OpenAI API context_length_exceeded: Maximum Context Length Error
Quick answer
A 400 context_length_exceeded means prompt plus output tokens exceed the model window. Trim history, summarize, or move to a longer-context model.
Most likely fix: Count tokens before sending
How to fix it
Try these in order — the most common fix is first.
1. Count tokens before sending
- Use tiktoken or the token counting endpoint
Did this fix it?2. Trim or summarize history
- Keep system prompt plus the last N turns
Did this fix it?3. Chunk or retrieve
- Split documents and send relevant chunks
Did this fix it?4. Use a longer-context model
- Pick a model with a larger window; requests above 272K input tokens cost more on some models
Did this fix it?
Symptoms
400 "This model's maximum context length is N tokens".
Why it happens
- Long chat history re-sent each call
- Large documents in the prompt
- Large max_tokens counted toward the window
This guide also covers
- openai context_length_exceeded
- openai maximum context length error
- openai too many tokens
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