diff --git a/mempalace/knowledge_graph.py b/mempalace/knowledge_graph.py
index 9096ab2..30055a1 100644
--- a/mempalace/knowledge_graph.py
+++ b/mempalace/knowledge_graph.py
@@ -171,6 +171,15 @@ class KnowledgeGraph:
             add_triple("Max", "does", "swimming", valid_from="2025-01-01")
             add_triple("Alice", "worried_about", "Max injury", valid_from="2026-01", valid_to="2026-02")
         """
+        # Reject inverted intervals: a triple with valid_to < valid_from
+        # would never satisfy `valid_from <= as_of AND valid_to >= as_of`,
+        # so it would be invisible to every query — silently corrupt.
+        if valid_from is not None and valid_to is not None and valid_to < valid_from:
+            raise ValueError(
+                f"valid_to={valid_to!r} is before valid_from={valid_from!r}; "
+                "an inverted interval would be invisible to every KG query"
+            )
+
         sub_id = self._entity_id(subject)
         obj_id = self._entity_id(obj)
         pred = predicate.lower().replace(" ", "_")
diff --git a/tests/test_knowledge_graph.py b/tests/test_knowledge_graph.py
index d7d9838..6eeb8d3 100644
--- a/tests/test_knowledge_graph.py
+++ b/tests/test_knowledge_graph.py
@@ -5,6 +5,8 @@ Covers: entity CRUD, triple CRUD, temporal queries, invalidation,
 timeline, stats, and edge cases (duplicate triples, ID collisions).
 """
 
+import pytest
+
 
 class TestEntityOperations:
     def test_add_entity(self, kg):
@@ -45,6 +47,38 @@ class TestTripleOperations:
         tid2 = kg.add_triple("Alice", "works_at", "Acme")
         assert tid1 != tid2  # new triple since old one was closed
 
+    def test_add_triple_rejects_inverted_interval(self, kg):
+        # valid_to before valid_from would never satisfy
+        # `valid_from <= as_of AND valid_to >= as_of` — silently invisible
+        # to every query. Reject at write time instead.
+        with pytest.raises(ValueError, match="before valid_from"):
+            kg.add_triple(
+                "Alice",
+                "worked_at",
+                "Acme",
+                valid_from="2026-03-01",
+                valid_to="2026-02-01",
+            )
+
+    def test_add_triple_accepts_equal_dates(self, kg):
+        # Same-day intervals are valid (point-in-time facts).
+        tid = kg.add_triple(
+            "Alice",
+            "joined",
+            "Acme",
+            valid_from="2026-03-15",
+            valid_to="2026-03-15",
+        )
+        assert tid.startswith("t_alice_joined_acme_")
+
+    def test_add_triple_allows_only_one_bound(self, kg):
+        # The guard only fires when BOTH bounds are set.
+        tid1 = kg.add_triple("Alice", "knows", "Bob", valid_from="2026-01-01")
+        assert tid1.startswith("t_alice_knows_bob_")
+        kg.invalidate("Alice", "knows", "Bob", ended="2026-02-01")
+        tid2 = kg.add_triple("Alice", "knew", "Bob", valid_to="2026-03-01")
+        assert tid2.startswith("t_alice_knew_bob_")
+
 
 class TestQueries:
     def test_query_outgoing(self, seeded_kg):