langchain-workflow-builder

What this skill does

# LangChain Workflow Builder

Build powerful LLM applications with chains, agents, and retrieval-augmented generation.

## Core Workflow

1. **Setup LangChain**: Install and configure
2. **Create chains**: Build processing pipelines
3. **Add memory**: Enable conversation context
4. **Define tools**: Extend agent capabilities
5. **Implement RAG**: Add knowledge retrieval
6. **Deploy**: Production-ready setup

## Installation

```bash
npm install langchain @langchain/openai @langchain/community
```

## Basic Chains

### Simple LLM Chain

```typescript
// chains/simple.ts
import { ChatOpenAI } from '@langchain/openai';
import { ChatPromptTemplate } from '@langchain/core/prompts';
import { StringOutputParser } from '@langchain/core/output_parsers';

const model = new ChatOpenAI({
  modelName: 'gpt-4-turbo-preview',
  temperature: 0.7,
});

const prompt = ChatPromptTemplate.fromMessages([
  ['system', 'You are a helpful assistant that {task}.'],
  ['human', '{input}'],
]);

const chain = prompt.pipe(model).pipe(new StringOutputParser());

// Usage
const result = await chain.invoke({
  task: 'summarizes text concisely',
  input: 'Summarize this article: ...',
});
```

### Sequential Chain

```typescript
// chains/sequential.ts
import { RunnableSequence } from '@langchain/core/runnables';

// Chain 1: Extract key points
const extractChain = ChatPromptTemplate.fromMessages([
  ['system', 'Extract the key points from the following text.'],
  ['human', '{text}'],
]).pipe(model).pipe(new StringOutputParser());

// Chain 2: Summarize key points
const summarizeChain = ChatPromptTemplate.fromMessages([
  ['system', 'Create a brief summary from these key points.'],
  ['human', '{keyPoints}'],
]).pipe(model).pipe(new StringOutputParser());

// Combined chain
const fullChain = RunnableSequence.from([
  {
    keyPoints: extractChain,
    originalText: (input) => input.text,
  },
  {
    summary: summarizeChain,
    keyPoints: (input) => input.keyPoints,
  },
]);

const result = await fullChain.invoke({ text: 'Long article...' });
// { summary: '...', keyPoints: '...' }
```

### Branching Chain

```typescript
// chains/branching.ts
import { RunnableBranch } from '@langchain/core/runnables';

const classifyChain = ChatPromptTemplate.fromMessages([
  ['system', 'Classify the query as: question, complaint, or feedback'],
  ['human', '{query}'],
]).pipe(model).pipe(new StringOutputParser());

const questionChain = ChatPromptTemplate.fromMessages([
  ['system', 'Answer this question helpfully.'],
  ['human', '{query}'],
]).pipe(model).pipe(new StringOutputParser());

const complaintChain = ChatPromptTemplate.fromMessages([
  ['system', 'Respond empathetically to this complaint.'],
  ['human', '{query}'],
]).pipe(model).pipe(new StringOutputParser());

const feedbackChain = ChatPromptTemplate.fromMessages([
  ['system', 'Thank the user for their feedback.'],
  ['human', '{query}'],
]).pipe(model).pipe(new StringOutputParser());

const routingChain = RunnableSequence.from([
  {
    classification: classifyChain,
    query: (input) => input.query,
  },
  RunnableBranch.from([
    [(input) => input.classification.includes('question'), questionChain],
    [(input) => input.classification.includes('complaint'), complaintChain],
    feedbackChain, // Default
  ]),
]);
```

## Memory & Conversation

### Buffer Memory

```typescript
// memory/conversation.ts
import { BufferMemory } from 'langchain/memory';
import { ConversationChain } from 'langchain/chains';

const memory = new BufferMemory({
  returnMessages: true,
  memoryKey: 'history',
});

const chain = new ConversationChain({
  llm: model,
  memory,
  prompt: ChatPromptTemplate.fromMessages([
    ['system', 'You are a helpful assistant.'],
    new MessagesPlaceholder('history'),
    ['human', '{input}'],
  ]),
});

// Conversation maintains context
await chain.invoke({ input: 'My name is Alice' });
await chain.invoke({ input: 'What is my name?' }); // Remembers Alice
```

