pay_invoicePay a BOLT11 Lightning invoice with a max-sats safety limit; returns the payment preimage on success.
Pay a BOLT11 Lightning invoice. Returns the payment preimage on success.
| Name | Required | Description | Default |
|---|---|---|---|
| invoice | Yes | BOLT11 Lightning invoice string (starts with lnbc…) | |
| maxSats | No | Maximum sats to pay (safety limit). Defaults to LIGHTNING_MAX_SATS env or 1000. |
Changes observed during successful MCP inspections.
v0.2.0Does the description disclose side effects, auth requirements, rate limits, or destructive behavior?
No annotations are provided, so the description carries the full behavioral burden. It mentions the success return value (payment preimage), which is useful since there is no output schema, but omits critical traits for a payment tool: irreversibility, failure behavior, and how maxSats acts as a safety limit.
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?
Two sentences with zero waste, front-loading the core action and following with the key return value. Every word earns its place.
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 financial transaction tool with no annotations and no output schema, the description is incomplete. It should disclose irreversibility, failure modes, or prerequisites, but instead provides only a minimal success case.
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 schema already documents both parameters thoroughly. The description adds no additional parameter semantics beyond what is in the schema, making the baseline score of 3 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?
States a specific verb (Pay) and resource type (BOLT11 Lightning invoice), clearly distinguishing it from invoice creation or address resolution. However, it does not explicitly differentiate from its closest sibling pay_lightning_address, which handles a different input format.
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 no guidance on when to use this tool versus alternatives like pay_lightning_address or pay_batch. It only restates the operation, leaving the agent to infer that BOLT11 invoices require this tool.
Agents often have multiple tools that could apply. Explicit usage guidance like "use X instead of Y when Z" prevents misuse.