diff --git a/src/plugin_system/plugin_state.py b/src/plugin_system/plugin_state.py
index d189d154..8b52823e 100644
--- a/src/plugin_system/plugin_state.py
+++ b/src/plugin_system/plugin_state.py
@@ -167,11 +167,16 @@ class PluginStateManager:
             plugin_id: Plugin identifier
 
         Returns:
-            List of recent state transitions, oldest first. A copy, so callers
-            cannot mutate the manager's own history.
+            List of recent state transitions, oldest first. Both the list and
+            the transition dicts are copies, so callers cannot mutate the
+            manager's own history. The values inside a transition are all
+            immutable, so a shallow copy per entry is enough.
         """
         with self._lock:
-            return list(self._state_history.get(plugin_id, ()))
+            return [
+                dict(transition)
+                for transition in self._state_history.get(plugin_id, ())
+            ]
     
     def set_error_info(self, plugin_id: str, error_info: Dict[str, Any]) -> None:
         """
@@ -290,11 +295,17 @@ class PluginStateManager:
         return info
     
     def clear_state(self, plugin_id: str) -> None:
-        """Clear all state information for a plugin."""
-        self._states.pop(plugin_id, None)
-        self._state_history.pop(plugin_id, None)
-        self._state_transition_counts.pop(plugin_id, None)
-        self._error_info.pop(plugin_id, None)
-        self._last_update.pop(plugin_id, None)
-        self._last_display.pop(plugin_id, None)
+        """Clear all state information for a plugin.
+
+        Held under ``_lock`` so the five dicts are dropped as one unit: every
+        other mutator takes the lock, and without it a concurrent set_state()
+        could interleave and leave a plugin with history but no state.
+        """
+        with self._lock:
+            self._states.pop(plugin_id, None)
+            self._state_history.pop(plugin_id, None)
+            self._state_transition_counts.pop(plugin_id, None)
+            self._error_info.pop(plugin_id, None)
+            self._last_update.pop(plugin_id, None)
+            self._last_display.pop(plugin_id, None)
 
diff --git a/test/test_plugin_state_history_cap.py b/test/test_plugin_state_history_cap.py
index ce1427cc..1d5e20d5 100644
--- a/test/test_plugin_state_history_cap.py
+++ b/test/test_plugin_state_history_cap.py
@@ -132,6 +132,24 @@ def test_get_state_history_returns_a_copy():
     assert len(manager.get_state_history("clock")) == 1
 
 
+def test_get_state_history_entries_are_copies():
+    """Copying the outer list is not enough -- the entries are handed out too.
+
+    A caller holding a returned transition must not be able to rewrite the
+    manager's record of what happened.
+    """
+    manager = PluginStateManager()
+    manager.set_state("clock", PluginState.ENABLED)
+
+    entry = manager.get_state_history("clock")[0]
+    entry["to"] = "tampered"
+    entry["error"] = "injected"
+
+    stored = manager.get_state_history("clock")[0]
+    assert stored["to"] == PluginState.ENABLED.value
+    assert stored["error"] is None
+
+
 def test_clear_state_drops_history():
     """Unloading a plugin still releases everything it accumulated."""
     manager = PluginStateManager()