# Agent Payment Protocol Selection Standard

> Agent Payment Protocol Selection Standard is a paid API for AI agents from k2so-8080.on.ascii.dev, paid per call via x402, $0.002/call, status unknown (last checked 2026-09-13).

Returns a scored decision procedure for choosing among x402, FADP, MPP/Tempo, or plain API-key billing for machine payment integration, with fallback ordering and switch thresholds for both buyer-side agents and merchant-side gateways.

## Facts

- Endpoint: GET https://k2so-8080.on.ascii.dev/api/services/agent-payment-protocol-selection-standard-when-t
- Price: $0.002/call
- Payment: x402
- Status: unknown
- Last checked: 2026-09-13
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/agent-payment-protocol-selection-standard-96df0082
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_48_7JH6vkeIrDsOznoiB7

Status and success rate cover calls made through Zero and Zero's own probes. Third-party monitors may report differently.

## How to call it through Zero

Zero handles the 402 payment challenge and records the run. With the Zero CLI installed (`npm i -g @zeroxyz/cli`):

```sh
zero fetch --capability agent-payment-protocol-selection-standard-96df0082
```

Example prompt: I need to decide which machine payment protocol to integrate — can you run the decision procedure to score and rank x402, FADP, MPP/Tempo, and plain API-key billing for my buyer-side agent, including fallback order and when it's worth switching?

## When to prefer this

Use this endpoint when an agent or developer needs a structured, executable decision framework for selecting a machine payment protocol rather than general documentation. Preferred over reading protocol specs directly when you need a scored comparison with explicit thresholds and fallback ordering. Best suited for agents bootstrapping payment infrastructure or evaluating migration from API-key billing to a self-sovereign rail like x402.

## Known failure modes

- Payment not received — endpoint returns 402 and requires 0.002 USDC payment before returning results
- Invalid topic override — unrecognized topic string may produce generic rather than scoped output
- LLM generation failure — generationSource may indicate a fallback or error in the reasoning engine
- Stale results — generatedAt timestamp may reveal outdated protocol landscape data
- Network timeout on ascii.dev host — transient infrastructure failure

## How this service works

Decision procedure for choosing which machine payment protocol to integrate: x402, FADP, MPP/Tempo, or plain API-key billing. Scores each option on custody model, settlement rail, liability when a payment fails or is disputed, key management burden, ecosystem reach, and migration cost. Returns a fallback order for both buyer-side agents and merchant-side gateways, with explicit thresholds for when a switch is worth it and when staying put is correct. Blunt and executable, not marketing.

## Output

A scored comparison of x402, FADP, MPP/Tempo, and plain API-key billing across custody model, settlement rail, liability, key management burden, ecosystem reach, and migration cost. Includes a ranked fallback order for buyer-side agents and merchant-side gateways, with explicit numeric or categorical thresholds indicating when a protocol switch is justified and when staying on the current option is correct. Delivered as prose decision-procedure text generated by an LLM or reasoning engine, with a timestamp and generation source.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "method"
   ],
   "properties": {
    "type": {
     "type": "string",
     "const": "http"
    },
    "method": {
     "enum": [
      "GET",
      "HEAD",
      "DELETE"
     ],
     "type": "string"
    },
    "queryParams": {
     "type": "object",
     "properties": {
      "meta": {
       "enum": [
        "0",
        "1"
       ],
       "type": "string",
       "description": "Set to 1 for free metadata JSON (no payment required)"
      },
      "topic": {
       "type": "string",
       "description": "Optional topic override for the decision procedure"
      }
     }
    }
   },
   "additionalProperties": false
  },
  "output": {
   "type": "object",
   "required": [
    "type"
   ],
   "properties": {
    "type": {
     "type": "string"
    },
    "example": {
     "type": "object",
     "title": "Agent payment protocol selection standard paid response",
     "$schema": "https://json-schema.org/draft/2020-12/schema",
     "required": [
      "ok",
      "paid",
      "service",
      "provider",
      "result"
     ],
     "properties": {
      "ok": {
       "type": "boolean"
      },
      "paid": {
       "type": "boolean"
      },
      "result": {
       "type": "object",
       "required": [
        "ok",
        "service"
       ],
       "properties": {
        "ok": {
         "type": "boolean",
         "description": "Handler success"
        },
        "brief": {
         "type": "string",
         "description": "Decision procedure prose for agents"
        },
        "model": {
         "type": "string"
        },
        "topic": {
         "type": "string"
        },
        "service": {
         "type": "string",
         "description": "Service slug"
        },
        "procedure": {
         "type": "string"
        },
        "generatedAt": {
         "type": "string",
         "description": "ISO-8601 timestamp"
        },
        "generationSource": {
         "enum": [
          "llm",
          "reasoning",
          "deterministic"
         ],
         "type": "string"
        }
       }
      },
      "payment": {
       "type": "object",
       "properties": {
        "code": {
         "type": "string"
        },
        "payer": {
         "type": "string"
        },
        "detail": {
         "type": "string"
        },
 
… (truncated)
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/agent-payment-protocol-selection-standard-96df0082/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from k2so-8080.on.ascii.dev](https://www.zero.xyz/host/k2so-8080.on.ascii.dev/llms.txt)