Responses API Overview

Comparison of Responses API and Chat Completions API, and use case descriptions

Responses API Overview

The Responses API is OpenAI’s next-generation API format, designed to simplify the development experience for multi-step tasks and tool calling. Crazyrouter fully supports the Responses API.

Responses API vs Chat Completions API

Feature	Responses API	Chat Completions API
Endpoint	/v1/responses	/v1/chat/completions
Message format	input (string or array)	messages (array)
Tool calling	Built-in support, automatic context management	Requires manual message history management
Streaming format	Rich event types	Simple delta format
Thinking mode	reasoning parameter	reasoning_effort parameter
Web search	Built-in web_search tool	Via tools parameter
Compatibility	Newer, supported by some models	Broadly supported by all models

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When to Use Responses API

Suitable for the following scenarios:

• Need built-in tools (web search, file search)
• Multi-step tasks and complex tool calling
• Need rich streaming events
• Using advanced features of newer models like GPT-5

When to Use Chat Completions API

Suitable for the following scenarios:

• Need the broadest model compatibility
• Simple conversations and text generation
• Existing code based on Chat Completions
• Using non-OpenAI models (Claude, Gemini, etc.)

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Quick Comparison Example

Responses API

from openai import OpenAI

client = OpenAI(
    api_key="YOUR_API_KEY",
    base_url="https://crazyrouter.com/v1"
)

response = client.responses.create(
    model="gpt-4o",
    input="Explain what a REST API is"
)

print(response.output_text)

Chat Completions API

from openai import OpenAI

client = OpenAI(
    api_key="YOUR_API_KEY",
    base_url="https://crazyrouter.com/v1"
)

response = client.chat.completions.create(
    model="gpt-4o",
    messages=[
        {"role": "user", "content": "Explain what a REST API is"}
    ]
)

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

Both APIs can be used together in the same project. Crazyrouter provides full support for both formats, and you can choose the appropriate API based on your specific needs.