docs(langgraph): add docs on Overwrite (#1202)

Add docs on new LangGraph `Overwrite` type, which is used to bypass a
reducer in a BinaryOperatorAggregate channel and write directly to the
channel.

[Overwrite LG
permalink](https://github.com/langchain-ai/langgraph/blob/c1661dd07fa6dca871e327b48caefc585e48e36e/libs/langgraph/langgraph/types.py#L522)

There is a problem with rendering the Overwrite example in the reference
doc:

<img width="500" alt="image"
src="https://github.com/user-attachments/assets/a9a6dc63-93f2-43f6-95f7-b7a9eeb1e168"
/>

Author: casparb

reference/python/docs/langgraph/types.md

@@ -11,4 +11,5 @@
         - StateSnapshot
         - Send
         - Command
+        - Overwrite
         - interrupt

src/oss/langgraph/graph-api.mdx

@@ -287,6 +287,13 @@ const State = z.object({
 In this example, we've used [Zod 4 registries](https://zod.dev/metadata) to specify a reducer function for the second key (`bar`). Note that the first key remains unchanged. Let's assume the input to the graph is `{ foo: 1, bar: ["hi"] }`. Let's then assume the first `Node` returns `{ foo: 2 }`. This is treated as an update to the state. Notice that the `Node` does not need to return the whole `State` schema - just an update. After applying this update, the `State` would then be `{ foo: 2, bar: ["hi"] }`. If the second node returns `{ bar: ["bye"] }` then the `State` would then be `{ foo: 2, bar: ["hi", "bye"] }`. Notice here that the `bar` key is updated by adding the two arrays together.
 :::
 
+:::python
+#### Overwrite
+<Tip>
+In some cases, you may want to bypass a reducer and directly overwrite a state value. LangGraph provides the [`Overwrite`](https://reference.langchain.com/python/langgraph/types/) type for this purpose. [Learn how to use `Overwrite` here](/oss/langgraph/use-graph-api#bypass-reducers-with-overwrite).
+</Tip>
+:::
+
 ### Working with Messages in Graph State
 
 #### Why use messages?

src/oss/langgraph/use-graph-api.mdx

@@ -446,6 +446,58 @@ const State = z.object({
 ```
 :::
 
+:::python
+### Bypass reducers with `Overwrite`
+
+In some cases, you may want to bypass a reducer and directly overwrite a state value. LangGraph provides the [`Overwrite`](https://reference.langchain.com/python/langgraph/types/) type for this purpose. When a node returns a value wrapped with `Overwrite`, the reducer is bypassed and the channel is set directly to that value.
+
+This is useful when you want to reset or replace accumulated state rather than merge it with existing values.
+
+```python
+from langgraph.graph import StateGraph, START, END
+from langgraph.types import Overwrite
+from typing_extensions import Annotated, TypedDict
+import operator
+
+class State(TypedDict):
+    messages: Annotated[list, operator.add]
+
+def add_message(state: State):
+    return {"messages": ["first message"]}
+
+def replace_messages(state: State):
+    # Bypass the reducer and replace the entire messages list
+    return {"messages": Overwrite(["replacement message"])}
+
+builder = StateGraph(State)
+builder.add_node("add_message", add_message)
+builder.add_node("replace_messages", replace_messages)
+builder.add_edge(START, "add_message")
+builder.add_edge("add_message", "replace_messages")
+builder.add_edge("replace_messages", END)
+
+graph = builder.compile()
+
+result = graph.invoke({"messages": ["initial"]})
+print(result["messages"])
+```
+
+```
+['replacement message']
+```
+
+You can also use JSON format with the special key `"__overwrite__"`:
+
+```python
+def replace_messages(state: State):
+    return {"messages": {"__overwrite__": ["replacement message"]}}
+```
+
+<Warning>
+When nodes execute in parallel, only one node can use `Overwrite` on the same state key in a given super-step. If multiple nodes attempt to overwrite the same key in the same super-step, an `InvalidUpdateError` will be raised.
+</Warning>
+:::
+
 ### 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.