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

Beyond the annotations (readOnlyHint=false, openWorldHint=true), the description discloses critical behavior: payment cost, that the key arrives in the paid response, no account/signup required, and that the user is only charged when a result actually comes back. This adds significant context not present in structured metadata.

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 dense sentences deliver purpose, cost, exact usage steps, and payment terms with zero waste. The most critical facts (paid, key, no signup) are front-loaded, and the step-by-step guidance is structured logically.

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 1-parameter tool with no output schema, the description covers the full call flow, the return value (key arrives in the paid response), payment mechanics, and caveats about charging. An agent has everything needed to invoke the tool correctly across both phases.

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?

The schema only says 'base64 X-PAYMENT payload' for x402_payment, but the description explains its role in the two-step payment flow: omit it first to receive an accepts array, then set it to the payment payload on the second call. This adds essential semantic 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?

The description clearly states the tool's action: 'Mint a 30-day HostDeFi Agent-plan API key' with specific attributes (5,000 calls, batch enabled, $5, one payment). This distinguishes it from informational siblings like get_x402_pricing and has no ambiguity about what is being bought.

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?

It provides an explicit two-call procedure: first call without x402_payment to receive payment requirements, then call with the x402_payment payload. While it doesn't compare to alternatives (no other buy tool exists), the usage flow is clearly specified and actionable.

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