What is the OpenAI Assistants API and how does it differ from the Chat Completions API?

The Assistants API (part of OpenAI's platform) provides a higher-level abstraction for building AI agents with persistent conversation threads, built-in tool use, and file handling — without managing state manually. Key concepts: an Assistant holds configuration (model, system prompt, tools); a Thread maintains conversation history automatically; a Run is an invocation of the assistant on a thread.

Unlike Chat Completions (stateless — you manage the message list), the Assistants API stores threads server-side. The built-in tools include code_interpreter (executes Python in a sandboxed environment), file_search (built-in RAG over uploaded files), and function calling. This makes it well-suited for multi-turn agentic workflows where you want OpenAI to manage state and tool execution loops.

from openai import OpenAI
import time

client = OpenAI()

# ── 1. Create an Assistant (once; reuse by ID)
assistant = client.beta.assistants.create(
    name='Data Analyst',
    instructions='You are a data analyst. Write and run Python code to answer questions.',
    model='gpt-4o',
    tools=[{'type': 'code_interpreter'}],
)

# ── 2. Create a Thread (conversation session)
thread = client.beta.threads.create()

# ── 3. Add a user message to the thread
client.beta.threads.messages.create(
    thread_id=thread.id,
    role='user',
    content='Calculate the mean and standard deviation of [4, 8, 15, 16, 23, 42]',
)

# ── 4. Run the assistant
run = client.beta.threads.runs.create(
    thread_id=thread.id,
    assistant_id=assistant.id,
)

# ── 5. Poll for completion
while run.status not in ('completed', 'failed'):
    time.sleep(1)
    run = client.beta.threads.runs.retrieve(thread_id=thread.id, run_id=run.id)

# ── 6. Retrieve the latest message
messages = client.beta.threads.messages.list(thread_id=thread.id)
print(messages.data[0].content[0].text.value)
# 'Mean: 18.0, Standard deviation: 13.29...'