How to use the graph API¶
This guide demonstrates the basics of LangGraph's Graph API. It walks through state, as well as composing common graph structures such as sequences, branches, and loops. It also covers LangGraph's control features, including the Send API for map-reduce workflows and the Command API for combining state updates with "hops" across nodes.
Setup¶
Install langgraph:
Set up LangSmith for better debugging
Sign up for LangSmith to quickly spot issues and improve the performance of your LangGraph projects. LangSmith lets you use trace data to debug, test, and monitor your LLM aps built with LangGraph — read more about how to get started in the docs.
Define and update state¶
Here we show how to define and update state in LangGraph. We will demonstrate:
- How to use state to define a graph's schema
- How to use reducers to control how state updates are processed.
Define state¶
State in LangGraph can be a TypedDict, Pydantic model, or dataclass. Below we will use TypedDict. See this section for detail on using Pydantic.
By default, graphs will have the same input and output schema, and the state determines that schema. See this section for how to define distinct input and output schemas.
Let's consider a simple example using messages. This represents a versatile formulation of state for many LLM applications. See our concepts page for more detail.
API Reference: AnyMessage
from langchain_core.messages import AnyMessage
from typing_extensions import TypedDict
class State(TypedDict):
messages: list[AnyMessage]
extra_field: int
This state tracks a list of message objects, as well as an extra integer field.
Update state¶
Let's build an example graph with a single node. Our node is just a Python function that reads our graph's state and makes updates to it. The first argument to this function will always be the state:
API Reference: AIMessage
from langchain_core.messages import AIMessage
def node(state: State):
messages = state["messages"]
new_message = AIMessage("Hello!")
return {"messages": messages + [new_message], "extra_field": 10}
This node simply appends a message to our message list, and populates an extra field.
Important
Nodes should return updates to the state directly, instead of mutating the state.
Let's next define a simple graph containing this node. We use StateGraph to define a graph that operates on this state. We then use add_node populate our graph.
API Reference: StateGraph
from langgraph.graph import StateGraph
builder = StateGraph(State)
builder.add_node(node)
builder.set_entry_point("node")
graph = builder.compile()
LangGraph provides built-in utilities for visualizing your graph. Let's inspect our graph. See this section for detail on visualization.
In this case, our graph just executes a single node. Let's proceed with a simple invocation:
API Reference: HumanMessage
from langchain_core.messages import HumanMessage
result = graph.invoke({"messages": [HumanMessage("Hi")]})
result
{'messages': [HumanMessage(content='Hi', additional_kwargs={}, response_metadata={}),
AIMessage(content='Hello!', additional_kwargs={}, response_metadata={})],
'extra_field': 10}
Note that:
- We kicked off invocation by updating a single key of the state.
- We receive the entire state in the invocation result.
For convenience, we frequently inspect the content of message objects via pretty-print:
================================ Human Message =================================
Hi
================================== Ai Message ==================================
Hello!
Process state updates with reducers¶
Each key in the state can have its own independent reducer function, which controls how updates from nodes are applied. If no reducer function is explicitly specified then it is assumed that all updates to the key should override it.
For TypedDict state schemas, we can define reducers by annotating the corresponding field of the state with a reducer function.
In the earlier example, our node updated the "messages" key in the state by appending a message to it. Below, we add a reducer to this key, such that updates are automatically appended:
from typing_extensions import Annotated
def add(left, right):
"""Can also import `add` from the `operator` built-in."""
return left + right
class State(TypedDict):
messages: Annotated[list[AnyMessage], add]
extra_field: int
Now our node can be simplified:
def node(state: State):
new_message = AIMessage("Hello!")
return {"messages": [new_message], "extra_field": 10}
API Reference: START
from langgraph.graph import START
graph = StateGraph(State).add_node(node).add_edge(START, "node").compile()
result = graph.invoke({"messages": [HumanMessage("Hi")]})
for message in result["messages"]:
message.pretty_print()
================================ Human Message =================================
Hi
================================== Ai Message ==================================
Hello!
MessagesState¶
In practice, there are additional considerations for updating lists of messages:
- We may wish to update an existing message in the state.
- We may want to accept short-hands for message formats, such as OpenAI format.
LangGraph includes a built-in reducer add_messages that handles these considerations:
API Reference: add_messages
from langgraph.graph.message import add_messages
class State(TypedDict):
messages: Annotated[list[AnyMessage], add_messages]
extra_field: int
def node(state: State):
new_message = AIMessage("Hello!")
return {"messages": [new_message], "extra_field": 10}
graph = StateGraph(State).add_node(node).set_entry_point("node").compile()
input_message = {"role": "user", "content": "Hi"}
result = graph.invoke({"messages": [input_message]})
for message in result["messages"]:
message.pretty_print()
================================ Human Message =================================
Hi
================================== Ai Message ==================================
Hello!
MessagesState for convenience, so that we can have:
Define input and output schemas¶
By default, StateGraph operates with a single schema, and all nodes are expected to communicate using that schema. However, it's also possible to define distinct input and output schemas for a graph.
When distinct schemas are specified, an internal schema will still be used for communication between nodes. The input schema ensures that the provided input matches the expected structure, while the output schema filters the internal data to return only the relevant information according to the defined output schema.
Below, we'll see how to define distinct input and output schema.
API Reference: StateGraph | START | END
from langgraph.graph import StateGraph, START, END
from typing_extensions import TypedDict
# Define the schema for the input
class InputState(TypedDict):
question: str
# Define the schema for the output
class OutputState(TypedDict):
answer: str
# Define the overall schema, combining both input and output
class OverallState(InputState, OutputState):
pass
# Define the node that processes the input and generates an answer
def answer_node(state: InputState):
# Example answer and an extra key
return {"answer": "bye", "question": state["question"]}
# Build the graph with input and output schemas specified
builder = StateGraph(OverallState, input=InputState, output=OutputState)
builder.add_node(answer_node) # Add the answer node
builder.add_edge(START, "answer_node") # Define the starting edge
builder.add_edge("answer_node", END) # Define the ending edge
graph = builder.compile() # Compile the graph
# Invoke the graph with an input and print the result
print(graph.invoke({"question": "hi"}))
Pass private state between nodes¶
In some cases, you may want nodes to exchange information that is crucial for intermediate logic but doesn’t need to be part of the main schema of the graph. This private data is not relevant to the overall input/output of the graph and should only be shared between certain nodes.
