# Agent Payment Protocol Selection Standard – Decision Procedure (K-2SO x402)

> Agent Payment Protocol Selection Standard – Decision Procedure (K-2SO x402) is a paid API for AI agents from k2so.wrong.systems, paid per call via x402, $0.002/call, status unknown (last checked 2026-09-13).

Returns a plain-prose decision procedure brief explaining when and how an AI agent should select and use the x402 payment protocol standard.

## Facts

- Endpoint: GET https://k2so.wrong.systems/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-decision-procedure-k-2so-x402-2651d0c5
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_DZaxRtiPwLNlp2un9nDT1

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-decision-procedure-k-2so-x402-2651d0c5
```

Example prompt: Give me a plain-language decision procedure for when an AI agent should select the x402 payment protocol standard — I need to know the criteria and logic my agent should follow when choosing between payment options.

## When to prefer this

Choose this endpoint when an AI agent needs authoritative, prose-formatted guidance on how to decide between payment protocols — specifically whether and when to use x402. It is particularly useful during agent onboarding, compliance reviews, or when building decision trees for autonomous payment logic. Prefer this over static documentation when you need a dynamically generated, model-backed brief tailored to a specific topic override.

## Known failure modes

- HTTP 402 returned if USDC payment is not settled before the request — agent must pay exact amount and retry with PAYMENT-SIGNATURE header
- Invalid or missing 'method' or 'type' fields in input result in a validation error
- Incorrect USDC payment amount causes payment failure and 402 re-prompt
- Service temporarily unavailable or heartbeat gap may cause transient 5xx errors
- meta=1 query param returns free metadata JSON instead of paid procedure content — agent must not confuse this with a paid response

## 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 JSON object containing a plain-prose decision procedure brief (120+ words) advising AI agents on when and how to select the x402 payment protocol standard. Includes the topic, service slug, the model or method used to generate the brief, an ISO-8601 generation timestamp, and a generation source indicator (llm, reasoning, or deterministic). Also includes payment confirmation fields (paid, payer, transaction).

## 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)
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "ok": true,
  "paid": true,
  "result": {
   "ok": true,
   "brief": "plain prose decision procedure (120+ words)",
   "model": "deepseek/deepseek-v4-flash-0731",
   "topic": "string topic",
   "service": "agent-payment-protocol-selection-standard-when-t",
   "procedure": "same as brief",
   "generatedAt": "2026-01-01T00:00:00.000Z",
   "generationSource": "deterministic"
  },
  "service": "agent-payment-protocol-selection-standard-when-t",
  "provider": "K-2SO"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/agent-payment-protocol-selection-standard-decision-procedure-k-2so-x402-2651d0c5/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from k2so.wrong.systems](https://www.zero.xyz/host/k2so.wrong.systems/llms.txt)