ChatGPT API Guide 2026

The ChatGPT API (officially the OpenAI API) lets you call GPT-5, GPT-4o, o3, and other models from your applications. This guide covers setup, Python usage, model selection, and APIMaster.ai's unified API access.

What Is the ChatGPT API?

The ChatGPT API is the OpenAI Chat Completions API—the same technology behind ChatGPT, exposed as a developer endpoint. You send a list of messages and receive a generated response. It's available through OpenAI directly or via providers like APIMaster.ai.

How to Get ChatGPT API Access

Direct from OpenAI:

• Billing and payment availability varies by account

Via APIMaster.ai:

1. Register with just an email
2. Top up balance (epay, PayPal, credit card, USDT)
3. Get API key from console
4. Use https://apimaster.ai/v1 as your base URL

ChatGPT API Python Quickstart

from openai import OpenAI

client = OpenAI(
    api_key="YOUR_APIMASTER_KEY",
    base_url="https://apimaster.ai/v1",
)

response = client.chat.completions.create(
    model="gpt-5.4",
    messages=[
        {"role": "system", "content": "You are a helpful assistant."},
        {"role": "user", "content": "Explain how the ChatGPT API works."},
    ],
)

print(response.choices[0].message.content)

ChatGPT API Models Comparison

Model	Context	Input	Output	Best For
gpt-5.5	128K	High	High	Complex tasks, reasoning
gpt-5.4	128K	Medium	High	General coding, writing
gpt-4o	128K	Medium	Medium	Multimodal, vision
gpt-4o-mini	128K	Low	Low	Fast, budget-friendly
o3	200K	High	High	Math, science, coding
o4-mini	128K	Medium	Medium	Fast reasoning

See APIMaster marketplace for current prices.

Streaming ChatGPT API Responses

Streaming delivers tokens as they're generated—essential for chat UIs:

stream = client.chat.completions.create(
    model="gpt-5.4",
    messages=[{"role": "user", "content": "Write a Python function to check if a number is prime."}],
    stream=True,
)

for chunk in stream:
    if chunk.choices[0].delta.content is not None:
        print(chunk.choices[0].delta.content, end="", flush=True)
print()

Vision: Send Images to ChatGPT API

GPT-4o and GPT-5 accept image inputs:

response = client.chat.completions.create(
    model="gpt-5.4",
    messages=[
        {
            "role": "user",
            "content": [
                {
                    "type": "image_url",
                    "image_url": {"url": "https://example.com/diagram.png"},
                },
                {"type": "text", "text": "Describe what you see in this diagram."},
            ],
        }
    ],
)
print(response.choices[0].message.content)

Function Calling (Tool Use)

tools = [
    {
        "type": "function",
        "function": {
            "name": "search_docs",
            "description": "Search the documentation",
            "parameters": {
                "type": "object",
                "properties": {
                    "query": {"type": "string"},
                },
                "required": ["query"],
            },
        },
    }
]

response = client.chat.completions.create(
    model="gpt-5.4",
    messages=[{"role": "user", "content": "Find docs about authentication."}],
    tools=tools,
    tool_choice="auto",
)

# Check if model wants to call a function
if response.choices[0].finish_reason == "tool_calls":
    tool_call = response.choices[0].message.tool_calls[0]
    print(f"Function: {tool_call.function.name}")
    print(f"Args: {tool_call.function.arguments}")

Building a Simple ChatGPT-like App

from openai import OpenAI

client = OpenAI(
    api_key="YOUR_APIMASTER_KEY",
    base_url="https://apimaster.ai/v1",
)

history = [{"role": "system", "content": "You are a helpful coding assistant."}]

def chat(user_input):
    history.append({"role": "user", "content": user_input})
    response = client.chat.completions.create(
        model="gpt-5.4",
        messages=history,
    )
    reply = response.choices[0].message.content
    history.append({"role": "assistant", "content": reply})
    return reply

# Interactive loop
while True:
    user = input("You: ")
    if user.lower() in ("quit", "exit"):
        break
    print(f"Assistant: {chat(user)}")

ChatGPT API Pricing

OpenAI charges per million tokens. APIMaster offers the same models at significant discounts. Visit the marketplace for current prices, or see the OpenAI API Pricing guide.

Why Use APIMaster for ChatGPT API Access

APIMaster provides a convenient alternative to the official OpenAI API:

• Discounted pricing on select models
• One key, all models: GPT, Claude, DeepSeek, Gemini through one endpoint
• Flexible payments: epay, PayPal, credit card, USDT—no subscription required
• Stable service: reliable infrastructure with weekly model authenticity verification

Frequently Asked Questions

What is the ChatGPT API? The ChatGPT API (OpenAI's Chat Completions API) lets you integrate GPT-5, GPT-4o, o3, and other OpenAI models into your applications. It's distinct from the ChatGPT web interface—you get full programmatic control.

How much does the ChatGPT API cost? GPT-5 series pricing varies by model version. Via APIMaster, you can access supported models at discounted rates. See current pricing.

How do I use the ChatGPT API in Python? Install the openai package (pip install openai), set your API key and base URL, then call client.chat.completions.create(). See the Python quickstart above.

What is the difference between ChatGPT and the ChatGPT API? ChatGPT is the consumer web product. The API gives you raw model access for building your own applications, with no chat UI, no usage limits from the web product, and full control over system prompts and parameters.

Can I use one API key for GPT, Claude, and DeepSeek? Yes—with APIMaster. One key and one endpoint covers OpenAI, Claude, DeepSeek, and Gemini models, so you can switch models with a single line change.

Get ChatGPT API — one key for GPT + Claude + DeepSeek →