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网上找到的，包括官方给的代码都是针对较低版本的，对目前最新的版本不适用。

首先说明版本号，package.json中相关依赖版本如下：

"dependencies": {
    "@langchain/core": "^0.3.39",
    "@langchain/openai": "^0.4.3",
    "langchain": "^0.3.15"
},
"resolutions": {
    "@langchain/core": "0.3.39"
}

构造Openrouter Model的核心代码如下：

const model = new ChatOpenAI({
  model: "gpt-4o",
  // modelName: "openai/gpt-4o",
  temperature: 0.8,
  streaming: true,
  openAIApiKey: OPENROUTER_API_KEY,
  configuration: {
    baseURL: "https://openrouter.ai/api/v1",
  },
});

踩坑注意：model名称不要使用openrouter文档中提供的openai/gpt-4o的类似写法，虽然结果可以返回，但token计算会出现错误，因为ChatOpenAI以及OpenAI的token计算方法使用了硬编码：

// @langchain/core/dist/utils/tiktoken.js:19:24
export async function encodingForModel(model) {
    return getEncoding(getEncodingNameForModel(model));
}

// js-tiktoken/dist/chunk-Z5MDQTGX.js:247:13
function getEncodingNameForModel(model) {
  switch (model) {
    case "gpt2": {
      return "gpt2";
    }
    case "code-cushman-001":
    case "code-cushman-002":
    case "code-davinci-001":
    case "code-davinci-002":
    case "cushman-codex":
    case "davinci-codex":
    case "davinci-002":
    case "text-davinci-002":
    case "text-davinci-003": {
      return "p50k_base";
    }
    case "code-davinci-edit-001":
    case "text-davinci-edit-001": {
      return "p50k_edit";
    }
    case "ada":
    case "babbage":
    case "babbage-002":
    case "code-search-ada-code-001":
    case "code-search-babbage-code-001":
    case "curie":
    case "davinci":
    case "text-ada-001":
    case "text-babbage-001":
    case "text-curie-001":
    case "text-davinci-001":
    case "text-search-ada-doc-001":
    case "text-search-babbage-doc-001":
    case "text-search-curie-doc-001":
    case "text-search-davinci-doc-001":
    case "text-similarity-ada-001":
    case "text-similarity-babbage-001":
    case "text-similarity-curie-001":
    case "text-similarity-davinci-001": {
      return "r50k_base";
    }
    case "gpt-3.5-turbo-instruct-0914":
    case "gpt-3.5-turbo-instruct":
    case "gpt-3.5-turbo-16k-0613":
    case "gpt-3.5-turbo-16k":
    case "gpt-3.5-turbo-0613":
    case "gpt-3.5-turbo-0301":
    case "gpt-3.5-turbo":
    case "gpt-4-32k-0613":
    case "gpt-4-32k-0314":
    case "gpt-4-32k":
    case "gpt-4-0613":
    case "gpt-4-0314":
    case "gpt-4":
    case "gpt-3.5-turbo-1106":
    case "gpt-35-turbo":
    case "gpt-4-1106-preview":
    case "gpt-4-vision-preview":
    case "gpt-3.5-turbo-0125":
    case "gpt-4-turbo":
    case "gpt-4-turbo-2024-04-09":
    case "gpt-4-turbo-preview":
    case "gpt-4-0125-preview":
    case "text-embedding-ada-002":
    case "text-embedding-3-small":
    case "text-embedding-3-large": {
      return "cl100k_base";
    }
    case "gpt-4o":
    case "gpt-4o-2024-05-13":
    case "gpt-4o-2024-08-06":
    case "gpt-4o-2024-11-20":
    case "gpt-4o-mini-2024-07-18":
    case "gpt-4o-mini":
    case "o1":
    case "o1-2024-12-17":
    case "o1-mini":
    case "o1-preview":
    case "o1-preview-2024-09-12":
    case "o1-mini-2024-09-12":
    case "o3-mini":
    case "o3-mini-2025-01-31":
    case "chatgpt-4o-latest":
    case "gpt-4o-realtime":
    case "gpt-4o-realtime-preview-2024-10-01": {
      return "o200k_base";
    }
    default:
      throw new Error("Unknown model");
  }
}

所以会同步抛出Failed to calculate number of tokens, falling back to approximate count Error: Unknown model异常。这理论上会影响llmOutput的estimatedTokenUsage相关数值计算准确性，当然不需要可以不用，也可以使用其他方法抑制这个异常输出。

当然最好的办法是写一个chat_model扩展，方便起见可以extend现成的model并重写对应token计算方法即可。