LangChain.js prompt + model + parser chain tested with a fake chat model: working versions (tested 2026-10-15)

{
  "name": "langchain-core-fake-model-chain",
  "private": true,
  "type": "module",
  "scripts": {
    "test": "node --import tsx --test test/*.test.ts"
  },
  "dependencies": {
    "@langchain/core": "1.2.17",
    "zod": "4.6.7"
  },
  "devDependencies": {
    "tsx": "4.23.15",
    "@types/node": "24.19.2"
  }
}

import { ChatPromptTemplate } from "@langchain/core/prompts";
import { StringOutputParser } from "@langchain/core/output_parsers";
import type { BaseChatModel } from "@langchain/core/language_models/chat_models";
import { z } from "zod";

const prompt = ChatPromptTemplate.fromMessages([
  ["system", "Classify the support ticket. Reply with JSON: {{\"category\": \"billing\"|\"bug\"|\"other\", \"urgent\": boolean}}"],
  ["human", "{ticket}"],
]);
const Result = z.object({ category: z.enum(["billing", "bug", "other"]), urgent: z.boolean() });

export function buildClassifier(model: BaseChatModel) {
  const chain = prompt.pipe(model).pipe(new StringOutputParser());
  return async (ticket: string) => {
    const raw = await chain.invoke({ ticket });
    const json = raw.replace(/^```(json)?\s*|```\s*$/g, "").trim(); // models often wrap JSON in fences
    return Result.parse(JSON.parse(json));
  };
}

export const formatPrompt = (ticket: string) => prompt.formatMessages({ ticket });

import { test } from "node:test";
import assert from "node:assert/strict";
import { FakeListChatModel } from "@langchain/core/utils/testing";
import { buildClassifier, formatPrompt } from "../src/chain.ts";

test("parses plain JSON output", async () => {
  const classify = buildClassifier(new FakeListChatModel({ responses: ['{"category":"billing","urgent":true}'] }));
  assert.deepEqual(await classify("I was charged twice!"), { category: "billing", urgent: true });
});

test("strips markdown fences", async () => {
  const classify = buildClassifier(new FakeListChatModel({ responses: ['```json\n{"category":"bug","urgent":false}\n```'] }));
  assert.deepEqual(await classify("button broken"), { category: "bug", urgent: false });
});

test("rejects out-of-schema output", async () => {
  const classify = buildClassifier(new FakeListChatModel({ responses: ['{"category":"sales","urgent":"yes"}'] }));
  await assert.rejects(classify("hi"));
});

test("prompt escapes literal braces and fills the variable", async () => {
  const msgs = await formatPrompt("hello");
  assert.match(String(msgs[0].content), /"category"/);
  assert.equal(msgs[1].content, "hello");
});