diff --git a/src/deep_agents_from_scratch/deep_agent.py b/src/deep_agents_from_scratch/deep_agent.py
new file mode 100644
index 0000000..6ae6cfe
--- /dev/null
+++ b/src/deep_agents_from_scratch/deep_agent.py
@@ -0,0 +1,194 @@
+"""Deep Agent implementation based on LangGraph StateGraph.
+
+The agent follows the architecture described in the original
+`deep-agents-from-scratch` notebook but is rewritten to satisfy the
+current project constraints:
+
+* All imports use the modern LangChain modules.
+* The graph is fully connected – the final node writes the virtual
+  files to disk.
+* A lightweight in‑memory virtual file system is used during the
+  conversation.
+* The search tool uses Tavily and returns only a minimal summary.
+* Summarization is performed by a small model (GPT‑4o‑mini).
+
+The module exposes a single helper ``build_deep_agent_graph`` which
+returns a ready‑to‑run :class:`StateGraph` instance.
+"""
+
+from __future__ import annotations
+
+from pathlib import Path
+from typing import Dict, List, Any
+
+# Modern LangChain imports
+from langchain_core.messages import BaseMessage, HumanMessage
+from langgraph.types import StateGraph
+
+# Local imports – the tools are defined in ``research_tools.py``
+from .research_tools import (
+    tavily_search,
+    think_tool,
+    summarize_webpage_content,
+)
+from .state import DeepAgentState
+
+# ---------------------------------------------------------------------------
+# Helper: virtual file system
+# ---------------------------------------------------------------------------
+class VirtualFileSystem:
+    """Simple in‑memory file system used by the graph.
+
+    The state already contains a ``files`` mapping, but the graph
+    interacts with this helper to keep the node implementations
+    clean.
+    """
+
+    def __init__(self, files: Dict[str, str]):
+        self._files = files
+
+    def write(self, filename: str, content: str) -> None:
+        self._files[filename] = content
+
+    def read(self, filename: str) -> str | None:
+        return self._files.get(filename)
+
+    def list(self) -> List[str]:
+        return list(self._files.keys())
+
+    def dump_to_disk(self, root: Path) -> None:
+        root.mkdir(parents=True, exist_ok=True)
+        for name, data in self._files.items():
+            (root / name).write_text(data, encoding="utf-8")
+
+# ---------------------------------------------------------------------------
+# Graph nodes
+# ---------------------------------------------------------------------------
+async def think_node(state: DeepAgentState) -> Dict[str, Any]:
+    """Ask the agent to decide what to search next.
+
+    The node receives the current conversation and returns a new
+    query string.
+    """
+    messages = state.messages
+    # Find the last user message
+    last_user = None
+    for msg in reversed(messages):
+        if msg.role == "user":
+            last_user = msg.content
+            break
+    if not last_user:
+        last_user = ""
+    # The tool expects a plain string
+    query = await think_tool.invoke(last_user)
+    return {"query": query}
+
+async def search_node(state: DeepAgentState) -> Dict[str, Any]:
+    """Perform a web search using the tavily tool.
+
+    The node receives the ``query`` from the previous node and returns
+    the raw search results.
+    """
+    query = state.query
+    if not query:
+        return {"search_results": []}
+    results = await tavily_search.invoke(query)
+    return {"search_results": results}
+
+async def summarize_node(state: DeepAgentState) -> Dict[str, Any]:
+    """Summarize each search result and store the content in the virtual FS.
+
+    The node writes full webpage content to the virtual file system and
+    returns a list of summary strings.
+    """
+    results = state.search_results
+    summaries: List[str] = []
+    vfs = VirtualFileSystem(state.files)
+    for i, res in enumerate(results):
+        content = res.get("content", "")
+        if not content:
+            continue
+        summary_obj = await summarize_webpage_content.invoke(content)
+        filename = f"search_result_{i+1}.md"
+        vfs.write(filename, content)
+        summaries.append(summary_obj["summary"])
+    return {"summaries": summaries}
+
+async def write_file_node(state: DeepAgentState) -> Dict[str, Any]:
+    """Write all virtual files to disk and add a final message to the chat.
+
+    The node is the final node of the graph. It writes the files to a
+    directory named ``output`` relative to the current working
+    directory.
+    """
+    vfs = VirtualFileSystem(state.files)
+    output_dir = Path("output")
+    vfs.dump_to_disk(output_dir)
+    final_msg = f"Files written to {output_dir.resolve()}"
+    # Append the final message to the conversation history
+    return {"messages": [HumanMessage(content=final_msg)]}
+
+# ---------------------------------------------------------------------------
+# Graph construction
+# ---------------------------------------------------------------------------
+def build_deep_agent_graph() -> StateGraph[DeepAgentState]:
+    """Build and return the StateGraph.
+
+    The graph follows the sequence:
+    1. think → 2. search → 3. summarize → 4. write_file
+    """
+    graph = StateGraph(DeepAgentState)
+
+    # Add nodes
+    graph.add_node("think", think_node)
+    graph.add_node("search", search_node)
+    graph.add_node("summarize", summarize_node)
+    graph.add_node("write_file", write_file_node)
+
+    # Connect nodes
+    graph.set_entry_point("think")
+    graph.add_edge("think", "search")
+    graph.add_edge("search", "summarize")
+    graph.add_edge("summarize", "write_file")
+
+    return graph
+
+# ---------------------------------------------------------------------------
+# Helper to run the graph with an initial user message
+# ---------------------------------------------------------------------------
+async def run_agent(user_input: str) -> str:
+    """Convenience wrapper that runs the graph for a single turn.
+
+    Parameters
+    ----------
+    user_input: str
+        The user message to start the conversation.
+
+    Returns
+    -------
+    str
+        The final message produced by the graph.
+    """
+    graph = build_deep_agent_graph()
+    # Initialise state
+    state: DeepAgentState = DeepAgentState(
+        messages=[HumanMessage(content=user_input)],
+        files={},
+        query=None,
+        search_results=[],
+        summaries=[],
+    )
+    # Run the graph – ``invoke`` returns the final state
+    final_state = await graph.invoke(state)
+    # The final message was appended to the conversation
+    return final_state.messages[-1].content
+
+# ---------------------------------------------------------------------------
+# CLI entry point
+# ---------------------------------------------------------------------------
+if __name__ == "__main__":
+    import asyncio
+
+    user_msg = input("Enter your research question: ")
+    result = asyncio.run(run_agent(user_msg))
+    print(result)
\ No newline at end of file