diff --git a/src/hqt/sessions/service.py b/src/hqt/sessions/service.py
index 44fdf62..0294371 100644
--- a/src/hqt/sessions/service.py
+++ b/src/hqt/sessions/service.py
@@ -440,6 +440,33 @@ class SessionService:
             raise ServiceError(f"{window_name!r} is not an hqt session window")
         return await self.open_tool_window(session_id, tool)
 
+    async def clone_session_for_window(
+        self, window_name: str
+    ) -> "CreateSessionResult":
+        """Open a fresh harness session cloning the one in `window_name`.
+
+        Same project, harness, and model as the source session, but a brand-new
+        conversation (a new hqt-<id> window at the next index). Raises
+        ServiceError if `window_name` is not an active hqt session window — so
+        invoking clone from a tool window (nvim/shell) or the TUI is a clean
+        no-op.
+        """
+        with self.factory() as db:
+            sess = (
+                db.query(Session)
+                .options(selectinload(Session.harness))
+                .filter_by(tmux_session_name=window_name, archived=False)
+                .first()
+            )
+            if sess is None:
+                raise ServiceError(f"{window_name!r} is not an hqt session window")
+            project_id = sess.project_id
+            harness_name = sess.harness.name
+            model = sess.model
+        return await self.create_session(
+            project_id, harness_name, nickname=None, model=model
+        )
+
     async def stop_session(self, session_id: int) -> None:
         with self.factory() as db:
             sess = db.get(Session, session_id)
diff --git a/tests/test_sessions.py b/tests/test_sessions.py
index 12d5563..ee2bf0b 100644
--- a/tests/test_sessions.py
+++ b/tests/test_sessions.py
@@ -1601,3 +1601,36 @@ async def test_open_tool_window_for_window_resolves_by_name(
 async def test_open_tool_window_for_window_unknown_window_raises(service):
     with pytest.raises(ServiceError):
         await service.open_tool_window_for_window("not-an-hqt-window", "nvim")
+
+
+# ---------------------------------------------------------------------------
+# Task 5: clone_session_for_window
+# ---------------------------------------------------------------------------
+
+
+@pytest.mark.asyncio
+async def test_clone_session_for_window_reuses_project_harness_model(
+    service, db, monkeypatch
+):
+    await service.create_session(
+        project_id=1, harness_name="claude-code", nickname="orig", model="opus"
+    )
+    captured = {}
+
+    async def fake_create(project_id, harness_name, nickname=None, model=None):
+        captured["args"] = (project_id, harness_name, nickname, model)
+        return "SENTINEL"
+
+    monkeypatch.setattr(service, "create_session", fake_create)
+
+    result = await service.clone_session_for_window("hqt-1")
+
+    assert result == "SENTINEL"
+    # Same project + harness + model; a fresh sibling, so no nickname.
+    assert captured["args"] == (1, "claude-code", None, "opus")
+
+
+@pytest.mark.asyncio
+async def test_clone_session_for_window_unknown_window_raises(service):
+    with pytest.raises(ServiceError):
+        await service.clone_session_for_window("not-an-hqt-window")