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

With no annotations provided, the description carries the burden of behavioral disclosure. It clearly states that the tool creates a Lightning invoice, that payment is external via a wallet, and that confirmation requires a follow-up call to check_payment. It also notes the free-to-call nature, which is useful for agents deciding whether invocation has side effects or cost. It could mention invoice expiry or irreversibility, but the core behavior is well covered.

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 and front-loaded with the core purpose. Every sentence earns its place: what it does, how to complete payment, why proof is needed, and that it is free to call. There is no fluff or duplication of schema content.

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 a rich input schema and an output schema, the description provides the essential workflow and prerequisites. It tells the agent to call check_payment afterward and explains why ownership proof matters. It could be more explicit about what the invoice response contains or that credits are only credited after payment, but the description is generally complete for safe invocation.

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 input schema already thoroughly documents all three parameters, especially dpop_token. The description adds modest context by explaining that npub ownership proof is required so credits land in the correct ledger, which reinforces the purpose of npub and dpop_token, but it doesn't add substantial parameter-level meaning beyond that. Baseline 3 is appropriate.

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 opens with a specific verb and resource: 'Buy credits via Bitcoin Lightning.' It clearly identifies what the tool does and distinguishes it from related sibling tools like check_payment and restore_credits by focusing on the purchase/invoice-creation step. An agent can immediately understand the tool's role.

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 actionable workflow guidance: create invoice, pay with any Lightning wallet, then call check_payment to confirm. It also explains the prerequisite that proof of npub ownership is required. It doesn't explicitly enumerate when not to use this tool vs alternatives, but the workflow is clear enough for an agent to select it for purchasing credits.

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