We designed the Cerebras API to be mostly compatible with OpenAI’s client libraries, making it simple to configure your existing applications to run on Cerebras and take advantage of our inference capabilities. We also offer dedicated Cerebras Python and Cerebras TypeScript SDKs.

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Configuring OpenAI to Use Cerebras API

To start using Cerebras with OpenAI’s client libraries, simply pass your Cerebras API key to the apiKey parameter and change the baseURL to https://api.cerebras.ai/v1: 
import os
import openai

client = openai.OpenAI(
    base_url="https://api.cerebras.ai/v1",
    api_key=os.environ.get("CEREBRAS_API_KEY")
)

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Developer-Level Instructions via System Role

This info is only applicable to the gpt-oss-120b model.
For gpt-oss-120b, our API maps the system role to a developer-level instruction layer in the prompt hierarchy. When you send messages with role: "system", these are elevated above normal user instructions and injected into the model’s internal system prompt. This gives you significant control over the assistant’s tone, style, and behavior while preserving the model’s built-in safety guardrails.

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Key Differences from OpenAI

OpenAI’s API distinguishes between system and developer roles. Our implementation does not expose developer directly. Instead, your system messages act at the developer level, meaning they have stronger influence than in OpenAI’s API. As a result, the same prompt may yield different behavior here compared to OpenAI. This is expected.

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Currently Unsupported OpenAI Features

Note that although Cerebras API is mostly OpenAI compatible, there are a few features we don’t support just yet: Text Completions
The following fields are currently not supported and will result in a 400 error if they are supplied:
  • frequency_penalty
  • logit_bias
  • presence_penalty
  • parallel_tool_calls
  • service_tier
Streaming with JSON Mode
While Cerebras supports a stream parameter, note that JSON mode is not compatible with streaming.