What is an OpenAI-compatible API? It is an API that accepts the same or a very similar request format as OpenAI's API, usually through a /v1 base URL. You can use an OpenAI-style SDK or client while choosing a different provider, hosted model, or local server.

OpenAI-compatible APIs are useful when you want to keep one application workflow while changing the model behind it. The provider still controls the model, pricing, authentication, and availability. Compatibility describes the interface, not identical behavior or identical features.

What "OpenAI-compatible" means

The OpenAI API popularized a request shape that many tools now support. A client typically sends a bearer token to an endpoint such as /v1/chat/completions, includes a model ID, and passes a list of messages:

{
  "model": "your-model-id",
  "messages": [
    { "role": "user", "content": "Explain this error" }
  ]
}

An OpenAI-compatible provider usually keeps the same high-level concepts:

A base URL, often ending in /v1
An API key sent as a bearer token
Chat messages with system, user, and assistant roles
A model ID selected in the request
A JSON response containing the generated message

Compatibility is not a guarantee that every OpenAI feature works. Streaming, tool calls, vision inputs, structured outputs, embeddings, and reasoning options may differ by provider. Check the provider's documentation for the supported subset.

Who offers an OpenAI-compatible API?

OpenAI-compatible endpoints are available from several kinds of providers:

OpenAI: The reference API format, with model-specific features and pricing.
OpenRouter: One endpoint and key for models from many providers. See the OpenRouter API key setup guide.
xAI: Grok's API supports OpenAI-style clients. See the xAI API key setup guide.
Hosted model providers: Many inference platforms expose a familiar /v1 interface for their hosted models.
Local model servers: Tools such as Ollama, LM Studio, and vLLM can expose OpenAI-style endpoints when configured to do so.

For a cloud provider with its own native API, "compatible" may only cover chat completions. A native SDK can still be required for provider-specific features.

OpenAI API vs OpenAI-compatible API

Detail	OpenAI API	OpenAI-compatible API
Model provider	OpenAI	OpenAI, a third party, or a local server
Base URL	OpenAI's endpoint	Provider-specific endpoint, often `/v1`
API key	Issued by OpenAI Platform	Issued by the selected provider
Request format	OpenAI's native format	Similar format, with provider-specific limits
Pricing and limits	OpenAI's published terms	The selected provider's terms
Feature support	Depends on the OpenAI model	Must be checked in the provider documentation

The API format does not make a third-party model an OpenAI model. It only lets compatible clients talk to it using familiar configuration.

Use an OpenAI-compatible endpoint in an SDK

Most OpenAI-style clients let you override the base URL. For example, a JavaScript client can be configured like this:

import OpenAI from "openai";

const client = new OpenAI({
  apiKey: process.env.PROVIDER_API_KEY,
  baseURL: "https://provider.example/v1",
});

const response = await client.chat.completions.create({
  model: "provider-model-id",
  messages: [{ role: "user", content: "Hello" }],
});

The same idea applies to command-line clients and many desktop applications. Replace the base URL, API key, and model ID with values from the provider you selected. Do not assume that gpt-4o or another OpenAI model name is valid on a different endpoint.

Connect an OpenAI-compatible API to MultimodelChat

MultimodelChat lets you add a compatible endpoint alongside native OpenAI, Anthropic, Google, xAI, and OpenRouter connections. Follow the OpenAI-compatible API setup guide, then:

Copy the provider's base URL from its API documentation. Include /v1 when the provider requires it, but do not add it twice.
Create or copy an API key from the provider's credentials page.
Open MultimodelChat settings and choose OpenAI-Compatible.
Paste the base URL and API key, then save the provider.
Select a model using the exact model ID returned by the endpoint.

You can also connect native providers directly with the OpenAI setup guide or use OpenRouter for access to many models with one key.

Common OpenAI-compatible API errors

404 Not Found

The base URL is usually missing a required path such as /v1, or it includes /chat/completions when the client expects only the base URL. Compare the configured URL with the provider's example request.

401 Unauthorized

The key may be invalid, revoked, expired, or sent to the wrong provider. Create a new key and confirm that the endpoint expects a bearer token.

Model not found

Compatible endpoints do not share one global model catalog. Use the exact model ID exposed by the provider, including any organization or version prefix.

Connection refused

This is common with local servers. Confirm that the server is running, the port is reachable, and the endpoint is accessible from the device making the request. A cloud-hosted application may need a secure public URL or tunnel instead of localhost.

FAQ

Is an OpenAI-compatible API the same as OpenAI?

No. It uses a similar request format, but the provider, model, billing, data handling, and supported features can be different.

Do I need an OpenAI API key for an OpenAI-compatible endpoint?

Usually not. Use the API key issued by the provider that owns the endpoint. An OpenAI key should only be sent to an OpenAI endpoint or a service that explicitly documents how it handles that key.

Can I use a local model with an OpenAI-compatible client?

Yes, if the local server exposes an OpenAI-style endpoint and the application can reach it. For a cloud application, localhost points to the cloud server, not your computer, so you may need a secure tunnel or public deployment.

Which base URL should I enter?

Enter the provider's documented API base URL, usually the host plus /v1. Do not paste the full /chat/completions path unless the client specifically asks for a complete endpoint URL.

Can I use OpenAI-compatible providers in a multi-model workflow?

Yes. Add the endpoint alongside your native provider keys and switch models in the same workspace. See the guide to using multiple AI models in one conversation.

Need a provider key first? Browse the AI API key setup guides, or start a free MultimodelChat trial and connect your endpoint.

What Is an OpenAI-Compatible API? (And How to Use Any Model with It)