OpenAI API Guide 2025: Setup, Models & Best Practices

# curl

curl https://api.openai.com/v1/chat/completions \
  -H "Authorization: Bearer $OPENAI_API_KEY" \
  -H "Content-Type: application/json" \
  -d '{
    "model": "gpt-4o",
    "messages": [
      {"role": "system", "content": "You are a helpful assistant."},
      {"role": "user", "content": "Explain REST APIs in 2 sentences."}
    ]
  }'

# Python (openai SDK)

pip install openai

from openai import OpenAI

client = OpenAI()  # uses OPENAI_API_KEY env var

response = client.chat.completions.create(
    model="gpt-4o",
    messages=[
        {"role": "system", "content": "You are a helpful assistant."},
        {"role": "user", "content": "Explain REST APIs in 2 sentences."}
    ]
)

print(response.choices[0].message.content)
print(f"Tokens used: {response.usage.total_tokens}")

stream = client.chat.completions.create(
    model="gpt-4o",
    messages=[{"role": "user", "content": "Tell me a joke."}],
    stream=True,
)

for chunk in stream:
    delta = chunk.choices[0].delta.content
    if delta:
        print(delta, end="", flush=True)

tools = [{
    "type": "function",
    "function": {
        "name": "get_weather",
        "description": "Get the current weather for a city",
        "parameters": {
            "type": "object",
            "properties": {
                "city": {"type": "string", "description": "City name"}
            },
            "required": ["city"]
        }
    }
}]

response = client.chat.completions.create(
    model="gpt-4o",
    messages=[{"role": "user", "content": "What's the weather in Paris?"}],
    tools=tools,
    tool_choice="auto"  # model decides when to call
)

pip install tiktoken

import tiktoken

enc = tiktoken.encoding_for_model("gpt-4o")
tokens = enc.encode("Your prompt text here...")
print(f"Token count: {len(tokens)}")  # GPT-4o limit: 128,000