Looking for the JS/TS version? Check out LangChain.js.
To help you ship LangChain apps to production faster, check out LangSmith. LangSmith is a unified developer platform for building, testing, and monitoring LLM applications.
uv add langchainLangChain is the easiest way to start building agents and applications powered by LLMs. With under 10 lines of code, you can connect to OpenAI, Anthropic, Google, and more. LangChain provides a pre-built agent architecture and model integrations to help you get started quickly and seamlessly incorporate LLMs into your agents and applications.
We recommend you use LangChain if you want to quickly build agents and autonomous applications. Use LangGraph, our low-level agent orchestration framework and runtime, when you have more advanced needs that require a combination of deterministic and agentic workflows, heavy customization, and carefully controlled latency.
LangChain agents are built on top of LangGraph in order to provide durable execution, streaming, human-in-the-loop, persistence, and more. (You do not need to know LangGraph for basic LangChain agent usage.)
For full documentation, see the API reference. For conceptual guides, tutorials, and examples on using LangChain, see the LangChain Docs. You can also chat with the docs using Chat LangChain.
See our Releases and Versioning policies.
As an open-source project in a rapidly developing field, we are extremely open to contributions, whether it be in the form of a new feature, improved infrastructure, or better documentation.
For detailed information on how to contribute, see the Contributing Guide.
Artifact attached to the ToolMessage produced by an MCP tool call.
Wrapping the structured content in a TypedDict leaves room for further
MCP result fields without changing the artifact's shape.
A request for data matching a schema.
A request for the human to visit an address.
Interrupt payload raised while an MCP tool call waits on input.
An answer to an MCPElicitationRequest.
A refusal to answer an MCPElicitationRequest, leaving the call to go on.
A refusal that abandons the tool call rather than just the question.
Value used to resume a run interrupted by an MCP elicitation.
Adapt an MCP target into LangChain tools.
MCPAdapter uses FastMCP for protocol negotiation and connection management,
then converts discovered MCP tools into asynchronous LangChain tools. The
resul
Typed sync handle for a nested named-agent execution.
Surfaces on run.subagents when a nested run's lc_agent_name differs
from its parent's (i.e., a create_agent(name=...) dispatched from a too
Typed async handle for a nested named-agent execution.
Promote nested named agents into typed handles on run.subagents.
The base _TasksLifecycleBase records each namespace's lc_agent_name
(set by create_agent(name=...)) and, on every task start,
Base class for structured output errors.
Raised when model returns multiple structured output tool calls when only one is expected.
Raised when structured output tool call arguments fail to parse according to the schema.
Use a tool calling strategy for model responses.
Use the model provider's native structured output method.
Information for tracking structured output tool metadata.
This contains all necessary information to handle structured responses generated via tool calls, including the original schema, its type clas
Information for tracking native structured output metadata.
This contains all necessary information to handle structured responses generated via native provider output, including the original schema,
Automatically select the best strategy for structured output.
Defer selected tools behind provider-native tool search.
Instead of sending every tool schema on every turn, this middleware marks
selected tools as deferred (via extras["defer_loading"]) and injec
Protocol describing a context editing strategy.
Configuration for clearing tool outputs when token limits are exceeded.
Automatically prune tool results to manage context size.
The middleware applies a sequence of edits when the total input token count exceeds configured thresholds.
Currently the ClearToolUsesEdit
Configuration contract for persistent shell sessions.
Concrete subclasses encapsulate how a shell process is launched and constrained.
Each policy documents its security guarantees and the operating
Run the shell directly on the host process.
This policy is best suited for trusted or single-tenant environments (CI jobs, developer workstations, pre-sandboxed containers) where the agent must acces
Launch the shell through the Codex CLI sandbox.
Ideal when you have the Codex CLI installed and want the additional syscall and filesystem restrictions provided by Anthropic's Seatbelt (macOS) or Lan
Run the shell inside a dedicated Docker container.
Choose this policy when commands originate from untrusted users or you require strong isolation between sessions. By default the workspace is bind-m
Middleware that automatically retries failed model calls with configurable backoff.
Supports retrying on specific exceptions and exponential backoff.
A single todo item with content and status.
State schema for the todo middleware.
Input schema for the write_todos tool.
Middleware that provides todo list management capabilities to agents.
This middleware adds a write_todos tool that allows agents to create and manage
structured task lists for complex multi-step op
Emulates specified tools using an LLM instead of executing them.
This middleware allows selective emulation of tools for testing purposes.
By default (when tools=None), all tools are emulated. You
Model request information for the agent.
Response from model execution including messages and optional structured output.
The result will usually contain a single AIMessage, but may include an additional
ToolMessage if the model used a
Model response with an optional 'Command' from 'wrap_model_call' middleware.
Use this to return a 'Command' alongside the model response from a 'wrap_model_call' handler. The command is applied as an
Annotation used to mark state attributes as omitted from input or output schemas.
State schema for the agent.
Input state schema for the agent.
Output state schema for the agent.
Base middleware class for an agent.
Subclass this and implement any of the defined methods to customize agent behavior between steps in the main agent loop.
State schema for ModelCallLimitMiddleware.
Extends AgentState with model call tracking fields.
Exception raised when model call limits are exceeded.
This exception is raised when the configured exit behavior is 'error' and either
the thread or run model call limit has been exceeded.
