diff --git a/src/open_cups/application_state.py b/src/open_cups/application_state.py
index 1d21dd6..cb0653e 100644
--- a/src/open_cups/application_state.py
+++ b/src/open_cups/application_state.py
@@ -15,6 +15,12 @@ def get_session_room(self, session_id: str) -> Room | None:
                 return room
         return None
 
+    def is_session_live(self, session_id: str, timeout_seconds: float) -> bool:
+        room = self.get_session_room(session_id)
+        if room is None:
+            return False
+        return room.is_session_live(session_id, timeout_seconds)
+
     def create_room(self, room_id: str, session_id: str) -> None:
         room = Room(room_id, session_id)
         self.rooms[room_id] = room
diff --git a/src/open_cups/room.py b/src/open_cups/room.py
index fb55ca5..f4054c6 100644
--- a/src/open_cups/room.py
+++ b/src/open_cups/room.py
@@ -47,6 +47,14 @@ def has_session(self, session_id: str) -> bool:
             return True
         return bool(self.is_host(session_id))
 
+    def is_session_live(self, session_id: str, timeout_seconds: float) -> bool:
+        current_time = time.time()
+        if self.is_host(session_id):
+            return current_time - self._host_last_seen <= timeout_seconds
+        if session_id not in self._sessions:
+            return False
+        return current_time - self._sessions[session_id].last_seen <= timeout_seconds
+
     def get_participants_by_activity(
         self,
         inactivity_timeout_seconds: float,
diff --git a/src/open_cups/session_state.py b/src/open_cups/session_state.py
index 626c159..3b9ad53 100644
--- a/src/open_cups/session_state.py
+++ b/src/open_cups/session_state.py
@@ -2,20 +2,40 @@
 
 import streamlit as st
 
+from open_cups.application_state import ApplicationState
+
+# A live session refreshes its last_seen every autorefresh tick (~2s), so this
+# threshold tolerates a couple of missed ticks before treating it as stale.
+SESSION_LIVENESS_TIMEOUT_SECONDS = 10
+
 
 class SessionState:
     """Per-user session state wrapper.
 
-    See https://docs.streamlit.io/develop/api-reference/caching-and-state/st.session_state
-    for more details.
+    The session id is kept in the URL so it survives a websocket drop (a phone
+    locking, a page reload). To stop a copy-pasted link from cloning identity,
+    a session id from the URL is only adopted when that session is currently
+    stale (a genuine reconnect); if it is still live, the visitor is treated as
+    a new user. See https://github.com/BayerC/open_cups/issues/165.
     """
 
-    def __init__(self) -> None:
+    def __init__(self, application_state: ApplicationState) -> None:
         if "session_id" not in st.session_state:
-            existing = st.query_params.get("session_id")
-            st.session_state.session_id = existing or str(uuid.uuid4())
+            st.session_state.session_id = _resolve_session_id(application_state)
         st.query_params["session_id"] = st.session_state.session_id
 
     @property
     def session_id(self) -> str:
         return str(st.session_state.session_id)
+
+
+def _resolve_session_id(application_state: ApplicationState) -> str:
+    url_session_id = st.query_params.get("session_id")
+    if url_session_id is None:
+        return str(uuid.uuid4())
+    if application_state.is_session_live(
+        url_session_id,
+        SESSION_LIVENESS_TIMEOUT_SECONDS,
+    ):
+        return str(uuid.uuid4())
+    return url_session_id
diff --git a/src/open_cups/state_provider.py b/src/open_cups/state_provider.py
index d93b0f3..fb1f538 100644
--- a/src/open_cups/state_provider.py
+++ b/src/open_cups/state_provider.py
@@ -105,7 +105,7 @@ def cleanup_all(self) -> None:
 class Context:
     def __init__(self) -> None:
         self.application_state: ApplicationState = self._get_application_state()
-        self.session_state = SessionState()
+        self.session_state = SessionState(self.application_state)
 
     @staticmethod
     @st.cache_resource
diff --git a/tests/unit/test_room.py b/tests/unit/test_room.py
index 73b77e7..b194842 100644
--- a/tests/unit/test_room.py
+++ b/tests/unit/test_room.py
@@ -104,6 +104,41 @@ def test_integration_with_stats_tracker(monkeypatch: pytest.MonkeyPatch) -> None
     assert room.get_status_history() == []
 
 
+def test_is_session_live_true_for_recent_host(mock_time: MockTime) -> None:
+    mock_time.current_time = 0.0
+    room = Room("room-id", "host-id")
+
+    mock_time.current_time = 5.0
+
+    assert room.is_session_live("host-id", timeout_seconds=10)
+
+
+def test_is_session_live_true_for_recent_client(mock_time: MockTime) -> None:
+    mock_time.current_time = 0.0
+    room = Room("room-id", "host-id")
+    room.set_session_status("user", UserStatus.GREEN)
+
+    mock_time.current_time = 5.0
+
+    assert room.is_session_live("user", timeout_seconds=10)
+
+
+def test_is_session_live_false_for_stale_client(mock_time: MockTime) -> None:
+    mock_time.current_time = 0.0
+    room = Room("room-id", "host-id")
+    room.set_session_status("user", UserStatus.GREEN)
+
+    mock_time.current_time = 20.0
+
+    assert not room.is_session_live("user", timeout_seconds=10)
+
+
+def test_is_session_live_false_for_unknown_session() -> None:
+    room = Room("room-id", "host-id")
+
+    assert not room.is_session_live("unknown-id", timeout_seconds=10)
+
+
 def test_get_participants_by_activity_separates_active_and_inactive(
     mock_time: MockTime,
 ) -> None:
diff --git a/tests/unit/test_session_state.py b/tests/unit/test_session_state.py
new file mode 100644
index 0000000..fbd2afb
--- /dev/null
+++ b/tests/unit/test_session_state.py
@@ -0,0 +1,40 @@
+from streamlit.testing.v1 import AppTest
+
+from tests.bdd.fixture import run_wrapper
+from tests.bdd.test_helper import get_room_id
+
+
+def _resolved_session_id(app: AppTest) -> str:
+    value = app.query_params["session_id"]
+    resolved = value[0] if isinstance(value, list) else value
+    return str(resolved)
+
+
+def test_fresh_visitor_gets_a_session_id() -> None:
+    app = AppTest.from_function(run_wrapper)
+    app.run()
+
+    assert _resolved_session_id(app)
+
+
+def test_url_session_id_adopted_when_not_live() -> None:
+    app = AppTest.from_function(run_wrapper)
+    app.query_params["session_id"] = "stale-or-unknown-id"
+    app.run()
+
+    assert _resolved_session_id(app) == "stale-or-unknown-id"
+
+
+def test_copy_pasted_live_session_forks_to_new_user() -> None:
+    host = AppTest.from_function(run_wrapper)
+    host.run()
+    host.button(key="start_room").click().run()
+    host_session_id = _resolved_session_id(host)
+    room_id = get_room_id(host)
+
+    visitor = AppTest.from_function(run_wrapper)
+    visitor.query_params["room_id"] = room_id
+    visitor.query_params["session_id"] = host_session_id
+    visitor.run()
+
+    assert _resolved_session_id(visitor) != host_session_id