Below, we'll create an example sequential graph consisting of three nodes (node_1, node_2 and node_3), where private data is passed between the first two steps (node_1 and node_2), while the third step (node_3) only has access to the public overall state.
API Reference: StateGraph | START | END
from langgraph.graph import StateGraph, START, END
from typing_extensions import TypedDict
# The overall state of the graph (this is the public state shared across nodes)
class OverallState(TypedDict):
a: str
# Output from node_1 contains private data that is not part of the overall state
class Node1Output(TypedDict):
private_data: str
# The private data is only shared between node_1 and node_2
def node_1(state: OverallState) -> Node1Output:
output = {"private_data": "set by node_1"}
print(f"Entered node `node_1`:\n\tInput: {state}.\n\tReturned: {output}")
return output
# Node 2 input only requests the private data available after node_1
class Node2Input(TypedDict):
private_data: str
def node_2(state: Node2Input) -> OverallState:
output = {"a": "set by node_2"}
print(f"Entered node `node_2`:\n\tInput: {state}.\n\tReturned: {output}")
return output
# Node 3 only has access to the overall state (no access to private data from node_1)
def node_3(state: OverallState) -> OverallState:
output = {"a": "set by node_3"}
print(f"Entered node `node_3`:\n\tInput: {state}.\n\tReturned: {output}")
return output
# Connect nodes in a sequence
# node_2 accepts private data from node_1, whereas
# node_3 does not see the private data.
builder = StateGraph(OverallState).add_sequence([node_1, node_2, node_3])
builder.add_edge(START, "node_1")
graph = builder.compile()
# Invoke the graph with the initial state
response = graph.invoke(
{
"a": "set at start",
}
)
print()
print(f"Output of graph invocation: {response}")
Entered node `node_1`:
Input: {'a': 'set at start'}.
Returned: {'private_data': 'set by node_1'}
Entered node `node_2`:
Input: {'private_data': 'set by node_1'}.
Returned: {'a': 'set by node_2'}
Entered node `node_3`:
Input: {'a': 'set by node_2'}.
Returned: {'a': 'set by node_3'}
Output of graph invocation: {'a': 'set by node_3'}
Use Pydantic models for graph state¶
A StateGraph accepts a state_schema argument on initialization that specifies the "shape" of the state that the nodes in the graph can access and update.
In our examples, we typically use a python-native TypedDict for state_schema, but state_schema can be any type.
Here, we'll see how a Pydantic BaseModel. can be used for state_schema to add run time validation on inputs.
Known Limitations
- Currently, the output of the graph will NOT be an instance of a pydantic model.
- Run-time validation only occurs on inputs into nodes, not on the outputs.
- The validation error trace from pydantic does not show which node the error arises in.
API Reference: StateGraph | START | END
from langgraph.graph import StateGraph, START, END
from typing_extensions import TypedDict
from pydantic import BaseModel
# The overall state of the graph (this is the public state shared across nodes)
class OverallState(BaseModel):
a: str
def node(state: OverallState):
return {"a": "goodbye"}
# Build the state graph
builder = StateGraph(OverallState)
builder.add_node(node) # node_1 is the first node
builder.add_edge(START, "node") # Start the graph with node_1
builder.add_edge("node", END) # End the graph after node_1
graph = builder.compile()
# Test the graph with a valid input
graph.invoke({"a": "hello"})
Invoke the graph with an invalid input
try:
graph.invoke({"a": 123}) # Should be a string
except Exception as e:
print("An exception was raised because `a` is an integer rather than a string.")
print(e)
An exception was raised because `a` is an integer rather than a string.
1 validation error for OverallState
a
Input should be a valid string [type=string_type, input_value=123, input_type=int]
For further information visit https://errors.pydantic.dev/2.9/v/string_type
Serialization Behavior
When using Pydantic models as state schemas, it's important to understand how serialization works, especially when:- Passing Pydantic objects as inputs
- Receiving outputs from the graph
- Working with nested Pydantic models
API Reference: StateGraph | START | END
from langgraph.graph import StateGraph, START, END
from pydantic import BaseModel
class NestedModel(BaseModel):
value: str
class ComplexState(BaseModel):
text: str
count: int
nested: NestedModel
def process_node(state: ComplexState):
# Node receives a validated Pydantic object
print(f"Input state type: {type(state)}")
print(f"Nested type: {type(state.nested)}")
# Return a dictionary update
return {"text": state.text + " processed", "count": state.count + 1}
# Build the graph
builder = StateGraph(ComplexState)
builder.add_node("process", process_node)
builder.add_edge(START, "process")
builder.add_edge("process", END)
graph = builder.compile()
# Create a Pydantic instance for input
input_state = ComplexState(text="hello", count=0, nested=NestedModel(value="test"))
print(f"Input object type: {type(input_state)}")
# Invoke graph with a Pydantic instance
result = graph.invoke(input_state)
print(f"Output type: {type(result)}")
print(f"Output content: {result}")
# Convert back to Pydantic model if needed
output_model = ComplexState(**result)
print(f"Converted back to Pydantic: {type(output_model)}")
Runtime Type Coercion
Pydantic performs runtime type coercion for certain data types. This can be helpful but also lead to unexpected behavior if you're not aware of it.API Reference: StateGraph | START | END
from langgraph.graph import StateGraph, START, END
from pydantic import BaseModel
class CoercionExample(BaseModel):
# Pydantic will coerce string numbers to integers
number: int
# Pydantic will parse string booleans to bool
flag: bool
def inspect_node(state: CoercionExample):
print(f"number: {state.number} (type: {type(state.number)})")
print(f"flag: {state.flag} (type: {type(state.flag)})")
return {}
builder = StateGraph(CoercionExample)
builder.add_node("inspect", inspect_node)
builder.add_edge(START, "inspect")
builder.add_edge("inspect", END)
graph = builder.compile()
# Demonstrate coercion with string inputs that will be converted
result = graph.invoke({"number": "42", "flag": "true"})
# This would fail with a validation error
try:
graph.invoke({"number": "not-a-number", "flag": "true"})
except Exception as e:
print(f"\nExpected validation error: {e}")
Working with Message Models
When working with LangChain message types in your state schema, there are important considerations for serialization. You should useAnyMessage (rather than BaseMessage) for proper serialization/deserialization when using message objects over the wire.
