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Alby Bitcoin Payments MCP Server

Official
by getAlby
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TypeScript
Hybrid

Pay Invoice

pay_invoice

Pays a Lightning invoice from your connected Lightning wallet; supports optional metadata and amount for zero-amount invoices.

Instructions

Pay a lightning invoice

Input Schema

TableJSON Schema
NameRequiredDescriptionDefault
invoiceYesThe lightning invoice to pay
metadataNo
amount_in_satsNo

Output Schema

TableJSON Schema
NameRequiredDescriptionDefault
preimageYesPayment preimage
fees_paid_in_satsNoFees paid in sats

Schema Changelog

Changes observed during successful MCP inspections.

  1. Changed2 schema fields changedv1.1.2
    • removedOutput schema / properties / fees_paid_in_sats / anyOf
      Removed value: -[
-  {
-    "type": "number"
-  },
-  {
-    "type": "null"
-  }
-]
    • addedOutput schema / properties / fees_paid_in_sats / type
      Added value: +[
+  "number",
+  "null"
+]
  2. Changed8 schema fields changedv1.0.0
    • removedInput schema / additionalProperties
      Removed value: -false
    • addedInput schema / properties / amount_in_sats / anyOf
      Added value: +[
+  {
+    "description": "Optional amount in sats, only provide if paying a zero-amount invoice",
+    "type": "number"
+  },
+  {
+    "type": "null"
+  }
+]
    • removedInput schema / properties / amount_in_sats / description
      Removed value: -"Optional amount in sats, only provide if paying a zero-amount invoice"
    • removedInput schema / properties / amount_in_sats / type
      Removed value: -[
-  "number",
-  "null"
-]
    • changedInput schema / properties / metadata / anyOf
      Previous value: -[
-  {
-    "additionalProperties": true,
-    "description": "Optional metadata to include with the payment",
-    "properties": {},
-    "type": "object"
-  },
-  {
-    "type": "null"
-  }
-]New value: +[
+  {
+    "additionalProperties": {},
+    "description": "Optional metadata to include with the payment",
+    "properties": {},
+    "type": "object"
+  },
+  {
+    "type": "null"
+  }
+]
    • removedInput schema / properties / metadata / description
      Removed value: -"Optional metadata to include with the payment"
    • addedOutput schema / properties / fees_paid_in_sats / anyOf
      Added value: +[
+  {
+    "type": "number"
+  },
+  {
+    "type": "null"
+  }
+]
    • removedOutput schema / properties / fees_paid_in_sats / type
      Removed value: -[
-  "number",
-  "null"
-]
  3. First observed

TDQS

C2.6/5.0
Behavior2/5

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

No annotations are provided, so the description carries the full behavioral burden. 'Pay a lightning invoice' implies an irreversible financial mutation but says nothing about balance requirements, fee handling, routing, or whether the operation can be undone; this is a major omission for a payment tool.

Agents need to know what a tool does to the world before calling it. Descriptions should go beyond structured annotations to explain consequences.

Conciseness4/5

Is the description appropriately sized, front-loaded, and free of redundancy?

It is a single front-loaded sentence with zero wasted words. The sentence is structurally clean, though its brevity contributes to the under-specification penalized in other dimensions.

Shorter descriptions cost fewer tokens and are easier for agents to parse. Every sentence should earn its place.

Completeness1/5

Given the tool's complexity, does the description cover enough for an agent to succeed on first attempt?

For a payment mutation with no annotations and only 33% schema description coverage, the description omits essential context: prerequisites, amount semantics, and safety behavior. The output schema covers return values, but the description is not complete enough for an agent to invoke this tool safely and correctly.

Complex tools with many parameters or behaviors need more documentation. Simple tools need less. This dimension scales expectations accordingly.

Parameters2/5

Does the description clarify parameter syntax, constraints, interactions, or defaults beyond what the schema provides?

The schema supplies some parameter documentation, especially for 'amount_in_sats' and its zero-amount caveat, but the description adds no parameter meaning at all. With schema description coverage at only 33%, the description fails to compensate for the undocumented or underspecified parameters such as metadata.

Input schemas describe structure but not intent. Descriptions should explain non-obvious parameter relationships and valid value ranges.

Purpose4/5

Does the description clearly state what the tool does and how it differs from similar tools?

The description pairs a specific verb ('Pay') with a specific resource ('lightning invoice'), so the action is immediately clear. It does not differentiate this tool from sibling invoice tools such as make_invoice, lookup_invoice, parse_invoice, or request_invoice, so it falls short of a 5.

Agents choose between tools based on descriptions. A clear purpose with a specific verb and resource helps agents select the right tool.

Usage Guidelines2/5

Does the description explain when to use this tool, when not to, or what alternatives exist?

No when-to-use, prerequisites, or alternatives are stated. It does not explain when an agent should pay an invoice versus create, look up, or parse one, leaving selection entirely to inference.

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

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