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*   How to create a custom LLM class

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How to create a custom LLM class
================================

Prerequisites

This guide assumes familiarity with the following concepts:

*   [LLMs](/v0.2/docs/concepts/#llms)

This notebook goes over how to create a custom LLM wrapper, in case you want to use your own LLM or a different wrapper than one that is directly supported in LangChain.

There are a few required things that a custom LLM needs to implement after extending the [`LLM` class](https://v02.api.js.langchain.com/classes/langchain_core_language_models_llms.LLM.html):

*   A `_call` method that takes in a string and call options (which includes things like `stop` sequences), and returns a string.
*   A `_llmType` method that returns a string. Used for logging purposes only.

You can also implement the following optional method:

*   A `_streamResponseChunks` method that returns an `AsyncIterator` and yields [`GenerationChunks`](https://v02.api.js.langchain.com/classes/langchain_core_outputs.GenerationChunk.html). This allows the LLM to support streaming outputs.

Let’s implement a very simple custom LLM that just echoes back the first `n` characters of the input.

    import { LLM, type BaseLLMParams } from "@langchain/core/language_models/llms";import type { CallbackManagerForLLMRun } from "langchain/callbacks";import { GenerationChunk } from "langchain/schema";export interface CustomLLMInput extends BaseLLMParams {  n: number;}export class CustomLLM extends LLM {  n: number;  constructor(fields: CustomLLMInput) {    super(fields);    this.n = fields.n;  }  _llmType() {    return "custom";  }  async _call(    prompt: string,    options: this["ParsedCallOptions"],    runManager: CallbackManagerForLLMRun  ): Promise<string> {    // Pass `runManager?.getChild()` when invoking internal runnables to enable tracing    // await subRunnable.invoke(params, runManager?.getChild());    return prompt.slice(0, this.n);  }  async *_streamResponseChunks(    prompt: string,    options: this["ParsedCallOptions"],    runManager?: CallbackManagerForLLMRun  ): AsyncGenerator<GenerationChunk> {    // Pass `runManager?.getChild()` when invoking internal runnables to enable tracing    // await subRunnable.invoke(params, runManager?.getChild());    for (const letter of prompt.slice(0, this.n)) {      yield new GenerationChunk({        text: letter,      });      // Trigger the appropriate callback      await runManager?.handleLLMNewToken(letter);    }  }}

We can now use this as any other LLM:

    const llm = new CustomLLM({ n: 4 });await llm.invoke("I am an LLM");

    I am

And support streaming:

    const stream = await llm.stream("I am an LLM");for await (const chunk of stream) {  console.log(chunk);}

    Iam

Richer outputs[​](#richer-outputs "Direct link to Richer outputs")
------------------------------------------------------------------

If you want to take advantage of LangChain's callback system for functionality like token tracking, you can extend the [`BaseLLM`](https://v02.api.js.langchain.com/classes/langchain_core_language_models_llms.BaseLLM.html) class and implement the lower level `_generate` method. Rather than taking a single string as input and a single string output, it can take multiple input strings and map each to multiple string outputs. Additionally, it returns a `Generation` output with fields for additional metadata rather than just a string.

    import { CallbackManagerForLLMRun } from "@langchain/core/callbacks/manager";import { LLMResult } from "@langchain/core/outputs";import {  BaseLLM,  BaseLLMCallOptions,  BaseLLMParams,} from "@langchain/core/language_models/llms";export interface AdvancedCustomLLMCallOptions extends BaseLLMCallOptions {}export interface AdvancedCustomLLMParams extends BaseLLMParams {  n: number;}export class AdvancedCustomLLM extends BaseLLM<AdvancedCustomLLMCallOptions> {  n: number;  constructor(fields: AdvancedCustomLLMParams) {    super(fields);    this.n = fields.n;  }  _llmType() {    return "advanced_custom_llm";  }  async _generate(    inputs: string[],    options: this["ParsedCallOptions"],    runManager?: CallbackManagerForLLMRun  ): Promise<LLMResult> {    const outputs = inputs.map((input) => input.slice(0, this.n));    // Pass `runManager?.getChild()` when invoking internal runnables to enable tracing    // await subRunnable.invoke(params, runManager?.getChild());    // One input could generate multiple outputs.    const generations = outputs.map((output) => [      {        text: output,        // Optional additional metadata for the generation        generationInfo: { outputCount: 1 },      },    ]);    const tokenUsage = {      usedTokens: this.n,    };    return {      generations,      llmOutput: { tokenUsage },    };  }}

This will pass the additional returned information in callback events and in the \`streamEvents method:

    const llm = new AdvancedCustomLLM({ n: 4 });const eventStream = await llm.streamEvents("I am an LLM", {  version: "v1",});for await (const event of eventStream) {  if (event.event === "on_llm_end") {    console.log(JSON.stringify(event, null, 2));  }}

    {  "event": "on_llm_end",  "name": "AdvancedCustomLLM",  "run_id": "a883a705-c651-4236-8095-cb515e2d4885",  "tags": [],  "metadata": {},  "data": {    "output": {      "generations": [        [          {            "text": "I am",            "generationInfo": {              "outputCount": 1            }          }        ]      ],      "llmOutput": {        "tokenUsage": {          "usedTokens": 4        }      }    }  }}

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