What Is OpenAI-Compatible API? Complete Guide for Developers

If you’ve used any AI tool in the past year—Cursor, Dify, LangChain, LobeChat, Open WebUI—you’ve probably seen the phrase “OpenAI-compatible API.” But what does that actually mean, and why does it matter?

This guide explains the concept clearly, so you can understand how it works and how to take advantage of it.

One: What “OpenAI-Compatible” Actually Means

When OpenAI released its API in 2020, it defined a specific HTTP interface:

Endpoint: POST /v1/chat/completions
Request format: JSON with model, messages, temperature, and other parameters
Response format: JSON with choices, usage, and streaming support via Server-Sent Events

This interface became so widely adopted that it evolved into an industry-wide standard. Today, dozens of providers—Anthropic, Google, DeepSeek, Mistral, and many others—offer endpoints that follow the exact same format.

“OpenAI-compatible” simply means: the API speaks the same language as OpenAI’s API. You can point your existing code at a different provider by changing one URL, and everything keeps working.

Two: Why This Standard Exists

The OpenAI interface won because of network effects:

OpenAI was first to market with a capable, well-documented API
Developers built tooling, SDKs, and frameworks around that interface
New providers adopted the same interface to be immediately compatible with all existing tools
Tool makers added “OpenAI-compatible” support as a default feature

The result is an ecosystem where any tool that supports OpenAI automatically supports hundreds of providers—as long as those providers implement the compatible interface.

Three: How It Works in Practice

The key parameters are:

from openai import OpenAI

client = OpenAI(
    api_key="your-api-key",       # provider-specific key
    base_url="https://api.apibox.cc/v1"  # swap this URL
)

response = client.chat.completions.create(
    model="claude-sonnet-4-6",    # use any supported model
    messages=[
        {"role": "user", "content": "Hello!"}
    ]
)

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

The only things that change between providers:

api_key — your key from that provider
base_url — the provider’s API endpoint
model — the model name as that provider defines it

Everything else—the SDK, your application logic, streaming handling—stays identical.

Four: What APIBox Provides

APIBox is an OpenAI-compatible API gateway that routes your requests to multiple underlying models:

Model	Access via APIBox
GPT-5, GPT-4o	`gpt-5`, `gpt-4o`
Claude Sonnet 4.6, Opus 4.6	`claude-sonnet-4-6`, `claude-opus-4-6`
Gemini 2.5 Pro, Flash	`gemini-2.5-pro`, `gemini-flash`
DeepSeek V3, R1	`deepseek-v3`, `deepseek-r1`

One API key. One endpoint. All models.

Endpoint: https://api.apibox.cc/v1

No code changes needed beyond updating base_url and api_key.

Five: Supported Tools and Frameworks

Because APIBox is OpenAI-compatible, it works out of the box with:

Development frameworks:

OpenAI Python SDK, Node.js SDK
LangChain, LlamaIndex
Vercel AI SDK

AI applications:

Cursor, Windsurf, Continue.dev
Dify, Flowise, n8n
LobeChat, Open WebUI, NextChat

Direct HTTP:

curl https://api.apibox.cc/v1/chat/completions \
  -H "Authorization: Bearer YOUR_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "claude-sonnet-4-6",
    "messages": [{"role": "user", "content": "Hello"}]
  }'

Six: Common Misconceptions

“OpenAI-compatible” doesn’t mean identical behavior. Different models respond differently. Claude is more verbose than GPT by default; DeepSeek has different token counting. Compatible interface, different output characteristics.

Not all features are supported everywhere. Vision, function calling, embeddings, and image generation support varies by model. Always check which capabilities each model supports before building on top of them.

The model name matters. You can’t use OpenAI model names like gpt-4o to call Claude. You must use the model name as defined by the provider you’re calling.

Seven: When to Use an OpenAI-Compatible Gateway

A gateway like APIBox makes sense when:

You want to switch models without rewriting your integration
You need access to models blocked in your region (OpenAI, Anthropic, Google)
You want a single billing relationship instead of multiple provider accounts
You want lower costs—APIBox pricing is approximately 1/7 of official rates

It’s not the right choice if you need features that require direct provider APIs, such as Anthropic’s Batch API or OpenAI’s Assistants API (stateful threads).

Eight: Summary

OpenAI-compatible API is the common language of the AI industry. If your code or tool works with OpenAI, it works with any compatible provider—including APIBox.

Change base_url to https://api.apibox.cc/v1, get your API key, and you have access to GPT-5, Claude, Gemini, and DeepSeek through one unified interface at a fraction of the official cost.

Get started with APIBox →