Maintaining Conversation State (Chat History)

We learned earlier that the API is stateless. It has amnesia. If you want to have a back-and-forth conversation, you have to do the heavy lifting.

The messages Array

When you send a payload to Claude, you don't just send a string. You send an array of objects called messages.

Each object in this array represents one turn in the conversation, and it requires a role and content.

• role: 'user' - This is you (or the person using your app).
• role: 'assistant' - This is Claude.

Building the History

If you are building a chat app, you must store the history of the conversation in a variable (or a database) and append to it every time someone speaks.

Here is what a conversation looks like in code:

// Step 1: Initialize the empty array
let chatHistory = [];

// Step 2: The user asks a question
chatHistory.push({ 
  role: 'user', 
  content: 'Hi, my name is Alex and my favorite color is blue.' 
});

// Step 3: Send the history to Claude
const response1 = await anthropic.messages.create({
  model: 'claude-3-haiku-20240307',
  max_tokens: 100,
  messages: chatHistory
});

// Step 4: Claude replies. We MUST save Claude's reply to the history!
chatHistory.push({ 
  role: 'assistant', 
  content: response1.content[0].text 
});

// Step 5: The user asks a follow-up
chatHistory.push({ 
  role: 'user', 
  content: 'What is my name and favorite color?' 
});

// Step 6: Send the ENTIRE history back to Claude again
const response2 = await anthropic.messages.create({
  model: 'claude-3-haiku-20240307',
  max_tokens: 100,
  messages: chatHistory
});

// Claude will correctly answer "Alex" and "blue" because it can read the history.

The Cost of Context

Because the API charges you per token (for both inputs and outputs), maintaining a long chat history gets exponentially more expensive.

If your chatHistory array has 20 messages in it, and you send a 5-word prompt, you aren't just paying for 5 words. You are paying Anthropic to re-read all 20 previous messages.

If you are building a production app, you will eventually need to implement a mechanism to drop old messages from the chatHistory array so it doesn't grow infinitely large.