Image generation

Call GPT Image, Gemini (Nano Banana) and other image models with the OpenAI Images format, at 1K / 2K / 4K.

Endpoint

POST https://api.onefast.ai/v1/images/generations   # text-to-image
POST https://api.onefast.ai/v1/images/edits         # image-to-image / inpainting (multipart)
Authorization: Bearer sk-xxx

All image models are called with the OpenAI Images format, and images are returned as Base64 (b64_json).

Text-to-image

cURL

curl https://api.onefast.ai/v1/images/generations \
  -H "Authorization: Bearer $API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-image-2.5",
    "prompt": "A cute corgi astronaut, studio lighting",
    "size": "1024x1024",
    "n": 1
  }' | python3 -c "import sys,json,base64; open('out.png','wb').write(base64.b64decode(json.load(sys.stdin)['data'][0]['b64_json']))"

Python

import base64
from openai import OpenAI

client = OpenAI(api_key="sk-xxx", base_url="https://api.onefast.ai/v1")
result = client.images.generate(
    model="gpt-image-2.5",
    prompt="A cute corgi astronaut, studio lighting",
    size="1024x1024",
)
with open("out.png", "wb") as f:
    f.write(base64.b64decode(result.data[0].b64_json))

Node.js

import fs from "node:fs";
import OpenAI from "openai";

const client = new OpenAI({ apiKey: "sk-xxx", baseURL: "https://api.onefast.ai/v1" });
const result = await client.images.generate({
  model: "gpt-image-2.5",
  prompt: "A cute corgi astronaut, studio lighting",
  size: "1024x1024",
});
fs.writeFileSync("out.png", Buffer.from(result.data[0].b64_json, "base64"));

Image generation usually takes 20–60 seconds. Set your HTTP client timeout to 300 seconds or more.

Common parameters

Gemini images (Nano Banana) and 4K

Gemini image models such as gemini-3.1-flash-image-preview and gemini-3-pro-image-preview use the same endpoint. The gateway decides the resolution tier and aspect ratio as follows:

Resolution (1K / 2K / 4K) — quality takes precedence; otherwise the longest side of size decides:

Aspect ratio — taken from the width-to-height ratio of size and snapped to the nearest standard ratio (1:1, 16:9, 9:16, 4:3, 3:4, 3:2, 2:3, 21:9).

cURL

curl https://api.onefast.ai/v1/images/generations \
  -H "Authorization: Bearer $API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gemini-3.1-flash-image-preview",
    "prompt": "A watercolor lighthouse at dawn",
    "size": "3840x2160"
  }'

The example above produces a 16:9 4K image. Exact pixel dimensions are determined by the model (for example, 2K at 16:9 is about 2752×1536). The output may be PNG or JPEG — check the file header.

Image-to-image / editing

Upload the source image (multipart form) and describe the change in prompt:

cURL

curl https://api.onefast.ai/v1/images/edits \
  -H "Authorization: Bearer $API_KEY" \
  -F model="gpt-image-2.5" \
  -F image="@input.png" \
  -F prompt="Add a red scarf to the dog"

Python

result = client.images.edit(
    model="gpt-image-2.5",
    image=open("input.png", "rb"),
    prompt="Add a red scarf to the dog",
)

Gemini image models accept several reference images in one request (up to 14, field name image[]) and do not support mask.

Billing

• Some groups charge per image (regardless of size), others per token (output image tokens at the output price). The listed price for that group in the model catalog applies.
• Failed requests are not billed.