Image Generation API

Guide to FastRouter's OpenAI-compatible Image Generation endpoint, including SDK and direct API examples in Python, Curl, and JavaScript for seamless LLM interactions.

Introduction

FastRouter allows you to generate high-quality images using an OpenAI-compatible Image Generation endpoint, supporting models from OpenAI and Google. You can create images in multiple sizes, formats, and quality levels — all while benefiting from FastRouter’s unified API layer and no vendor lock-in.

FastRouter supports:

• Native Image Generation API for OpenAI models (openai/gpt-image-1 , openai/gpt-image-1-mini , openai/dall-e-2 , openai/dall-e-3 )

• Image generation via Chat Completions for Google’s Gemini Nano Banana (google/gemini-2.5-flash-image) including aspect ratio support

Before getting started, replace placeholders like <FASTROUTER_API_KEY> with your actual API key found in your dashboard (see Keys & Settings).

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Endpoint

POST https://go.fastrouter.ai/api/v1/images/generations

Base URL

https://go.fastrouter.ai/api/v1

---

Supported Models

OpenAI Image Models (via Image Generation API)

• openai/gpt-image-1

• openai/gpt-image-1-mini

• openai/dall-e-2

• openai/dall-e-3

Google Image Models (via Chat Completion API)

• google/gemini-2.5-flash-image (Nano Banana)

Note: Nano Banana supports image generation only through the Chat Completions API and includes support for custom aspect ratios.

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Parameters (Image Generation Endpoint)

Parameter	Type	Required	Default	Supported Values	Notes
n	integer or null	Optional	1	1–10	Only n = 1 is supported for dall-e-3.
output_format	string or null	Optional	png	png, jpeg, webp	Only supported for gpt-image-1 and openai/gpt-image-1-mini
quality	string or null	Optional	auto	auto, high, medium, low, hd, standard	gpt-image-1and gpt-image-1-mini: auto/high/medium/low dall-e-3: auto/hd/standard dall-e-2: standard only
size	string or null	Optional	auto	See model support	gpt-image-1 and gpt-image-1-mini auto, 1024×1024, 1536×1024, 1024×1536 dall-e-2: 256×256, 512×512, 1024×1024 dall-e-3: 1024×1024, 1792×1024, 1024×1792
background	string or null	Optional	auto	transparent, opaque, auto	Only for gpt-image-1 and gpt-image-1-miniTransparent requires png/webp.

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OpenAI-Compatible Image Request Format

FastRouter follows the same schema as OpenAI’s Image Generation API.

Python

import base64
from openai import OpenAI

client = OpenAI(
    api_key="<FASTROUTER_API_KEY>",
    base_url="https://go.fastrouter.ai/api/v1"
)

img = client.images.generate(
    model="openai/gpt-image-1",
    prompt="A cute baby sea otter",
    n=1,
    size="1024x1024"
)

image_base64 = img.data[0].b64_json
with open("otter.png", "wb") as f:
    f.write(base64.b64decode(image_base64))

---

Calling FastRouter Directly (No SDK)

cURL

curl --location 'https://go.fastrouter.ai/api/v1/images/generations' \
  --header 'Content-Type: application/json' \
  --header 'Authorization: Bearer <FASTROUTER_API_KEY>' \
  --data '{
    "model": "openai/dall-e-3",
    "prompt": "A group of children playing cricket",
    "n": 1,
    "size": "1024x1024"
  }'

JavaScript

Python

---

Image Generation via Google Gemini Nano Banana (using Chat Completions Endpoint)

FastRouter supports google/gemini-2.5-flash-image — also known as Nano Banana — through the Chat Completions API.

Nano Banana supports:

• High-quality image generation

• Custom aspect ratios

• Flexible prompt inputs using Chat-style messages

---

Nano Banana Chat Completion Example

curl --location 'https://go.fastrouter.ai/api/v1/chat/completions' \
--header 'Content-Type: application/json' \
--header 'Authorization: Bearer <FASTROUTER_API_KEY>' \
--data '{
  "stream": false,
  "model": "google/gemini-2.5-flash-image",
  "messages": [
    {
      "role": "user",
      "content": [
        {
          "type": "text",
          "text": "Give me a picture of a yellow bicycle against a red brick wall"
        }
      ]
    }
  ],
  "aspectRatio": "3:2"
}'

---

Supported Aspect Ratios (Nano Banana)

Aspect Ratio	Resolution	Tokens
1:1	1024×1024	1290
2:3	832×1248	1290
3:2	1248×832	1290
3:4	864×1184	1290
4:3	1184×864	1290
4:5	896×1152	1290
5:4	1152×896	1290
9:16	768×1344	1290
16:9	1344×768	1290
21:9	1536×672	1290

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Key Takeaway

FastRouter’s Image Generation features give you:

• OpenAI-compatible Image API

• Support for OpenAI’s latest image models

• Google Gemini Nano Banana via Chat Completions

• Custom aspect ratios

• Unified API + zero vendor lock-in