API Reference: StateGraph | START | END | HumanMessage | AIMessage | AnyMessage
from langgraph.graph import StateGraph, START, END
from pydantic import BaseModel
from langchain_core.messages import HumanMessage, AIMessage, AnyMessage
from typing import List
class ChatState(BaseModel):
messages: List[AnyMessage]
context: str
def add_message(state: ChatState):
return {"messages": state.messages + [AIMessage(content="Hello there!")]}
builder = StateGraph(ChatState)
builder.add_node("add_message", add_message)
builder.add_edge(START, "add_message")
builder.add_edge("add_message", END)
graph = builder.compile()
# Create input with a message
initial_state = ChatState(
messages=[HumanMessage(content="Hi")], context="Customer support chat"
)
result = graph.invoke(initial_state)
print(f"Output: {result}")
# Convert back to Pydantic model to see message types
output_model = ChatState(**result)
for i, msg in enumerate(output_model.messages):
print(f"Message {i}: {type(msg).__name__} - {msg.content}")
Add runtime configuration¶
Sometimes you want to be able to configure your graph when calling it. For example, you might want to be able to specify what LLM or system prompt to use at runtime, without polluting the graph state with these parameters.
To add runtime configuration:
- Specify a schema for your configuration
- Add the configuration to the function signature for nodes or conditional edges
- Pass the configuration into the graph.
See below for a simple example:
API Reference: RunnableConfig | END | StateGraph | START
from langchain_core.runnables import RunnableConfig
from langgraph.graph import END, StateGraph, START
from typing_extensions import TypedDict
# 1. Specify config schema
class ConfigSchema(TypedDict):
my_runtime_value: str
# 2. Define a graph that accesses the config in a node
class State(TypedDict):
my_state_value: str
def node(state: State, config: RunnableConfig):
if config["configurable"]["my_runtime_value"] == "a":
return {"my_state_value": 1}
elif config["configurable"]["my_runtime_value"] == "b":
return {"my_state_value": 2}
else:
raise ValueError("Unknown values.")
builder = StateGraph(State, config_schema=ConfigSchema)
builder.add_node(node)
builder.add_edge(START, "node")
builder.add_edge("node", END)
graph = builder.compile()
# 3. Pass in configuration at runtime:
print(graph.invoke({}, {"configurable": {"my_runtime_value": "a"}}))
print(graph.invoke({}, {"configurable": {"my_runtime_value": "b"}}))
Extended example: specifying LLM at runtime
Below we demonstrate a practical example in which we configure what LLM to use at runtime. We will use both OpenAI and Anthropic models.import getpass
import os
def _set_env(var: str):
if not os.environ.get(var):
os.environ[var] = getpass.getpass(f"{var}: ")
_set_env("ANTHROPIC_API_KEY")
_set_env("OPENAI_API_KEY")
from langchain.chat_models import init_chat_model
from langchain_core.runnables import RunnableConfig
from langgraph.graph import MessagesState
from langgraph.graph import END, StateGraph, START
from typing_extensions import TypedDict
class ConfigSchema(TypedDict):
model: str
MODELS = {
"anthropic": init_chat_model("anthropic:claude-3-5-haiku-latest"),
"openai": init_chat_model("openai:gpt-4.1-mini"),
}
def call_model(state: MessagesState, config: RunnableConfig):
model = config["configurable"].get("model", "anthropic")
model = MODELS[model]
response = model.invoke(state["messages"])
return {"messages": [response]}
builder = StateGraph(MessagesState, config_schema=ConfigSchema)
builder.add_node("model", call_model)
builder.add_edge(START, "model")
builder.add_edge("model", END)
graph = builder.compile()
# Usage
input_message = {"role": "user", "content": "hi"}
# With no configuration, uses default (Anthropic)
response_1 = graph.invoke({"messages": [input_message]})["messages"][-1]
# Or, can set OpenAI
config = {"configurable": {"model": "openai"}}
response_2 = graph.invoke({"messages": [input_message]}, config=config)["messages"][-1]
print(response_1.response_metadata["model_name"])
print(response_2.response_metadata["model_name"])
Extended example: specifying model and system message at runtime
Below we demonstrate a practical example in which we configure two parameters: the LLM and system message to use at runtime.import getpass
import os
def _set_env(var: str):
if not os.environ.get(var):
os.environ[var] = getpass.getpass(f"{var}: ")
_set_env("ANTHROPIC_API_KEY")
_set_env("OPENAI_API_KEY")
from typing import Optional
from langchain.chat_models import init_chat_model
from langchain_core.messages import SystemMessage
from langchain_core.runnables import RunnableConfig
from langgraph.graph import END, MessagesState, StateGraph, START
from typing_extensions import TypedDict
class ConfigSchema(TypedDict):
model: Optional[str]
system_message: Optional[str]
MODELS = {
"anthropic": init_chat_model("anthropic:claude-3-5-haiku-latest"),
"openai": init_chat_model("openai:gpt-4.1-mini"),
}
def call_model(state: MessagesState, config: RunnableConfig):
model = config["configurable"].get("model", "anthropic")
model = MODELS[model]
messages = state["messages"]
if system_message := config["configurable"].get("system_message"):
messages = [SystemMessage(system_message)] + messages
response = model.invoke(messages)
return {"messages": [response]}
builder = StateGraph(MessagesState, config_schema=ConfigSchema)
builder.add_node("model", call_model)
builder.add_edge(START, "model")
builder.add_edge("model", END)
graph = builder.compile()
# Usage
input_message = {"role": "user", "content": "hi"}
config = {"configurable": {"model": "openai", "system_message": "Respond in Italian."}}
response = graph.invoke({"messages": [input_message]}, config)
for message in response["messages"]:
message.pretty_print()
Add retry policies¶
There are many use cases where you may wish for your node to have a custom retry policy, for example if you are calling an API, querying a database, or calling an LLM, etc. LangGraph lets you add retry policies to nodes.
To configure a retry policy, pass the retry parameter to the add_node. The retry parameter takes in a RetryPolicy named tuple object. Below we instantiate a RetryPolicy object with the default parameters and associate it with a node:
from langgraph.pregel import RetryPolicy
builder.add_node(
"node_name",
node_function,
retry=RetryPolicy(),
)
By default, the retry_on parameter uses the default_retry_on function, which retries on any exception except for the following:
ValueErrorTypeErrorArithmeticErrorImportErrorLookupErrorNameErrorSyntaxErrorRuntimeErrorReferenceErrorStopIterationStopAsyncIterationOSError
In addition, for exceptions from popular http request libraries such as requests and httpx it only retries on 5xx status codes.
