diff --git a/surfsense_backend/app/services/chat_session_state_service.py b/surfsense_backend/app/services/chat_session_state_service.py
new file mode 100644
index 000000000..d82fff3a7
--- /dev/null
+++ b/surfsense_backend/app/services/chat_session_state_service.py
@@ -0,0 +1,65 @@
+"""
+Service layer for chat session state (live collaboration).
+"""
+
+from datetime import UTC, datetime
+from uuid import UUID
+
+from sqlalchemy import select
+from sqlalchemy.dialects.postgresql import insert
+from sqlalchemy.ext.asyncio import AsyncSession
+from sqlalchemy.orm import selectinload
+
+from app.db import ChatSessionState
+
+
+async def get_session_state(
+    session: AsyncSession,
+    thread_id: int,
+) -> ChatSessionState | None:
+    """Get the current session state for a thread."""
+    result = await session.execute(
+        select(ChatSessionState)
+        .options(selectinload(ChatSessionState.ai_responding_to_user))
+        .filter(ChatSessionState.thread_id == thread_id)
+    )
+    return result.scalar_one_or_none()
+
+
+async def set_ai_responding(
+    session: AsyncSession,
+    thread_id: int,
+    user_id: UUID,
+) -> ChatSessionState:
+    """Mark AI as responding to a specific user. Uses upsert for atomicity."""
+    now = datetime.now(UTC)
+    upsert_query = insert(ChatSessionState).values(
+        thread_id=thread_id,
+        ai_responding_to_user_id=user_id,
+        updated_at=now,
+    )
+    upsert_query = upsert_query.on_conflict_do_update(
+        index_elements=["thread_id"],
+        set_={
+            "ai_responding_to_user_id": user_id,
+            "updated_at": now,
+        },
+    )
+    await session.execute(upsert_query)
+    await session.commit()
+
+    return await get_session_state(session, thread_id)
+
+
+async def clear_ai_responding(
+    session: AsyncSession,
+    thread_id: int,
+) -> ChatSessionState | None:
+    """Clear AI responding state when response is complete."""
+    state = await get_session_state(session, thread_id)
+    if state:
+        state.ai_responding_to_user_id = None
+        state.updated_at = datetime.now(UTC)
+        await session.commit()
+        await session.refresh(state)
+    return state