Tracks model call counts and enforces limits.
This middleware monitors the number of model calls made during agent execution and can terminate the agent when specified limits are reached. It supports
Middleware that automatically retries failed tool calls with configurable backoff.
Supports retrying on specific exceptions and exponential backoff.
Automatic fallback to alternative models on errors.
Retries failed model calls with alternative models in sequence until
success or all models exhausted. Primary model specified in create_agent.
Dictionary-based trigger specification for AND conditions.
All specified thresholds in a single TriggerClause must be met for the clause to
trigger summarization (AND semantics). When multiple clau
Summarizes conversation history when token limits are approached.
This middleware monitors message token counts and automatically summarizes older messages when a threshold is reached, preserving rec
Uses an LLM to select relevant tools before calling the main model.
When an agent has many tools available, this middleware filters them down to only the most relevant ones for the user's query. This
Represents an individual match of sensitive data.
Raised when configured to block on detected sensitive values.
Configuration for handling a single PII type.
Resolved redaction rule ready for execution.
State schema for ToolCallLimitMiddleware.
Extends AgentState with tool call tracking fields.
The count fields are dictionaries mapping tool names to execution counts. This allows multiple middle
Exception raised when tool call limits are exceeded.
This exception is raised when the configured exit behavior is 'error' and either
the thread or run tool call limit has been exceeded.
Track tool call counts and enforces limits during agent execution.
This middleware monitors the number of tool calls made and can terminate or restrict execution when limits are exceeded. It supports
Provides Glob and Grep search over filesystem files.
This middleware adds two tools that search through local filesystem:
Represents an action with a name and args.
Represents an action request with a name, args, and description.
Policy for reviewing a HITL request.
Request for human feedback on a sequence of actions requested by a model.
Response when a human approves the action.
Response when a human edits the action.
Response when a human rejects the action.
Response when a human answers on behalf of the tool, skipping execution.
Used for "ask user" style tools whose real implementation is the human's response. The tool is not executed; instead, a synthe
Response payload for a HITLRequest.
Configuration for an action requiring human in the loop.
This is the configuration format used in the HumanInTheLoopMiddleware.__init__
method.
Human in the loop middleware.
Detect and handle Personally Identifiable Information (PII) in conversations.
This middleware detects common PII types and applies configurable strategies to handle them. It can detect emails, credit
Return selected tool-execution exceptions to the model as error ToolMessages.
on_error is called for each exception raised by tool execution. Return content
(a str or a list of content blocks)
Keep internal model calls out of run.messages and the raw event log.
Used by middleware that makes internal model calls and runs before built-in transformers.
For tagged events, streamed `message-
Agent state extension for tracking shell session resources.
Structured result from command execution.
Persistent shell session that supports sequential command execution.
Middleware that registers a persistent shell tool for agents.
The middleware exposes a single long-lived shell session. Use the execution policy to match your deployment's security posture:
Convert one MCP tool into a LangChain tool.
The returned tool calls the MCP tool through client on every invocation.
FastMCP clients are reentrant, so the tool can open the client itself
whether or
Initialize an embedding model from a model name and optional provider.
Requires the integration package for the chosen model provider to be installed.
See the model_provider para
Creates an agent graph that calls tools in a loop until a stopping condition is met.
For more details on using create_agent,
visit the [Agents](https://docs.langchain.com/oss/python/langchain/agent
Set the process-wide default TracePolicy for agent middleware hook spans.
Call once at startup. A middleware's own trace_policy overrides this wholesale
(no field-level merge). Pass None to cle
Create and manage a structured task list for your current work session.
Decorator to configure hook behavior in middleware methods.
Use this decorator on before_model or after_model methods in middleware classes
to configure their behavior. Currently supports specify
Decorator used to dynamically create a middleware with the before_model hook.
Decorator used to dynamically create a middleware with the after_model hook.
Decorator used to dynamically create a middleware with the before_agent hook.
Decorator used to dynamically create a middleware with the after_agent hook.
Async version is aafter_agent.
Decorator used to dynamically generate system prompts for the model.
This is a convenience decorator that creates middleware using wrap_model_call
specifically for dynamic prompt generation. The de
Create middleware with wrap_model_call hook from a function.
Converts a function with handler callback into middleware that can intercept model calls, implement retry logic, handle errors, and rewr
Create middleware with wrap_tool_call hook from a function.
Async version is awrap_tool_call.
Converts a function with handler callback into middleware that can intercept tool calls, implement r
Validate retry parameters.
Return whether an exception should be retried by default.
Check if an exception should trigger a retry.
Calculate delay for a retry attempt with exponential backoff and optional jitter.
Detect email addresses in content.
Detect credit card numbers in content using Luhn validation.
Detect IPv4 or IPv6 addresses in content.
Detect MAC addresses in content.
Detect URLs in content using regex and stdlib validation.
Apply the configured strategy to matches within content.
Return a callable detector for the given configuration.
Fast file pattern matching tool that works with any codebase size.
Supports glob patterns like **/*.js or src/**/*.ts.
Returns matching file paths sorted by modification time.
Use this tool whe
Fast content search tool that works with any codebase size.
Searches file contents using regular expressions. Supports full regex syntax and filters files by pattern with the include parameter.
Return metadata that marks a model call as internal to middleware.
Initialize a chat model from any supported provider using a unified interface.
Two main use cases: