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Before integrating a chatbot into your application, it’s essential to understand the difference between word completion and chat completion. Although both use the same underlying GPT-3.5-Turbo model, their usage patterns—and the way they manage context—are very different.

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What Is Word Completion?

Word completion is a stateless API call. You send a single prompt, and the model returns a continuation without retaining any memory of previous interactions. Key characteristics:
  • Single-shot responses
  • No conversation history
  • Simpler, lower token usage
The image compares "Word Completion" and "Chat Completion," highlighting that word completion doesn't need memory, while chat completion expects context from previous conversations.

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What Is Chat Completion?

Chat completion is stateful. You maintain a record of all messages in a messages array, allowing the model to build on past user and assistant exchanges.

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How Chat Completion Works

  1. You create an ordered messages list.
  2. Each entry has a role: system, user, or assistant.
  3. You submit the full history each time you call the API.
  4. The model returns a response that accounts for all previous context.

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Roles in the messages Array

RoleDescriptionExample
systemSets global instructions or persona“You are a physics professor.”
userRepresents the human’s input at each turn“Explain quantum mechanics in simple terms.”
assistantModel-generated responses that continue the conversation context“Quantum mechanics is the study of matter at very small scales…”
The image illustrates the difference between word completion and chat completion, showing a user interacting with a system and an LLM (Large Language Model) using chat history as a message parameter.
  • Use word completion for simple text continuations, code generation, or single-turn tasks.
  • Use chat completion for multi-turn conversations, contextual assistants, and applications that require stateful interaction.

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Sample Chat Completion Request

POST https://api.openai.com/v1/chat/completions
Content-Type: application/json
Authorization: Bearer YOUR_API_KEY

{
  "model": "gpt-3.5-turbo",
  "messages": [
    { "role": "system", "content": "You are a helpful assistant." },
    { "role": "user",   "content": "Explain photosynthesis." }
  ]
}

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Comparison at a Glance

FeatureWord CompletionChat Completion
Context HandlingStatelessStateful via messages array
Best forSingle-turn promptsMulti-turn conversations
Token EfficiencyFewer overhead tokensMore overhead tokens for history
Recommended ModelGPT-3.5-TurboGPT-3.5-Turbo

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Further Reading and References

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