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

With no annotations provided, the description carries the full behavioral burden. It discloses that the tool creates an invoice, requires proof of ownership, and requires a follow-up call to confirm payment. This gives the agent a realistic model of the asynchronous purchase flow. It does not mention invoice expiry or whether repeated calls create multiple invoices, 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 short, front-loaded with the core action, and each sentence earns its place: what it does, how the flow works, the prerequisite, and the cost. The line breaks separate distinct facts cleanly without padding.

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 is complete enough for an agent to follow the purchase flow: it names the follow-up tool (check_payment), states the ownership proof requirement, and clarifies the free-to-call aspect. An output schema exists, so return values need not be described. Minor omissions like invoice expiry or idempotency keep it from a 5.

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%, so the baseline is 3; the description adds context around the npub/dpop_token relationship ('Proof of npub ownership is required') but does not add new parameter-level semantics beyond what the schema already provides. It does not discuss amount_sats, but the schema documents it fully.

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 then clarifies the concrete first step ('Creates a Lightning invoice'), making the tool's function unmistakable and distinct from siblings like check_payment or restore_credits.

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 clearly lays out the usage flow: create the invoice, pay with any Lightning wallet, then call check_payment to confirm. It also states a key prerequisite ('Proof of npub ownership is required') and a cost condition ('Free — no credits required to call'), though it does not explicitly discuss when not to use this tool or name alternatives.

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