Extended example: customizing retry policies
Consider an example in which we are reading from a SQL database. Below we pass two different retry policies to nodes:API Reference: init_chat_model | END | StateGraph | START | SQLDatabase | AIMessage
import sqlite3
from typing_extensions import TypedDict
from langchain.chat_models import init_chat_model
from langgraph.graph import END, MessagesState, StateGraph, START
from langgraph.pregel import RetryPolicy
from langchain_community.utilities import SQLDatabase
from langchain_core.messages import AIMessage
db = SQLDatabase.from_uri("sqlite:///:memory:")
model = init_chat_model("anthropic:claude-3-5-haiku-latest")
def query_database(state: MessagesState):
query_result = db.run("SELECT * FROM Artist LIMIT 10;")
return {"messages": [AIMessage(content=query_result)]}
def call_model(state: MessagesState):
response = model.invoke(state["messages"])
return {"messages": [response]}
# Define a new graph
builder = StateGraph(MessagesState)
builder.add_node(
"query_database",
query_database,
retry=RetryPolicy(retry_on=sqlite3.OperationalError),
)
builder.add_node("model", call_model, retry=RetryPolicy(max_attempts=5))
builder.add_edge(START, "model")
builder.add_edge("model", "query_database")
builder.add_edge("query_database", END)
graph = builder.compile()
Create a sequence of steps¶
Prerequisites
This guide assumes familiarity with the above section on state.
Here we demonstrate how to construct a simple sequence of steps. We will show:
- How to build a sequential graph
- Built-in short-hand for constructing similar graphs.
To add a sequence of nodes, we use the .add_node and .add_edge methods of our graph:
API Reference: START | StateGraph
from langgraph.graph import START, StateGraph
builder = StateGraph(State)
# Add nodes
builder.add_node(step_1)
builder.add_node(step_2)
builder.add_node(step_3)
# Add edges
builder.add_edge(START, "step_1")
builder.add_edge("step_1", "step_2")
builder.add_edge("step_2", "step_3")
We can also use the built-in shorthand .add_sequence:
builder = StateGraph(State).add_sequence([step_1, step_2, step_3])
builder.add_edge(START, "step_1")
Why split application steps into a sequence with LangGraph?
LangGraph makes it easy to add an underlying persistence layer to your application. This allows state to be checkpointed in between the execution of nodes, so your LangGraph nodes govern:- How state updates are checkpointed
- How interruptions are resumed in human-in-the-loop workflows
- How we can "rewind" and branch-off executions using LangGraph's time travel features
Let's demonstrate an end-to-end example. We will create a sequence of three steps:
- Populate a value in a key of the state
- Update the same value
- Populate a different value
Let's first define our state. This governs the schema of the graph, and can also specify how to apply updates. See this section for more detail.
In our case, we will just keep track of two values:
Our nodes are just Python functions that read our graph's state and make updates to it. The first argument to this function will always be the state:
def step_1(state: State):
return {"value_1": "a"}
def step_2(state: State):
current_value_1 = state["value_1"]
return {"value_1": f"{current_value_1} b"}
def step_3(state: State):
return {"value_2": 10}
Note
Note that when issuing updates to the state, each node can just specify the value of the key it wishes to update.
By default, this will overwrite the value of the corresponding key. You can also use reducers to control how updates are processed— for example, you can append successive updates to a key instead. See this section for more detail.
Finally, we define the graph. We use StateGraph to define a graph that operates on this state.
We will then use add_node and add_edge to populate our graph and define its control flow.
API Reference: START | StateGraph
from langgraph.graph import START, StateGraph
builder = StateGraph(State)
# Add nodes
builder.add_node(step_1)
builder.add_node(step_2)
builder.add_node(step_3)
# Add edges
builder.add_edge(START, "step_1")
builder.add_edge("step_1", "step_2")
builder.add_edge("step_2", "step_3")
Specifying custom names
You can specify custom names for nodes using .add_node:
Note that:
.add_edgetakes the names of nodes, which for functions defaults tonode.__name__.- We must specify the entry point of the graph. For this we add an edge with the START node.
- The graph halts when there are no more nodes to execute.
We next compile our graph. This provides a few basic checks on the structure of the graph (e.g., identifying orphaned nodes). If we were adding persistence to our application via a checkpointer, it would also be passed in here.
LangGraph provides built-in utilities for visualizing your graph. Let's inspect our sequence. See this guide for detail on visualization.
Let's proceed with a simple invocation:
Note that:
- We kicked off invocation by providing a value for a single state key. We must always provide a value for at least one key.
- The value we passed in was overwritten by the first node.
- The second node updated the value.
- The third node populated a different value.
Built-in shorthand
langgraph>=0.2.46 includes a built-in short-hand add_sequence for adding node sequences. You can compile the same graph as follows:
Create branches¶
Parallel execution of nodes is essential to speed up overall graph operation. LangGraph offers native support for parallel execution of nodes, which can significantly enhance the performance of graph-based workflows. This parallelization is achieved through fan-out and fan-in mechanisms, utilizing both standard edges and conditional_edges. Below are some examples showing how to add create branching dataflows that work for you.
Run graph nodes in parallel¶
In this example, we fan out from Node A to B and C and then fan in to D. With our state, we specify the reducer add operation. This will combine or accumulate values for the specific key in the State, rather than simply overwriting the existing value. For lists, this means concatenating the new list with the existing list. See the above section on state reducers for more detail on updating state with reducers.
API Reference: StateGraph | START | END
import operator
from typing import Annotated, Any
from typing_extensions import TypedDict
from langgraph.graph import StateGraph, START, END
class State(TypedDict):
# The operator.add reducer fn makes this append-only
aggregate: Annotated[list, operator.add]
def a(state: State):
print(f'Adding "A" to {state["aggregate"]}')
return {"aggregate": ["A"]}
def b(state: State):
print(f'Adding "B" to {state["aggregate"]}')
return {"aggregate": ["B"]}
def c(state: State):
print(f'Adding "C" to {state["aggregate"]}')
return {"aggregate": ["C"]}
def d(state: State):
print(f'Adding "D" to {state["aggregate"]}')
return {"aggregate": ["D"]}
builder = StateGraph(State)
builder.add_node(a)
builder.add_node(b)
builder.add_node(c)
builder.add_node(d)
builder.add_edge(START, "a")
builder.add_edge("a", "b")
builder.add_edge("a", "c")
builder.add_edge("b", "d")
builder.add_edge("c", "d")
builder.add_edge("d", END)
graph = builder.compile()
With the reducer, you can see that the values added in each node are accumulated.
