# GPT-5.3 Codex (via x402 Gateway)

> GPT-5.3 Codex (via x402 Gateway) is a paid API for AI agents from x402-gateway-production.up.railway.app, paid per call via x402, $0.08/call, status unknown (last checked 2026-10-11).

Calls OpenAI's GPT-5.3 Codex model — a state-of-the-art agentic coding model — for software engineering and code generation tasks, paid per-call via x402/USDC.

## Facts

- Endpoint: POST https://x402-gateway-production.up.railway.app/api/llm/gpt-5.3-codex?utm_source=zero.xyz
- Price: $0.08/call
- Payment: x402
- Status: unknown
- Last checked: 2026-10-11
- Success rate: 0% of calls made through Zero
- Activations on Zero: 1
- Tags: x402
- Canonical page: https://www.zero.xyz/c/x402-gateway-production-up-railway-app-3e55b479
- Structured record (JSON): https://api.zero.xyz/v1/capabilities/cap_F7quJLxJ-enhA2hzJsXRe

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 x402-gateway-production-up-railway-app-3e55b479 -d '<json body>'
```

Example prompt: Write me a Python function that takes a list of dictionaries and sorts them by a specified key, handling missing keys gracefully — include type hints and a docstring.

## When to prefer this

Choose this endpoint when you need OpenAI's best-in-class coding model (GPT-5.3 Codex) for complex software engineering tasks, agentic coding workflows, or high-quality code generation — especially when you want SOTA performance and are willing to pay $0.08/call. Prefer over cheaper GPT-5 tiers for difficult or nuanced coding problems.

## Known failure modes

- Invalid or missing prompt returns 400 error
- Insufficient USDC balance or x402 payment failure returns 402
- Model unavailable or overloaded returns 503
- Prompt too long exceeds context window, returning 400
- Malformed request body returns 422

## How this service works

OpenAI's SOTA agentic coding model — best-in-class for software engineering and code generation

## Output

Returns AI-generated code, refactored code, explanations, or debugging suggestions produced by OpenAI's GPT-5.3 Codex model, optimized for software engineering tasks.

## Example request

```json
{
 "model": "gpt-5.3-codex",
 "messages": [
  {
   "role": "user",
   "content": "Write a Python function that validates email addresses using regex, includes proper error handling, and returns a boolean indicating validity. Include type hints and a docstring."
  }
 ],
 "max_tokens": 500
}
```

## 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)
```

## More

- Live health (JSON, refreshed every minute): https://www.zero.xyz/c/x402-gateway-production-up-railway-app-3e55b479/health.json
- [Zero catalog index](https://www.zero.xyz/llms.txt)
- [Other services from x402-gateway-production.up.railway.app](https://www.zero.xyz/host/x402-gateway-production.up.railway.app/llms.txt)