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

With no annotations provided, the description carries the full burden of behavioral disclosure. It clearly explains the payment requirement, the recharge versus new-key behavior, the one-time key return, and the critical caveat that the key cannot be recovered. This is rich, honest behavioral context.

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?

The description is compact yet information-dense, with every sentence contributing essential details. The key caveat about key recovery is front-loaded near the end, and 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 tool with no output schema and no annotations, the description provides enough context to invoke it correctly: payment amount, minimum, auth modes, and outcome. It does not describe error cases or the exact response format beyond key return, but the critical operational details are 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 coverage is 100%, so the baseline is 3. The description adds meaningful context beyond the schema by explaining how the amount ties to prepaid credits, how the Authorization header changes the effect of the call, and that a new key is returned only once and must be stored as a secret. This enhances the parameter understanding.

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 clear action ('Buy prepaid credits with x402') and a specific resource, making the tool's purpose immediately identifiable. It also explains the benefit ('later calls skip the per-call payment round trip'), which helps distinguish it from the other listed tools.

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 description gives explicit instructions for two use cases: with an Authorization header it recharges the key, and without one it mints a new API key. It does not explicitly compare against sibling tools, but the usage context is clear and actionable.

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