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Thread message limit in OpenAI Assistants API

Q: Thread message limit in OpenAI Assistants API

The OpenAI Assistants API supports up to 100 messages per thread to maintain context. Exceeding this limit requires truncating or summarizing earlier messages to keep the conversation within the allowed message window.

Quick answer

The OpenAI Assistants API supports up to 100 messages per thread to maintain context. Exceeding this limit requires truncating or summarizing earlier messages to keep the conversation within the allowed message window.

PREREQUISITES

Python 3.8+
OpenAI API key (free tier works)
pip install openai>=1.0

Setup

Install the official openai Python SDK version 1.0 or higher and set your API key as an environment variable.

bash

pip install openai>=1.0

Step by step

This example demonstrates creating a conversation thread with the OpenAI Assistants API and managing the message limit by truncating older messages.

python

import os
from openai import OpenAI

client = OpenAI(api_key=os.environ["OPENAI_API_KEY"])

# Simulate a conversation thread with up to 100 messages
messages = []

# Add 105 messages to simulate exceeding the limit
for i in range(105):
    messages.append({"role": "user", "content": f"Message {i + 1}"})

# Truncate to last 100 messages to respect the thread limit
if len(messages) > 100:
    messages = messages[-100:]

response = client.chat.completions.create(
    model="gpt-4o",
    messages=messages
)

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

output

<response text from model>

Common variations

Use gpt-4o-mini or other models with the same message limit.
Implement message summarization to reduce context size instead of truncation.
Use async calls with asyncio and await for concurrency.

Troubleshooting

If you receive errors about context length or message limits, ensure your thread does not exceed 100 messages. Remove or summarize older messages before sending the request.

✅

Key Takeaways

OpenAI Assistants API supports a maximum of 100 messages per thread to maintain context.
Always truncate or summarize older messages to stay within the thread message limit.
Use the official openai SDK v1+ with environment-based API keys for integration.
Consider async calls or smaller models for efficient usage within message limits.

Verified 2026-04 · gpt-4o, gpt-4o-mini

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