Every sentence contributes useful information without 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?

The description covers pricing, credit consumption, and payment methods, but for a purchase tool with no annotations and no output schema it omits what happens after buying (e.g., credits added, receipt, balance update) and does not explain same-day credit behavior.

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 50%, and the description compensates by mapping three of the five bundle enum values to prices and grouping same-day credits. The api_key parameter is already documented in the schema, so the description adds meaningful value where the bundle parameter lacks schema description.

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 states a specific verb and resource: 'Buy a bundle of letter credits' and further specifies same-day credits. It clearly distinguishes the two bundle families, though it does not explicitly name or differentiate from siblings like top_up or get_prices.

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 through the credit-consumption context ('Every arriving letter uses 1 credit, a parcel 3') and payment options, but there is no explicit guidance on when to choose buy_bundle versus top_up, get_prices, or other payment-related siblings.

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