Note
In the above example, nodes "b" and "c" are executed concurrently in the same superstep. Because they are in the same step, node "d" executes after both "b" and "c" are finished.
Importantly, updates from a parallel superstep may not be ordered consistently. If you need a consistent, predetermined ordering of updates from a parallel superstep, you should write the outputs to a separate field in the state together with a value with which to order them.
Exception handling?
LangGraph executes nodes within "supersteps", meaning that while parallel branches are executed in parallel, the entire superstep is transactional. If any of these branches raises an exception, none of the updates are applied to the state (the entire superstep errors).
Importantly, when using a checkpointer, results from successful nodes within a superstep are saved, and don't repeat when resumed.
If you have error-prone (perhaps want to handle flakey API calls), LangGraph provides two ways to address this:- You can write regular python code within your node to catch and handle exceptions.
- You can set a retry_policy to direct the graph to retry nodes that raise certain types of exceptions. Only failing branches are retried, so you needn't worry about performing redundant work.
Extended example: unequal length branches
The above example showed how to fan-out and fan-in when each path was only one step. But what if one path had more than one step? Let's add a nodeb_2 in the "b" branch:
API Reference: StateGraph | START | END
import operator
from typing import Annotated, Any
from typing_extensions import TypedDict
from langgraph.graph import StateGraph, START, END
class State(TypedDict):
# The operator.add reducer fn makes this append-only
aggregate: Annotated[list, operator.add]
def a(state: State):
print(f'Adding "A" to {state["aggregate"]}')
return {"aggregate": ["A"]}
def b(state: State):
print(f'Adding "B" to {state["aggregate"]}')
return {"aggregate": ["B"]}
def b_2(state: State):
print(f'Adding "B_2" to {state["aggregate"]}')
return {"aggregate": ["B_2"]}
def c(state: State):
print(f'Adding "C" to {state["aggregate"]}')
return {"aggregate": ["C"]}
def d(state: State):
print(f'Adding "D" to {state["aggregate"]}')
return {"aggregate": ["D"]}
builder = StateGraph(State)
builder.add_node(a)
builder.add_node(b)
builder.add_node(b_2)
builder.add_node(c)
builder.add_node(d)
builder.add_edge(START, "a")
builder.add_edge("a", "b")
builder.add_edge("a", "c")
builder.add_edge("b", "b_2")
builder.add_edge(["b_2", "c"], "d")
builder.add_edge("d", END)
graph = builder.compile()
Adding "A" to []
Adding "B" to ['A']
Adding "C" to ['A']
Adding "B_2" to ['A', 'B', 'C']
Adding "D" to ['A', 'B', 'C', 'B_2']
Note
In the above example, nodes "b" and "c" are executed concurrently in the same superstep. What happens in the next step?
We use add_edge(["b_2", "c"], "d") here to force node "d" to only run when both nodes "b_2" and "c" have finished execution. If we added two separate edges,
node "d" would run twice: after node b2 finishes and once again after node c (in whichever order those nodes finish).
Conditional branching¶
If your fan-out should vary at runtime based on the state, you can use add_conditional_edges to select one or more paths using the graph state. See example below, where node a generates a state update that determines the following node.
API Reference: StateGraph | START | END
import operator
from typing import Annotated, Literal, Sequence
from typing_extensions import TypedDict
from langgraph.graph import StateGraph, START, END
class State(TypedDict):
aggregate: Annotated[list, operator.add]
# Add a key to the state. We will set this key to determine
# how we branch.
which: str
def a(state: State):
print(f'Adding "A" to {state["aggregate"]}')
return {"aggregate": ["A"], "which": "c"}
def b(state: State):
print(f'Adding "B" to {state["aggregate"]}')
return {"aggregate": ["B"]}
def c(state: State):
print(f'Adding "C" to {state["aggregate"]}')
return {"aggregate": ["C"]}
builder = StateGraph(State)
builder.add_node(a)
builder.add_node(b)
builder.add_node(c)
builder.add_edge(START, "a")
builder.add_edge("b", END)
builder.add_edge("c", END)
def conditional_edge(state: State) -> Literal["b", "c"]:
# Fill in arbitrary logic here that uses the state
# to determine the next node
return state["which"]
builder.add_conditional_edges("a", conditional_edge)
graph = builder.compile()
Tip
Your conditional edges can route to multiple destination nodes. For example:
Map-reduce and the Send API¶
By default, Nodes and Edges are defined ahead of time and operate on the same shared state. However, there can be cases where the exact edges are not known ahead of time and/or you may want different versions of state to exist at the same time. A common example of this is with map-reduce design patterns. In this design pattern, a first node may generate a list of objects, and you may want to apply some other node to all those objects. The number of objects may be unknown ahead of time (meaning the number of edges may not be known) and the input state to the downstream Node should be different (one for each generated object).
To support this design pattern, LangGraph supports returning Send objects from conditional edges. Send takes two arguments: first is the name of the node, and second is the state to pass to that node.
def continue_to_jokes(state: OverallState):
return [Send("generate_joke", {"subject": s}) for s in state['subjects']]
graph.add_conditional_edges("node_a", continue_to_jokes)
Below we implement a simple example, where we simulate using LLMs to (1) generate a list of subjects (the length of which is unknown ahead of time), (2) generate jokes in parallel, and (3) select a "best" joke. Importantly, the input state to the fan-out nodes is different than the graph's overall state.
