diff --git a/services/collab_doc/Dockerfile b/services/collab_doc/Dockerfile
new file mode 100644
index 000000000..2a6ede7d7
--- /dev/null
+++ b/services/collab_doc/Dockerfile
@@ -0,0 +1,13 @@
+FROM python:3.12-slim
+
+WORKDIR /app
+
+COPY requirements.txt /app/requirements.txt
+RUN pip install --no-cache-dir -r /app/requirements.txt
+
+COPY app /app/app
+
+EXPOSE 8000
+
+CMD ["uvicorn", "app.main:app", "--host", "0.0.0.0", "--port", "8000"]
+
diff --git a/services/collab_doc/README.md b/services/collab_doc/README.md
new file mode 100644
index 000000000..6cf72f5f5
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+++ b/services/collab_doc/README.md
@@ -0,0 +1,19 @@
+# Collab Doc (MVP)
+
+This is a minimal realtime collaboration **relay** server used by `webui/collab_doc.html`.
+
+- It does **not** store documents yet (in-memory only, and the CRDT state lives in clients).
+- It simply forwards websocket text/binary messages to other peers in the same `{doc_id}` room.
+
+## Run locally (docker-compose)
+
+If you use `docker/run/docker-compose.yml`, a `collab-doc` service is exposed on port `50081`.
+
+Health check:
+
+`http://localhost:50081/health`
+
+Websocket endpoint:
+
+`ws://localhost:50081/ws/<doc_id>`
+
diff --git a/services/collab_doc/app/main.py b/services/collab_doc/app/main.py
new file mode 100644
index 000000000..f76072bc8
--- /dev/null
+++ b/services/collab_doc/app/main.py
@@ -0,0 +1,83 @@
+from __future__ import annotations
+
+import asyncio
+from collections import defaultdict
+from dataclasses import dataclass
+from typing import DefaultDict, Dict, Set
+
+from fastapi import FastAPI, WebSocket, WebSocketDisconnect
+from fastapi.middleware.cors import CORSMiddleware
+
+
+@dataclass(frozen=True)
+class Client:
+    websocket: WebSocket
+
+
+app = FastAPI(title="Collab Doc Relay", version="0.1.0")
+
+# For browser-based clients hosted on a different port.
+app.add_middleware(
+    CORSMiddleware,
+    allow_origins=["*"],
+    allow_credentials=False,
+    allow_methods=["*"],
+    allow_headers=["*"],
+)
+
+
+# doc_id -> set of connected clients
+_rooms: DefaultDict[str, Set[Client]] = defaultdict(set)
+_locks: DefaultDict[str, asyncio.Lock] = defaultdict(asyncio.Lock)
+
+
+@app.get("/health")
+def health() -> Dict[str, str]:
+    return {"ok": "true"}
+
+
+async def _broadcast(doc_id: str, sender: Client, message: str | bytes) -> None:
+    # Snapshot recipients under lock, then send without holding lock.
+    async with _locks[doc_id]:
+        recipients = [c for c in _rooms[doc_id] if c != sender]
+
+    # Best-effort fanout: a failed client doesn't block others.
+    for c in recipients:
+        try:
+            if isinstance(message, bytes):
+                await c.websocket.send_bytes(message)
+            else:
+                await c.websocket.send_text(message)
+        except Exception:
+            # Ignore send errors; disconnect handling will clean up eventually.
+            pass
+
+
+@app.websocket("/ws/{doc_id}")
+async def ws_doc(doc_id: str, websocket: WebSocket) -> None:
+    await websocket.accept()
+    client = Client(websocket=websocket)
+
+    async with _locks[doc_id]:
+        _rooms[doc_id].add(client)
+
+    try:
+        while True:
+            msg = await websocket.receive()
+            if msg.get("bytes") is not None:
+                await _broadcast(doc_id, client, msg["bytes"])
+            elif msg.get("text") is not None:
+                await _broadcast(doc_id, client, msg["text"])
+            else:
+                # Ignore ping/pong or other frames we don't care about.
+                pass
+    except WebSocketDisconnect:
+        pass
+    finally:
+        async with _locks[doc_id]:
+            _rooms[doc_id].discard(client)
+            if not _rooms[doc_id]:
+                # Cleanup empty rooms to avoid unbounded growth.
+                _rooms.pop(doc_id, None)
+                _locks.pop(doc_id, None)
+
diff --git a/services/collab_doc/requirements.txt b/services/collab_doc/requirements.txt
new file mode 100644
index 000000000..ad4ca5437
--- /dev/null
+++ b/services/collab_doc/requirements.txt
@@ -0,0 +1,2 @@
+fastapi==0.115.6
+uvicorn[standard]==0.32.1