diff --git a/semantica/kg/graph_validator.py b/semantica/kg/graph_validator.py
index a8d126ca..a24fd16e 100644
--- a/semantica/kg/graph_validator.py
+++ b/semantica/kg/graph_validator.py
@@ -166,15 +166,24 @@ class GraphValidator:
             
             # Check ID uniqueness
             if eid:
-                if eid in entity_ids:
+                try:
+                    if eid in entity_ids:
+                        issues.append(ValidationIssue(
+                            code="DUPLICATE_ID",
+                            message=f"Duplicate entity ID found: {eid}",
+                            severity=ValidationSeverity.CRITICAL,
+                            element_id=eid,
+                            element_type="entity"
+                        ))
+                    entity_ids.add(eid)
+                except TypeError:
                     issues.append(ValidationIssue(
-                        code="DUPLICATE_ID",
-                        message=f"Duplicate entity ID found: {eid}",
-                        severity=ValidationSeverity.CRITICAL,
-                        element_id=eid,
+                        code="INVALID_ID",
+                        message=f"Entity ID is not hashable: {eid}",
+                        severity=ValidationSeverity.ERROR,
+                        element_id=str(eid),
                         element_type="entity"
                     ))
-                entity_ids.add(eid)
             
             # Schema Check (if schema provided)
             if self.schema and "entity_types" in self.schema:
diff --git a/tests/kg/test_graph_validator.py b/tests/kg/test_graph_validator.py
index a0c57add..103e67b5 100644
--- a/tests/kg/test_graph_validator.py
+++ b/tests/kg/test_graph_validator.py
@@ -32,3 +32,43 @@ def test_entity_id_aliases_are_validated_across_graph_structure():
         issue.code in {"MISSING_FIELD", "DANGLING_EDGE", "ORPHAN_NODES"}
         for issue in result.issues
     )
+
+
+def test_entity_id_and_relationship_aliases_validate_without_builder():
+    """Validator endpoint fallbacks should be tested without normalization."""
+    result = GraphValidator().validate(
+        {
+            "entities": [
+                {"entity_id": "alice:1", "name": "Alice", "type": "Person"},
+                {"entity_id": "org:1", "name": "Acme", "type": "Organization"},
+            ],
+            "relationships": [
+                {
+                    "source_id": "alice:1",
+                    "target_id": "org:1",
+                    "type": "WORKS_FOR",
+                }
+            ],
+        }
+    )
+
+    assert result.is_valid
+    assert not any(
+        issue.code in {"MISSING_FIELD", "DANGLING_EDGE", "ORPHAN_NODES"}
+        for issue in result.issues
+    )
+
+
+def test_unhashable_entity_id_returns_validation_issue():
+    """Invalid unhashable IDs should produce an issue instead of crashing."""
+    result = GraphValidator().validate(
+        {
+            "entities": [
+                {"entity_id": ["alice:1"], "name": "Alice", "type": "Person"}
+            ],
+            "relationships": [],
+        }
+    )
+
+    assert not result.is_valid
+    assert any(issue.code == "INVALID_ID" for issue in result.issues)