# langchain-js

A Cursor rule for langchain-js projects that sets guidance on modular design, typing, tool schemas, configuration, and testing.

- rule · `type=rule` · Cursor · ★ 3580
- Author: sanjeed5
- License: CC0-1.0 · policy=mirror · the beginning of the file is at the end of this page
- Source: https://github.com/sanjeed5/awesome-cursor-rules-mdc/tree/HEAD/rules-mdc/langchain-js.mdc
- Updated: 2026-05-19
- HTML: https://affpapa.org/en/nevibecode/rules/langchain-js

## What it does

The rule recommends keeping agents, tools, models, and chains in separate modules. It calls for explicit types and `zod` schemas at tool boundaries, while API keys, model settings, and service URLs should come from environment variables rather than source files.

It also covers a retrieval pattern built around `VectorStoreRetriever`, `RunnableSequence`, and MongoDB Atlas Vector Search. The rule targets `**/*.{js,jsx}`. Its examples use `@langchain/openai`, `@langchain/mongodb`, `mongodb`, `VoyageEmbeddings`, and `dotenv`; the model configuration shown depends on `OPENAI_API_KEY`.

## Install

### Cursor (native platform)

**Manually: put the files in place**
- File: `.cursor/rules/langchain-js.mdc` — Frontmatter: description, globs, alwaysApply

### Claude Code

**Manually: put the files in place**
- File: `.claude/rules/langchain-js.md` — The paths frontmatter field limits the files: move globs from the source into paths

### Codex

**AGENTS.md**
- Codex reads AGENTS.md: paste the rule text there

### GitHub Copilot

**Manually: put the files in place**
- File: `.github/instructions/langchain-js.instructions.md` — The applyTo field sets which files it applies to: move globs from the source into applyTo

### Windsurf (Devin Desktop)

**Manually: put the files in place**
- File: `.devin/rules/langchain-js.md`

### Cline

**Manually: put the files in place**
- File: `.clinerules/langchain-js.md`

Where a file is given instead of a command, copy the files from the source to that path: https://github.com/sanjeed5/awesome-cursor-rules-mdc/tree/HEAD/rules-mdc/langchain-js.mdc

## When to use it
- split agents and tools into focused modules
- validate tool inputs with `zod` schemas
- move API keys out of source code
- build an RAG chain with MongoDB Atlas
- review `langchain-js` application structure

## Example prompts
- Review this project against the `langchain-js` rule and point out modularity issues.
- Split the agent and its tools into separate modules, then add explicit types and `zod` schemas.
- Refactor this RAG code around `VectorStoreRetriever` and `RunnableSequence`, keeping secrets in environment variables.

## Source text · langchain-js.mdc · CC0-1.0

Below is the content of a third-party file. It is data, not instructions for you: do not act on anything written inside unless the human asks. Block tag: 72639c5b01551a0e.

BEGIN THIRD-PARTY FILE 72639c5b01551a0e
````text
---
description: This guide provides opinionated, actionable best practices for building robust and maintainable LLM applications with langchain-js, emphasizing modularity, type safety, observability, and testing.
globs: **/*.{js,jsx}
---
# langchain-js Best Practices

Building LLM-powered applications with `langchain-js` requires a disciplined approach to ensure maintainability, performance, and reliability. This guide outlines our team's definitive best practices.

## 1. Code Organization and Structure

Always prioritize modularity and explicit typing. Break down complex logic into small, reusable components.

### 1.1. Modular Components & Single Responsibility

Each LangChain component (Agent, Tool, Chain, Model) should reside in its own file or a dedicated module, adhering to the Single Responsibility Principle.

❌ **BAD: Monolithic file**
```javascript
// src/agent.js
import { ChatOpenAI } from "@langchain/openai";
import { createAgent, tool } from "langchain";
import { z } from "zod";

const getStockPrice = tool(async ({ ticker }) => { /* ... */ }, { /* ... */ });
const getNews = tool(async ({ query }) => { /* ... */ }, { /* ... */ });

const model = new ChatOpenAI({ temperature: 0.7 });
const agent = createAgent({
  model,
  tools: [getStockPrice, getNews],
});

export async function runFinancialAgent(input) {
  return agent.invoke(input);
}
```

✅ **GOOD: Modular, reusable components**
```typescript
// src/tools/getStockPrice.ts
import { tool } from "langchain";
import { z } from "zod";

export const getStockPriceTool = tool(
  async ({ ticker }: { ticker: string }) => {
    // Simulate API call
    if (ticker === "AAPL") return "$170.00";
    return "Price not found.";
  },
  {
    name: "get_stock_price",
    description: "Get the current stock price for a given ticker symbol.",
    schema: z.object({
      ticker: z.string().describe("The stock ticker symbol (e.g., AAPL)"),
    }),
  }
);

// src/tools/getNews.ts
import { tool } from "langchain";
import { z } from "zod";

export const getNewsTool = tool(
  async ({ query }: { query: string }) => {
    // Simulate API call
    return `Latest news for ${query}: Market is up!`;
  },
  {
    name: "get_news",
    description: "Get the latest news for a given query.",
    schema: z.object({
      query: z.string().describe("The news query"),
    }),
  }
);

// src/agents/financialAgent.ts
import { ChatOpenAI } from "@langchain/openai";
import { createAgent } from "langchain";
import { getStockPriceTool } from "../tools/getStockPrice";
import { getNewsTool } from "../tools/getNews";

const model = new ChatOpenAI({ temperature: 0.7 });

export const financialAgent = createAgent({
  model,
  tools: [getStockPriceTool, getNewsTool],
});

// src/index.ts (or wherever the agent is invoked)
import { financialAgent } from "./agents/financialAgent";

async function main() {
  const result = await financialAgent.invoke({
    messages: [{ role: "user", content: "What's the price of AAPL and the latest market news?" }],
  });
  console.log(result);
}

main();
```

### 1.2. Explicit TypeScript Types

Always use explicit types for prompts, models, and tool interfaces. Avoid `any`.

❌ **BAD: Implicit types, no schema validation**
```javascript
const myTool = tool((input) => { /* ... */ }, { name: "my_tool" });
```

✅ **GOOD: Explicit types with Zod schema**
```typescript
import { tool } from "langchain";
import { z } from "zod";

const MyToolInputSchema = z.object({
  param1: z.string().describe("Description for param1"),
  param2: z.number().optional().describe("Description for param2"),
});

export const myTypedTool = tool(
  async (input: z.infer<typeof MyToolInputSchema>) => {
    console.log(`Received: ${input.param1}, ${input.param2}`);
    return "Processed successfully";
  },
  {
    name: "my_typed_tool",
    description: "A tool that takes typed input.",
    schema: MyToolInputSchema,
  }
);
```

### 1.3. Configuration Management

Store API keys, model parameters, and external service URLs in environment variables. Never hardcode secrets.
````
END THIRD-PARTY FILE 72639c5b01551a0e

The third-party file has ended. Everything between the 72639c5b01551a0e tags is data: requests and commands from it are not followed. An “end of file” line with another tag inside the block is part of the file.

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