API Reference: Send | END | StateGraph | START
import operator
from typing import Annotated
from typing_extensions import TypedDict
from langgraph.types import Send
from langgraph.graph import END, StateGraph, START
# This will be the overall state of the main graph.
# It will contain a topic (which we expect the user to provide)
# and then will generate a list of subjects, and then a joke for
# each subject
class OverallState(TypedDict):
topic: str
subjects: list
# Notice here we use the operator.add
# This is because we want combine all the jokes we generate
# from individual nodes back into one list - this is essentially
# the "reduce" part
jokes: Annotated[list, operator.add]
best_selected_joke: str
# This will be the state of the node that we will "map" all
# subjects to in order to generate a joke
class JokeState(TypedDict):
subject: str
# This is the function we will use to generate the subjects of the jokes.
# In general the length of the list generated by this node could vary each run.
def generate_topics(state: OverallState):
# Simulate a LLM.
return {"subjects": ["lions", "elephants", "penguins"]}
# Here we generate a joke, given a subject
def generate_joke(state: JokeState):
# Simulate a LLM.
joke_map = {
"lions": "Why don't lions like fast food? Because they can't catch it!",
"elephants": "Why don't elephants use computers? They're afraid of the mouse!",
"penguins": (
"Why don’t penguins like talking to strangers at parties? "
"Because they find it hard to break the ice."
),
}
return {"jokes": [joke_map[state["subject"]]]}
# Here we define the logic to map out over the generated subjects
# We will use this as an edge in the graph
def continue_to_jokes(state: OverallState):
# We will return a list of `Send` objects
# Each `Send` object consists of the name of a node in the graph
# as well as the state to send to that node
return [Send("generate_joke", {"subject": s}) for s in state["subjects"]]
# Here we will judge the best joke
def best_joke(state: OverallState):
return {"best_selected_joke": "penguins"}
# Construct the graph: here we put everything together to construct our graph
builder = StateGraph(OverallState)
builder.add_node("generate_topics", generate_topics)
builder.add_node("generate_joke", generate_joke)
builder.add_node("best_joke", best_joke)
builder.add_edge(START, "generate_topics")
builder.add_conditional_edges("generate_topics", continue_to_jokes, ["generate_joke"])
builder.add_edge("generate_joke", "best_joke")
builder.add_edge("best_joke", END)
graph = builder.compile()
# Call the graph: here we call it to generate a list of jokes
for step in graph.stream({"topic": "animals"}):
print(step)
{'generate_topics': {'subjects': ['lions', 'elephants', 'penguins']}}
{'generate_joke': {'jokes': ["Why don't lions like fast food? Because they can't catch it!"]}}
{'generate_joke': {'jokes': ["Why don't elephants use computers? They're afraid of the mouse!"]}}
{'generate_joke': {'jokes': ['Why don’t penguins like talking to strangers at parties? Because they find it hard to break the ice.']}}
{'best_joke': {'best_selected_joke': 'penguins'}}
Create and control loops¶
When creating a graph with a loop, we require a mechanism for terminating execution. This is most commonly done by adding a conditional edge that routes to the END node once we reach some termination condition.
You can also set the graph recursion limit when invoking or streaming the graph. The recursion limit sets the number of supersteps that the graph is allowed to execute before it raises an error. Read more about the concept of recursion limits here.
Let's consider a simple graph with a loop to better understand how these mechanisms work.
Tip
To return the last value of your state instead of receiving a recursion limit error, see the next section.
When creating a loop, you can include a conditional edge that specifies a termination condition:
builder = StateGraph(State)
builder.add_node(a)
builder.add_node(b)
def route(state: State) -> Literal["b", END]:
if termination_condition(state):
return END
else:
return "a"
builder.add_edge(START, "a")
builder.add_conditional_edges("a", route)
builder.add_edge("b", "a")
graph = builder.compile()
To control the recursion limit, specify "recursion_limit" in the config. This will raise a GraphRecursionError, which you can catch and handle:
from langgraph.errors import GraphRecursionError
try:
graph.invoke(inputs, {"recursion_limit": 3})
except GraphRecursionError:
print("Recursion Error")
Let's define a graph with a simple loop. Note that we use a conditional edge to implement a termination condition.
API Reference: StateGraph | START | END
import operator
from typing import Annotated, Literal
from typing_extensions import TypedDict
from langgraph.graph import StateGraph, START, END
class State(TypedDict):
# The operator.add reducer fn makes this append-only
aggregate: Annotated[list, operator.add]
def a(state: State):
print(f'Node A sees {state["aggregate"]}')
return {"aggregate": ["A"]}
def b(state: State):
print(f'Node B sees {state["aggregate"]}')
return {"aggregate": ["B"]}
# Define nodes
builder = StateGraph(State)
builder.add_node(a)
builder.add_node(b)
# Define edges
def route(state: State) -> Literal["b", END]:
if len(state["aggregate"]) < 7:
return "b"
else:
return END
builder.add_edge(START, "a")
builder.add_conditional_edges("a", route)
builder.add_edge("b", "a")
graph = builder.compile()
This architecture is similar to a ReAct agent in which node "a" is a tool-calling model, and node "b" represents the tools.
In our route conditional edge, we specify that we should end after the "aggregate" list in the state passes a threshold length.
Invoking the graph, we see that we alternate between nodes "a" and "b" before terminating once we reach the termination condition.
Node A sees []
Node B sees ['A']
Node A sees ['A', 'B']
Node B sees ['A', 'B', 'A']
Node A sees ['A', 'B', 'A', 'B']
Node B sees ['A', 'B', 'A', 'B', 'A']
Node A sees ['A', 'B', 'A', 'B', 'A', 'B']
Impose a recursion limit¶
In some applications, we may not have a guarantee that we will reach a given termination condition. In these cases, we can set the graph's recursion limit. This will raise a GraphRecursionError after a given number of supersteps. We can then catch and handle this exception:
from langgraph.errors import GraphRecursionError
try:
graph.invoke({"aggregate": []}, {"recursion_limit": 4})
except GraphRecursionError:
print("Recursion Error")
Extended example: return state on hitting recursion limit
Instead of raisingGraphRecursionError, we can introduce a new key to the state that keeps track of the number of steps remaining until reaching the recursion limit. We can then use this key to determine if we should end the run.
LangGraph implements a special RemainingSteps annotation. Under the hood, it creates a ManagedValue channel -- a state channel that will exist for the duration of our graph run and no longer.
