# x402engine GPT-4o LLM API

> x402engine GPT-4o LLM API is a paid API for AI agents from x402engine.app, paid per call via x402, $0.018586/call, status unknown (last checked 2026-10-12).

Calls OpenAI GPT-4o via HTTP 402 micropayment, returning an OpenAI-compatible chat completion response for a given message array.

## Facts

- Endpoint: POST https://x402engine.app/api/llm/gpt-4o?utm_source=zero.xyz
- Price: $0.018586/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-12
- Activations on Zero: 0
- Tags: x402
- Canonical page: https://www.zero.xyz/c/x402engine-gpt-4o-llm-api-5541a00f
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_WgmAUOFkAqDKMWi0YuEdC

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 x402engine-gpt-4o-llm-api-5541a00f -d '<json body>'
```

Example prompt: Ask GPT-4o to write a concise executive summary of the following meeting transcript, using the system prompt 'You are a professional business writer' and stream the response back as it generates.

## When to prefer this

Choose this endpoint when you need GPT-4o specifically (not a generic LLM), want to pay per-call in USDC without an OpenAI subscription, are building a crypto-native or agent-to-agent workflow that benefits from HTTP 402 micropayments, or need OpenAI-compatible streaming and tool/function calling without managing API keys.

## Known failure modes

- Payment not settled — 402 response if USDC funds are insufficient
- Invalid messages array — 400 if required 'messages' field is missing or malformed
- Unsupported parameter for model — some optional fields (seed, logprobs, top_p) may not be supported by all model variants
- Streaming not supported in some client environments — SSE requires persistent HTTP connection
- Tool call format errors — malformed function definitions will cause 400 or unexpected behavior
- Rate limiting or upstream OpenAI outage — may return 503 or timeout

## How this service works

125 production APIs behind HTTP 402 micropayments. 87 LLMs, Kling V3 and Hailuo video generation, image generation, code execution, travel search, web scraping, TTS, and transaction simulation. Pay with USDC on Base, USDm on MegaETH, or USDC on Solana.

## Output

Returns an OpenAI-compatible JSON chat completion object containing the model name, an array of choices with the assistant's message content and finish reason, and a top-level 'content' shortcut field. When streaming is enabled, delivers OpenAI-compatible server-sent events (SSE) incrementally.

## Request schema (JSON Schema)

```json
{
 "type": "object",
 "$schema": "https://json-schema.org/draft/2020-12/schema",
 "required": [
  "input"
 ],
 "properties": {
  "input": {
   "type": "object",
   "required": [
    "type",
    "bodyType",
    "body",
    "method"
   ],
   "properties": {
    "body": {
     "type": "object",
     "required": [
      "messages"
     ],
     "properties": {
      "seed": {
       "type": "integer",
       "description": "Model-supported seed; see /docs/llm-capabilities.md."
      },
      "stop": {
       "oneOf": [
        {
         "type": "string"
        },
        {
         "type": "array",
         "items": {
          "type": "string"
         },
         "maxItems": 4
        }
       ],
       "description": "Model-supported stop; see /docs/llm-capabilities.md."
      },
      "tools": {
       "type": "array",
       "items": {
        "type": "object",
        "required": [
         "function"
        ],
        "properties": {
         "type": {
          "enum": [
           "function"
          ],
          "type": "string"
         },
         "function": {
          "type": "object",
          "properties": {
           "name": {
            "type": "string"
           },
           "strict": {
            "type": "boolean"
           },
           "parameters": {
            "type": "object"
           },
           "description": {
            "type": "string"
           }
          }
         }
        }
       },
       "description": "Client-executed function definitions; return results using role=tool and tool_call_id."
      },
      "top_p": {
       "type": "number",
       "description": "Model-supported top_p; see /docs/llm-capabilities.md."
      },
      "stream": {
       "type": "boolean",
       "description": "Return live OpenAI-compatible SSE after payment settlement."
      },
      "plugins": {
       "type": "array",
       "items": {
        "type": "object",
        "required": [
         "pdf"
        ],
        "properties": {
         "id": {
          "enum": [
           "file-parser"
          ],
          "type": "string"
         },
         "pdf": {
          "type": "object",
          "properties": {
           "engine": {
            "enum": [
             "native"
            ],
            "type": "string"
           }
          }
         }
        }
       },
       "description": "Optional native PDF parser only; automatically enabled for file inputs."
      },
      "logprobs": {
       "type": "bool
… (truncated)
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "model": "openai/gpt-4o",
  "choices": [
   {
    "index": 0,
    "message": {
     "role": "assistant",
     "content": "Hello!"
    },
    "finish_reason": "stop"
   }
  ],
  "content": "Hello!"
 }
}
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/x402engine-gpt-4o-llm-api-5541a00f/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from x402engine.app](https://www.zero.xyz/host/x402engine.app/llms.txt)