Auto-fixing parser

This output parser wraps another output parser, and in the event that the first one fails it calls out to another LLM to fix any errors.

But we can do other things besides throw errors. Specifically, we can pass the misformatted output, along with the formatted instructions, to the model and ask it to fix it.

For this example, we'll use the structured output parser. Here's what happens if we pass it a result that does not comply with the schema:

tip

See this section for general instructions on installing integration packages.

npm

npm install @langchain/openai

Yarn

yarn add @langchain/openai

pnpm

pnpm add @langchain/openai

import { z } from "zod";
import { ChatOpenAI } from "@langchain/openai";
import {
  StructuredOutputParser,
  OutputFixingParser,
} from "langchain/output_parsers";

export const run = async () => {
  const parser = StructuredOutputParser.fromZodSchema(
    z.object({
      answer: z.string().describe("answer to the user's question"),
      sources: z
        .array(z.string())
        .describe("sources used to answer the question, should be websites."),
    })
  );
  /** This is a bad output because sources is a string, not a list */
  const badOutput = `\`\`\`json
  {
    "answer": "foo",
    "sources": "foo.com"
  }
  \`\`\``;
  try {
    await parser.parse(badOutput);
  } catch (e) {
    console.log("Failed to parse bad output: ", e);
    /*
    Failed to parse bad output:  OutputParserException [Error]: Failed to parse. Text: ```json
      {
        "answer": "foo",
        "sources": "foo.com"
      }
      ```. Error: [
      {
        "code": "invalid_type",
        "expected": "array",
        "received": "string",
        "path": [
          "sources"
        ],
        "message": "Expected array, received string"
      }
    ]
    at StructuredOutputParser.parse (/Users/ankushgola/Code/langchainjs/langchain/src/output_parsers/structured.ts:71:13)
    at run (/Users/ankushgola/Code/langchainjs/examples/src/prompts/fix_parser.ts:25:18)
    at <anonymous> (/Users/ankushgola/Code/langchainjs/examples/src/index.ts:33:22)
   */
  }
  const fixParser = OutputFixingParser.fromLLM(
    new ChatOpenAI({ temperature: 0 }),
    parser
  );
  const output = await fixParser.parse(badOutput);
  console.log("Fixed output: ", output);
  // Fixed output:  { answer: 'foo', sources: [ 'foo.com' ] }
};

API Reference:

• ChatOpenAI from @langchain/openai
• StructuredOutputParser from langchain/output_parsers
• OutputFixingParser from langchain/output_parsers