diff --git a/python/samples/getting_started/chat_store/third_party_chat_store_cosmos_db.py b/python/samples/getting_started/chat_store/third_party_chat_store_cosmos_db.py
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+++ b/python/samples/getting_started/chat_store/third_party_chat_store_cosmos_db.py
@@ -0,0 +1,187 @@
+# Copyright (c) Microsoft. All rights reserved.
+
+import os
+from time import time
+from uuid import uuid4
+from typing import Sequence, Any
+import enum, datetime, uuid
+from pydantic import BaseModel
+from azure.cosmos.aio import CosmosClient
+from agent_framework import ChatMessage, ChatAgent
+from agent_framework.azure import AzureOpenAIChatClient
+from azure.identity.aio import DefaultAzureCredential
+
+"""Cosmos DB Chat Message Store Example
+
+Demonstrates how to store and retrieve chat history using Azure Cosmos DB
+as an external message store for the Microsoft Agent Framework.
+
+Scenarios:
+  1) Persist chat messages in Cosmos DB with thread-based partitioning.
+  2) Retrieve messages in chronological order for conversation continuity.
+
+Requirements:
+  - Azure Cosmos DB with an existing database and container.
+  - Container partition key must be /thread_id.
+  - Environment variables:
+      COSMOS_DB_ENDPOINT,
+      AZURE_OPENAI_ENDPOINT, AZURE_OPENAI_API_KEY,
+      AZURE_OPENAI_API_VERSION, AZURE_OPENAI_CHAT_DEPLOYMENT_NAME
+  - Dependencies:
+      pip install azure-cosmos "agent-framework"
+"""
+
+class CosmosDBStoreState(BaseModel):
+    """Serializable state for CosmosDB chat message store."""
+    thread_id: str
+    database_name: str = "agent_framework"
+    container_name: str = "chat_messages"
+
+
+class CosmosDBChatMessageStore:
+    """
+    Lightweight Cosmos DB-backed chat history store for Microsoft Agent Framework.
+
+    This implementation:
+        - Uses Azure AD authentication with an injected CosmosClient.
+        - Stores one conversation thread per partition (partition key = /thread_id).
+        - Appends messages and retrieves them chronologically.
+        - Uses ChatMessage.to_dict() and ChatMessage.from_dict() for serialization.
+        - Assumes database and container already exist.
+    """
+
+    def __init__(
+        self,
+        cosmos_client: CosmosClient,
+        *,
+        thread_id: str | None = None,
+        database_name: str = "agent_framework",
+        container_name: str = "chat_messages",
+    ) -> None:
+        self.thread_id = thread_id or f"thread_{uuid4()}"
+        self.database_name = database_name
+        self.container_name = container_name
+        self._client: CosmosClient | None = cosmos_client
+        self._container = None
+        self._ready = False
+
+    async def _ensure(self) -> None:
+        if self._ready:
+            return
+        if self._client is None:
+            raise RuntimeError("CosmosDBChatMessageStore requires an injected CosmosClient.")
+        db = self._client.get_database_client(self.database_name)
+        self._container = db.get_container_client(self.container_name)
+        self._ready = True
+
+
+    async def add_messages(self, messages: Sequence[ChatMessage]) -> None:
+        """Persist new chat messages for the current thread."""
+        if not messages:
+            return
+        await self._ensure()
+        for msg in messages:
+            doc = {
+                "id": f"{self.thread_id}_{uuid4()}",
+                "thread_id": self.thread_id,
+                "ts": time(),
+                "message": self._to_dict(msg),
+            }
+            await self._container.upsert_item(doc)
+
+    async def list_messages(self) -> list[ChatMessage]:
+        """Retrieve all chat messages for the thread in chronological order."""
+        await self._ensure()
+        query = """
+            SELECT c.message FROM c
+            WHERE c.thread_id = @thread_id
+            ORDER BY c.ts ASC
+        """
+        params = [{"name": "@thread_id", "value": self.thread_id}]
+        messages: list[ChatMessage] = []
+        async for row in self._container.query_items(
+            query=query, parameters=params, partition_key=self.thread_id
+        ):
+            messages.append(self._from_dict(row["message"]))
+        return messages
+
+    async def clear(self) -> None:
+        """Delete all chat messages for the current thread."""
+        await self._ensure()
+        query = "SELECT c.id FROM c WHERE c.thread_id = @thread_id"
+        params = [{"name": "@thread_id", "value": self.thread_id}]
+        async for row in self._container.query_items(
+            query=query, parameters=params, partition_key=self.thread_id
+        ):
+            await self._container.delete_item(row["id"], partition_key=self.thread_id)
+
+    async def serialize_state(self, **kwargs: Any) -> dict:
+        """Serialize the store configuration for persistent thread state."""
+        return CosmosDBStoreState(
+            thread_id=self.thread_id,
+            database_name=self.database_name,
+            container_name=self.container_name,
+        ).model_dump(**kwargs)
+
+    async def deserialize_state(self, state: dict | None, **_: Any) -> None:
+        """Restore store configuration from serialized thread state."""
+        if not state:
+            return
+
+        s = CosmosDBStoreState.model_validate(state)
+        self.thread_id = s.thread_id
+
+        if s.database_name != self.database_name or s.container_name != self.container_name:
+            self.database_name = s.database_name
+            self.container_name = s.container_name
+            self._ready = False
+
+    def _to_dict(self, message: ChatMessage) -> dict:
+        """Convert ChatMessage into a JSON-safe dictionary."""
+        return message.to_dict() if hasattr(message, "to_dict") else vars(message)
+
+    def _from_dict(self, data: dict) -> ChatMessage:
+        """Reconstruct a ChatMessage from a stored dictionary."""
+        return ChatMessage.from_dict(data)
+
+async def main() -> None:
+    """Demonstration of CosmosDBChatMessageStore with ChatAgent."""
+    
+    credential = DefaultAzureCredential()
+
+    cosmos_client = CosmosClient(
+        url=os.getenv("COSMOS_DB_ENDPOINT"),
+        credential=credential
+    )
+        
+    store = CosmosDBChatMessageStore(
+        cosmos_client=cosmos_client,
+        database_name="agent-chat-conversation",
+        container_name="chat_messages",
+    )
+
+    chat_client = AzureOpenAIChatClient(
+        deployment_name=os.getenv("AZURE_OPENAI_CHAT_DEPLOYMENT_NAME"),
+        api_key=os.getenv("AZURE_OPENAI_API_KEY"),
+        endpoint=os.getenv("AZURE_OPENAI_ENDPOINT"),
+        api_version=os.getenv("AZURE_OPENAI_API_VERSION"),
+    )
+
+    agent = ChatAgent(
+        chat_client=chat_client,
+        name="Joker",
+        instructions="You are good at telling jokes.",
+        chat_message_store_factory=lambda: store,
+    )
+
+    try:
+        thread = agent.get_new_thread()
+        await agent.run("Tell me a pirate joke.", thread=thread)
+        await agent.run("One more!", thread=thread)
+    finally:
+        await cosmos_client.close()
+        await credential.close()
+
+if __name__ == "__main__":
+    import asyncio
+    asyncio.run(main())
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