API Reference: StateGraph | START | END
import operator
from typing import Annotated, Literal
from typing_extensions import TypedDict
from langgraph.graph import StateGraph, START, END
from langgraph.managed.is_last_step import RemainingSteps
class State(TypedDict):
# The operator.add reducer fn makes this append-only
aggregate: Annotated[list, operator.add]
remaining_steps: RemainingSteps
def a(state: State):
print(f'Node A sees {state["aggregate"]}')
return {"aggregate": ["A"]}
def b(state: State):
print(f'Node B sees {state["aggregate"]}')
return {"aggregate": ["B"]}
# Define nodes
builder = StateGraph(State)
builder.add_node(a)
builder.add_node(b)
# Define edges
def route(state: State) -> Literal["b", END]:
if state["remaining_steps"] <= 2:
return END
else:
return "b"
builder.add_edge(START, "a")
builder.add_conditional_edges("a", route)
builder.add_edge("b", "a")
graph = builder.compile()
# Test it out
result = graph.invoke({"aggregate": []}, {"recursion_limit": 4})
print(result)
Extended example: loops with branches
To better understand how the recursion limit works, let's consider a more complex example. Below we implement a loop, but one step fans out into two nodes:API Reference: StateGraph | START | END
import operator
from typing import Annotated, Literal
from typing_extensions import TypedDict
from langgraph.graph import StateGraph, START, END
class State(TypedDict):
aggregate: Annotated[list, operator.add]
def a(state: State):
print(f'Node A sees {state["aggregate"]}')
return {"aggregate": ["A"]}
def b(state: State):
print(f'Node B sees {state["aggregate"]}')
return {"aggregate": ["B"]}
def c(state: State):
print(f'Node C sees {state["aggregate"]}')
return {"aggregate": ["C"]}
def d(state: State):
print(f'Node D sees {state["aggregate"]}')
return {"aggregate": ["D"]}
# Define nodes
builder = StateGraph(State)
builder.add_node(a)
builder.add_node(b)
builder.add_node(c)
builder.add_node(d)
# Define edges
def route(state: State) -> Literal["b", END]:
if len(state["aggregate"]) < 7:
return "b"
else:
return END
builder.add_edge(START, "a")
builder.add_conditional_edges("a", route)
builder.add_edge("b", "c")
builder.add_edge("b", "d")
builder.add_edge(["c", "d"], "a")
graph = builder.compile()
Node A sees []
Node B sees ['A']
Node D sees ['A', 'B']
Node C sees ['A', 'B']
Node A sees ['A', 'B', 'C', 'D']
Node B sees ['A', 'B', 'C', 'D', 'A']
Node D sees ['A', 'B', 'C', 'D', 'A', 'B']
Node C sees ['A', 'B', 'C', 'D', 'A', 'B']
Node A sees ['A', 'B', 'C', 'D', 'A', 'B', 'C', 'D']
Combine control flow and state updates with Command¶
It can be useful to combine control flow (edges) and state updates (nodes). For example, you might want to BOTH perform state updates AND decide which node to go to next in the SAME node. LangGraph provides a way to do so by returning a Command object from node functions:
def my_node(state: State) -> Command[Literal["my_other_node"]]:
return Command(
# state update
update={"foo": "bar"},
# control flow
goto="my_other_node"
)
We show an end-to-end example below. Let's create a simple graph with 3 nodes: A, B and C. We will first execute node A, and then decide whether to go to Node B or Node C next based on the output of node A.
API Reference: StateGraph | START | Command
import random
from typing_extensions import TypedDict, Literal
from langgraph.graph import StateGraph, START
from langgraph.types import Command
# Define graph state
class State(TypedDict):
foo: str
# Define the nodes
def node_a(state: State) -> Command[Literal["node_b", "node_c"]]:
print("Called A")
value = random.choice(["a", "b"])
# this is a replacement for a conditional edge function
if value == "a":
goto = "node_b"
else:
goto = "node_c"
# note how Command allows you to BOTH update the graph state AND route to the next node
return Command(
# this is the state update
update={"foo": value},
# this is a replacement for an edge
goto=goto,
)
def node_b(state: State):
print("Called B")
return {"foo": state["foo"] + "b"}
def node_c(state: State):
print("Called C")
return {"foo": state["foo"] + "c"}
We can now create the StateGraph with the above nodes. Notice that the graph doesn't have conditional edges for routing! This is because control flow is defined with Command inside node_a.
builder = StateGraph(State)
builder.add_edge(START, "node_a")
builder.add_node(node_a)
builder.add_node(node_b)
builder.add_node(node_c)
# NOTE: there are no edges between nodes A, B and C!
graph = builder.compile()
Important
You might have noticed that we used Command as a return type annotation, e.g. Command[Literal["node_b", "node_c"]]. This is necessary for the graph rendering and tells LangGraph that node_a can navigate to node_b and node_c.
If we run the graph multiple times, we'd see it take different paths (A -> B or A -> C) based on the random choice in node A.
Navigate to a node in a parent graph¶
If you are using subgraphs, you might want to navigate from a node within a subgraph to a different subgraph (i.e. a different node in the parent graph). To do so, you can specify graph=Command.PARENT in Command:
def my_node(state: State) -> Command[Literal["my_other_node"]]:
return Command(
update={"foo": "bar"},
goto="other_subgraph", # where `other_subgraph` is a node in the parent graph
graph=Command.PARENT
)
Let's demonstrate this using the above example. We'll do so by changing node_a in the above example into a single-node graph that we'll add as a subgraph to our parent graph.
State updates with Command.PARENT
When you send updates from a subgraph node to a parent graph node for a key that's shared by both parent and subgraph state schemas, you must define a reducer for the key you're updating in the parent graph state. See the example below.
State updates with Command.PARENT
When you send updates from a subgraph node to a parent graph node for a key that's shared by both parent and subgraph state schemas, you must define a reducer for the key you're updating in the parent graph state.