### Window Memory

```typescript
// memory/window.ts
import { BufferWindowMemory } from 'langchain/memory';

const memory = new BufferWindowMemory({
  k: 5, // Keep last 5 exchanges
  returnMessages: true,
  memoryKey: 'history',
});
```

### Summary Memory

```typescript
// memory/summary.ts
import { ConversationSummaryMemory } from 'langchain/memory';

const memory = new ConversationSummaryMemory({
  llm: model,
  memoryKey: 'history',
});
// Summarizes conversation to save tokens
```

### Persistent Memory with Redis

```typescript
// memory/redis.ts
import { BufferMemory } from 'langchain/memory';
import { RedisChatMessageHistory } from '@langchain/community/stores/message/redis';

const memory = new BufferMemory({
  chatHistory: new RedisChatMessageHistory({
    sessionId: `user:${userId}:session:${sessionId}`,
    client: redisClient,
    ttl: 3600, // 1 hour
  }),
  returnMessages: true,
  memoryKey: 'history',
});
```

## Tools & Agents

### Define Custom Tools

```typescript
// tools/custom.ts
import { DynamicTool, DynamicStructuredTool } from '@langchain/core/tools';
import { z } from 'zod';

// Simple tool
const searchTool = new DynamicTool({
  name: 'search',
  description: 'Search the web for information',
  func: async (query: string) => {
    const results = await searchAPI.search(query);
    return JSON.stringify(results);
  },
});

// Structured tool with schema
const calculatorTool = new DynamicStructuredTool({
  name: 'calculator',
  description: 'Perform mathematical calculations',
  schema: z.object({
    expression: z.string().describe('Mathematical expression to evaluate'),
  }),
  func: async ({ expression }) => {
    try {
      const result = eval(expression); // Use safer math parser in production
      return String(result);
    } catch {
      return 'Error: Invalid expression';
    }
  },
});

// Database query tool
const dbQueryTool = new DynamicStructuredTool({
  name: 'query_database',
  description: 'Query the database for user or order information',
  schema: z.object({
    table: z.enum(['users', 'orders', 'products']),
    filter: z.record(z.string()).optional(),
    limit: z.number().default(10),
  }),
  func: async ({ table, filter, limit }) => {
    const results = await db[table].findMany({
      where: filter,
      take: limit,
    });
    return JSON.stringify(results);
  },
});
```

### Create Agent

```typescript
// agents/react.ts
import { createReactAgent, AgentExecutor } from 'langchain/agents';
import { pull } from 'langchain/hub';

// Get standard ReAct prompt
const prompt = await pull('hwchase17/react');

// Create agent
const agent = await createReactAgent({
  llm: model,
  tools: [searchTool, calculatorTool, dbQueryTool],
  prompt,
});

// Create executor
const executor = new AgentExecutor({
  agent,
  tools: [searchTool, calculatorTool, dbQueryTool],
  verbose: true,
  maxIterations: 5,
});

// Run agent
const result = await executor.invoke({
  input: 'What is the square root of the number of users in our database?',
});
```

### OpenAI Functions Agent

```typescript
// agents/openai-functions.ts
import { createOpenAIFunctionsAgent, AgentExecutor } from 'langchain/agents';

const prompt = ChatPromptTemplate.fromMessages([
  ['system', 'You are a helpful assistant with access to tools.'],
  new MessagesPlaceholder('chat_history'),
  ['human', '{input}'],
  new MessagesPlaceholder('agent_scratchpad'),
]);

const agent = await createOpenAIFunctionsAgent({
  llm: new ChatOpenAI({ modelName: 'gpt-4-turbo-preview' }),
  tools: [searchTool, calculatorTool],
  prompt,
});

const executor = new AgentExecutor({
  agent,
  tools: [searchTool, calculatorTool],
  memory: new BufferMemory({
    returnMessages: true,
    memoryKey: 'chat_history',
  }),
});
```

## RAG Pipeline

### Document Loading

```typescript
// rag/loader.ts
import { DirectoryLoader } from 'langchain/document_loaders/fs/directory';
import { PDFLoader } from 'langchain/document_loaders/fs/pdf';
import { TextLoader } from 'langchain/document_loa