API Reference Index

Complete API reference for all LangChain components. Each section provides detailed type definitions, parameter descriptions, and usage examples.

Want task-based examples instead? See the Task Index for "I want to..." → code mappings.

Core APIs

Agents

Complete API for creating and executing ReAct agents.

Key APIs:

• createAgent() - Create production-ready agents
• ReactAgent class - Agent execution methods (invoke, stream, batch)
• Agent configuration types
• State and runtime types
• Type inference utilities
• Error classes

View Agent API Reference →

Tools

API for creating tools that agents can use.

Key APIs:

• tool() - Create structured tools
• DynamicTool - String-input tools
• StructuredTool - Schema-based tools
• DynamicStructuredTool - Dynamic structured tools
• Tool configuration types

View Tools API Reference →

Models

Universal model initialization API.

Key APIs:

• initChatModel() - Initialize chat models from any provider
• Model configuration types
• Supported providers

View Models API Reference →

Messages

Message types and utilities for agent communication.

Key APIs:

• HumanMessage - User messages
• AIMessage - Assistant messages
• SystemMessage - System instructions
• ToolMessage - Tool results
• filterMessages() - Filter message arrays
• trimMessages() - Trim messages to fit context

View Messages API Reference →

Structured Outputs

Type-safe structured response generation.

Key APIs:

• toolStrategy() - Tool-based structured output
• providerStrategy() - Provider-native structured output
• ToolStrategy class
• ProviderStrategy class
• Response format types
• Error classes

View Structured Outputs API Reference →

Storage

Storage implementations for persisting data.

Key APIs:

• InMemoryStore - In-memory key-value store
• Storage interface types

View Storage API Reference →

Context

Execution context utilities.

Key APIs:

• context.get() - Get context values
• context.set() - Set context values
• context.run() - Run with context
• context.runAsync() - Async context execution

View Context API Reference →

Documents

Document representation for text processing.

Key APIs:

• Document class - Document container
• Document types

View Documents API Reference →

Quick Lookups

Most Common APIs

// Import commonly used APIs
import {
  createAgent,
  tool,
  initChatModel,
  HumanMessage,
  AIMessage,
  SystemMessage,
  toolStrategy,
  providerStrategy,
  createMiddleware,
  InMemoryStore,
} from "langchain";

Agent Creation

function createAgent<TConfig>(params: CreateAgentParams): ReactAgent<TConfig>;

Full documentation →

Tool Creation

function tool<T = any>(
  func: (input: T, config?: ToolConfig) => any | Promise<any>,
  fields: {
    name: string;
    description: string;
    schema: ZodType<T>;
  }
): StructuredTool<T>;

Full documentation →

Model Initialization

function initChatModel<RunInput = any, CallOptions extends BaseChatModelCallOptions = BaseChatModelCallOptions>(
  model?: string | ChatModel,
  fields?: InitChatModelFields
): ChatModel<RunInput, CallOptions>;

Full documentation →

Type Inference

// Infer types from agents
type InferAgentState<T>
type InferAgentResponse<T>
type InferAgentContext<T>
type InferAgentTools<T>
type InferAgentMiddleware<T>

// Infer types from middleware
type InferMiddlewareState<T>
type InferMiddlewareStates<T extends readonly AgentMiddleware[]>
type InferMiddlewareContext<T>
type InferMiddlewareContexts<T extends readonly AgentMiddleware[]>
type InferMiddlewareTools<T extends readonly AgentMiddleware[]>

// Schema inference
type InferSchemaInput<A>
type InferContextInput<ContextSchema>

Full documentation →

Error Reference

Quick lookup for all error classes:

Agent Errors

• MultipleToolsBoundError - Tools already bound to model
• MultipleStructuredOutputsError - Multiple outputs returned
• StructuredOutputParsingError - Failed to parse output
• ToolInvocationError - Tool execution failed

Middleware Errors

• ToolCallLimitExceededError - Tool call limit exceeded
• PIIDetectionError - PII detection failed

Complete error handling guide →

Type System

Core Types

// Agent types
type CreateAgentParams
type ReactAgent<TConfig>
type UserInput<TStateSchema>
type State<TConfig>
type InvokeConfiguration<ContextSchema>
type StreamConfiguration<ContextSchema>
type BatchConfiguration<ContextSchema>

// Tool types
type Tool
type ToolCall
type ToolResult
type ToolConfig
type ToolRuntime

// Message types
type BaseMessage
type MessageInput
type ContentBlock

// Middleware types
type AgentMiddleware<TSchema, TContextSchema, TFullContext, TTools>
type MiddlewareConfig<TSchema, TContextSchema, TTools>

// Response format types
type ResponseFormat
type JsonSchemaFormat

// Runtime types
type Runtime<TContext>

Navigation

By Category

Getting Started

• Quick Reference - Fast lookups
• Agent Guide - Task-oriented guide
• Tool Guide - Tool patterns

API References

• Agents - Complete agent API
• Tools - Tool API
• Models - Model API
• Messages - Message API
• Structured Outputs - Response formats
• Storage - Storage API
• Context - Context API
• Documents - Document API

Advanced Topics

• Type Inference - TypeScript types
• Error Handling - Error patterns

Middleware

• Overview - Middleware system
• Built-in Catalog - All pre-built middleware
• Human-in-the-Loop - HITL middleware
• Custom Middleware - Creating middleware

Integrations

• LangChain Hub - Prompt sharing
• Serialization - Module loading

By Use Case

Building Agents

1. Start: Agent Guide
2. Reference: Agent API
3. Examples: Quick Reference

Adding Tools

1. Start: Tool Guide
2. Reference: Tool API
3. Patterns: Tool Guide - Patterns

Structured Outputs

1. Start: Structured Output Guide
2. Reference: Structured Output API
3. Examples: Quick Reference

Using Middleware

1. Start: Middleware Guide
2. Reference: Middleware Overview
3. Catalog: Built-in Middleware

Type Safety

1. Start: Type Inference
2. Reference: Agent Types
3. Examples: Type Inference - Usage

Search Tips

Finding APIs

1. By name: Use browser search (Ctrl/Cmd+F) on this index page
2. By category: Navigate to specific API reference pages
3. By use case: Follow the "By Use Case" navigation above
4. Quick lookup: Check Quick Reference first

Finding Examples

1. Quick patterns: Quick Reference
2. Task-oriented: Guides in guides/
3. Comprehensive: Full examples in API references
4. Advanced: Type Inference examples

Finding Types

1. Inference utilities: Type Inference
2. Agent types: Agent API - Types
3. Tool types: Tool API - Types
4. Middleware types: Middleware Overview - Types