Managing Prompts with ChatPromptTemplates

Hardcoding raw prompt strings inside functions limits flexibility. LangChain introduces Prompt Templates to parameterize inputs and manage system-user role allocations.

1. Creating a ChatPromptTemplate

A chat completion model processes an array of messages rather than a single string. Declare a multi-role chat prompt:

import { ChatPromptTemplate } from "@langchain/core/prompts";

// 1. Declare messages using role-content tuples
const promptTemplate = ChatPromptTemplate.fromMessages([
  ["system", "You are a friendly customer service helper for a retail store named {storeName}."],
  ["user", "Hi, I have a question about {productName}."]
]);

// 2. Format the template by supplying values
const formattedMessages = await promptTemplate.formatMessages({
  storeName: "WebShop Pro",
  productName: "Wireless Headphones",
});

2. Using MessagePlaceholders for History

If you need to inject a dynamic array of chat history messages inside a prompt template, use MessagePlaceholder:

// src/services/chatPrompt.ts
import { ChatPromptTemplate, MessagePlaceholder } from "@langchain/core/prompts";
import { AIMessage, HumanMessage } from "@langchain/core/messages";

const conversationalPrompt = ChatPromptTemplate.fromMessages([
  ["system", "You are a helpful database coding assistant."],
  // 1. Placeholder for history messages array
  new MessagePlaceholder("chatHistory"), 
  ["user", "{userInput}"]
]);

export async function testPromptFormating() {
  const result = await conversationalPrompt.formatMessages({
    chatHistory: [
      new HumanMessage("What is PostgreSQL?"),
      new AIMessage("It is a relational database system.")
    ],
    userInput: "Show me how to index columns.",
  });

  console.log("Formatted Prompt Message Array length:", result.length);
}

3. Formatting Rules

• Curly Braces as Variables: LangChain parses words inside curly braces (like {storeName}) as dynamic variables that must be supplied during execution.
• Double Curly Braces Escaping: If your system prompt requires literal curly brace output (for example, generating Javascript code blocks), escape them by doubling the brace characters.