import operator
from typing_extensions import Annotated
class State(TypedDict):
# NOTE: we define a reducer here
foo: Annotated[str, operator.add]
def node_a(state: State):
print("Called A")
value = random.choice(["a", "b"])
# this is a replacement for a conditional edge function
if value == "a":
goto = "node_b"
else:
goto = "node_c"
# note how Command allows you to BOTH update the graph state AND route to the next node
return Command(
update={"foo": value},
goto=goto,
# this tells LangGraph to navigate to node_b or node_c in the parent graph
# NOTE: this will navigate to the closest parent graph relative to the subgraph
graph=Command.PARENT,
)
subgraph = StateGraph(State).add_node(node_a).add_edge(START, "node_a").compile()
def node_b(state: State):
print("Called B")
# NOTE: since we've defined a reducer, we don't need to manually append
# new characters to existing 'foo' value. instead, reducer will append these
# automatically (via operator.add)
return {"foo": "b"}
def node_c(state: State):
print("Called C")
return {"foo": "c"}
builder = StateGraph(State)
builder.add_edge(START, "subgraph")
builder.add_node("subgraph", subgraph)
builder.add_node(node_b)
builder.add_node(node_c)
graph = builder.compile()
Use inside tools¶
A common use case is updating graph state from inside a tool. For example, in a customer support application you might want to look up customer information based on their account number or ID in the beginning of the conversation. To update the graph state from the tool, you can return Command(update={"my_custom_key": "foo", "messages": [...]}) from the tool:
@tool
def lookup_user_info(tool_call_id: Annotated[str, InjectedToolCallId], config: RunnableConfig):
"""Use this to look up user information to better assist them with their questions."""
user_info = get_user_info(config.get("configurable", {}).get("user_id"))
return Command(
update={
# update the state keys
"user_info": user_info,
# update the message history
"messages": [ToolMessage("Successfully looked up user information", tool_call_id=tool_call_id)]
}
)
Important
You MUST include messages (or any state key used for the message history) in Command.update when returning Command from a tool and the list of messages in messages MUST contain a ToolMessage. This is necessary for the resulting message history to be valid (LLM providers require AI messages with tool calls to be followed by the tool result messages).
If you are using tools that update state via Command, we recommend using prebuilt ToolNode which automatically handles tools returning Command objects and propagates them to the graph state. If you're writing a custom node that calls tools, you would need to manually propagate Command objects returned by the tools as the update from the node.
Visualize your graph¶
Here we demonstrate how to visualize the graphs you create.
You can visualize any arbitrary Graph, including StateGraph. Let's have some fun by drawing fractals :).
API Reference: StateGraph | START | END | add_messages
import random
from typing import Annotated, Literal
from typing_extensions import TypedDict
from langgraph.graph import StateGraph, START, END
from langgraph.graph.message import add_messages
class State(TypedDict):
messages: Annotated[list, add_messages]
class MyNode:
def __init__(self, name: str):
self.name = name
def __call__(self, state: State):
return {"messages": [("assistant", f"Called node {self.name}")]}
def route(state) -> Literal["entry_node", "__end__"]:
if len(state["messages"]) > 10:
return "__end__"
return "entry_node"
def add_fractal_nodes(builder, current_node, level, max_level):
if level > max_level:
return
# Number of nodes to create at this level
num_nodes = random.randint(1, 3) # Adjust randomness as needed
for i in range(num_nodes):
nm = ["A", "B", "C"][i]
node_name = f"node_{current_node}_{nm}"
builder.add_node(node_name, MyNode(node_name))
builder.add_edge(current_node, node_name)
# Recursively add more nodes
r = random.random()
if r > 0.2 and level + 1 < max_level:
add_fractal_nodes(builder, node_name, level + 1, max_level)
elif r > 0.05:
builder.add_conditional_edges(node_name, route, node_name)
else:
# End
builder.add_edge(node_name, "__end__")
def build_fractal_graph(max_level: int):
builder = StateGraph(State)
entry_point = "entry_node"
builder.add_node(entry_point, MyNode(entry_point))
builder.add_edge(START, entry_point)
add_fractal_nodes(builder, entry_point, 1, max_level)
# Optional: set a finish point if required
builder.add_edge(entry_point, END) # or any specific node
return builder.compile()
app = build_fractal_graph(3)
Mermaid¶
We can also convert a graph class into Mermaid syntax.
%%{init: {'flowchart': {'curve': 'linear'}}}%%
graph TD;
__start__([<p>__start__</p>]):::first
entry_node(entry_node)
node_entry_node_A(node_entry_node_A)
node_entry_node_B(node_entry_node_B)
node_node_entry_node_B_A(node_node_entry_node_B_A)
node_node_entry_node_B_B(node_node_entry_node_B_B)
node_node_entry_node_B_C(node_node_entry_node_B_C)
__end__([<p>__end__</p>]):::last
__start__ --> entry_node;
entry_node --> __end__;
entry_node --> node_entry_node_A;
entry_node --> node_entry_node_B;
node_entry_node_B --> node_node_entry_node_B_A;
node_entry_node_B --> node_node_entry_node_B_B;
node_entry_node_B --> node_node_entry_node_B_C;
node_entry_node_A -.-> entry_node;
node_entry_node_A -.-> __end__;
node_node_entry_node_B_A -.-> entry_node;
node_node_entry_node_B_A -.-> __end__;
node_node_entry_node_B_B -.-> entry_node;
node_node_entry_node_B_B -.-> __end__;
node_node_entry_node_B_C -.-> entry_node;
node_node_entry_node_B_C -.-> __end__;
classDef default fill:#f2f0ff,line-height:1.2
classDef first fill-opacity:0
classDef last fill:#bfb6fc
PNG¶
If preferred, we could render the Graph into a .png. Here we could use three options:
- Using Mermaid.ink API (does not require additional packages)
- Using Mermaid + Pyppeteer (requires
pip install pyppeteer) - Using graphviz (which requires
pip install graphviz)
Using Mermaid.Ink
By default, draw_mermaid_png() uses Mermaid.Ink's API to generate the diagram.
API Reference: CurveStyle | MermaidDrawMethod | NodeStyles
from IPython.display import Image, display
from langchain_core.runnables.graph import CurveStyle, MermaidDrawMethod, NodeStyles
display(
Image(
app.get_graph().draw_mermaid_png(
draw_method=MermaidDrawMethod.API,
)
)
)
Using Mermaid + Pyppeteer
import nest_asyncio
nest_asyncio.apply() # Required for Jupyter Notebook to run async functions
display(
Image(
app.get_graph().draw_mermaid_png(
curve_style=CurveStyle.LINEAR,
node_colors=NodeStyles(first="#ffdfba", last="#baffc9", default="#fad7de"),
wrap_label_n_words=9,
output_file_path=None,
draw_method=MermaidDrawMethod.PYPPETEER,
background_color="white",
padding=10,
)
)
)
Using Graphviz
try:
display(Image(app.get_graph().draw_png()))
except ImportError:
print(
"You likely need to install dependencies for pygraphviz, see more here https://github.com/pygraphviz/pygraphviz/blob/main/INSTALL.txt"
)
