# x402engine GPT-5.1 LLM API

> x402engine GPT-5.1 LLM API is a paid API for AI agents from x402engine.app, paid per call via x402, $0.010015/call, status unknown (last checked 2026-10-12).

Calls OpenAI's GPT-5.1 model via a pay-per-call HTTP 402 micropayment endpoint, returning OpenAI-compatible chat completions

## Facts

- Endpoint: POST https://x402engine.app/api/llm/gpt-5.1?utm_source=zero.xyz
- Price: $0.010015/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-5-1-llm-api-254b4a11
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_kqH0fDrYOevnay_kcdMFU

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-5-1-llm-api-254b4a11 -d '<json body>'
```

Example prompt: Ask GPT-5.1 to explain the tradeoffs between relational and vector databases in plain English, streaming the response back to me, and pay per call with USDC.

## When to prefer this

Choose this endpoint when you need GPT-5.1 specifically (OpenAI's latest flagship model) and want pay-per-call pricing with no API key management, billed in USDC on Base, USDm on MegaETH, or USDC on Solana. Ideal for autonomous agents, serverless pipelines, or situations where you want to avoid OpenAI subscription overhead. Prefer alternatives if you need a cheaper model for high-volume simple tasks (use Claude Haiku or MiMo-Flash siblings), need video or image generation (use Kling/FLUX siblings), or need audio transcription.

## Known failure modes

- Payment not settled — 402 returned if USDC/USDm balance is insufficient
- Invalid messages array — 400 if required role or content fields are missing
- Token limit exceeded — model returns an error or truncated finish_reason
- Tool call schema mismatch — malformed function definitions cause 400 or unexpected behavior
- Streaming timeout — SSE connection dropped on slow clients
- Unsupported seed value — model may ignore or error on out-of-range seeds

## 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 object containing the model name (openai/gpt-5.1), a choices array with the assistant message (role, content, tool_calls if any), and a finish_reason. Also exposes a top-level content shortcut field. Supports SSE streaming when stream=true.

## 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."
      },
      "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."
      },
      "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."
      },
      "messages": {
       "type": "array",
       "items": {
        "type": "object",
        "anyOf": [
         {
          "required": [
           "content"
          ]
         },
         {
          "required": [
           "tool_calls"
          ]
         }
        ],
        "required": [
         "role"
        ],
        "properties": {
         "name": {
          "type": "string"
         },
         "role": {
          "enum": [
           "system",
   
… (truncated)
```

## Response schema (JSON Schema)

```json
{
 "type": "json",
 "example": {
  "model": "openai/gpt-5.1",
  "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-5-1-llm-api-254b4a11/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)