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skills/langgraph-master/01_core_concepts_state.md

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+# State
+
+Memory shared across all nodes in the graph.
+
+## Overview
+
+State is like a "notebook" that records everything the agent learns and decides. It is a **shared data structure** accessible to all nodes and edges in the graph.
+
+## Definition Methods
+
+### Using TypedDict
+
+```python
+from typing import TypedDict
+
+class State(TypedDict):
+    messages: list[str]
+    user_name: str
+    count: int
+```
+
+### Using Pydantic Model
+
+```python
+from pydantic import BaseModel
+
+class State(BaseModel):
+    messages: list[str]
+    user_name: str
+    count: int = 0  # Default value
+```
+
+## Reducer (Controlling Update Methods)
+
+A function that specifies how each key is updated. If not specified, it defaults to **value overwrite**.
+
+### Addition (Adding to List)
+
+```python
+from typing import Annotated
+from operator import add
+
+class State(TypedDict):
+    messages: Annotated[list[str], add]  # Add to existing list
+    count: int  # Overwrite
+```
+
+### Custom Reducer
+
+```python
+def concat_strings(existing: str, new: str) -> str:
+    return existing + " " + new
+
+class State(TypedDict):
+    text: Annotated[str, concat_strings]
+```
+
+## MessagesState (LLM Preset)
+
+For LLM conversations, LangChain's `MessagesState` is convenient:
+
+```python
+from langgraph.graph import MessagesState
+
+# This is equivalent to:
+class MessagesState(TypedDict):
+    messages: Annotated[list[AnyMessage], add_messages]
+```
+
+The `add_messages` reducer:
+- Adds new messages
+- Updates existing messages (ID-based)
+- Supports OpenAI format shorthand
+
+## Important Principles
+
+1. **Store Raw Data**: Format prompts within nodes
+2. **Clear Schema**: Define types with TypedDict or Pydantic
+3. **Control with Reducer**: Explicitly specify update methods
+
+## Example
+
+```python
+from typing import Annotated, TypedDict
+from operator import add
+
+class AgentState(TypedDict):
+    # Messages are added to the list
+    messages: Annotated[list[str], add]
+
+    # User information is overwritten
+    user_id: str
+    user_name: str
+
+    # Counter is also overwritten
+    iteration_count: int
+```
+
+## Related Pages
+
+- [01_core_concepts_node.md](01_core_concepts_node.md) - How to use State in nodes
+- [03_memory_management_overview.md](03_memory_management_overview.md) - State persistence