LangChain

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import { ChatOpenAI } from "@langchain/openai";
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import { HumanMessage, SystemMessage } from "@langchain/core/messages";
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const chat = new ChatOpenAI(
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  {
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    model: '<model_name>',
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    temperature: 0.8,
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    streaming: true,
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    apiKey: '${API_KEY_REF}',
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  },
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  {
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    baseURL: 'https://openrouter.ai/api/v1',
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    defaultHeaders: {
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      'HTTP-Referer': '<YOUR_SITE_URL>', // Optional. Site URL for rankings on openrouter.ai.
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      'X-Title': '<YOUR_SITE_NAME>', // Optional. Site title for rankings on openrouter.ai.
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    },
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  },
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);
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// Example usage
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const response = await chat.invoke([
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  new SystemMessage("You are a helpful assistant."),
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  new HumanMessage("Hello, how are you?"),
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]);