diff --git a/src/scenic/syntax/scenic.gram b/src/scenic/syntax/scenic.gram
index 5a3d7b3fe..ec3e63f10 100644
--- a/src/scenic/syntax/scenic.gram
+++ b/src/scenic/syntax/scenic.gram
@@ -1721,18 +1721,23 @@ scenic_specifiers: ss=','.scenic_specifier+ { ss }
 scenic_specifier:
     | scenic_valid_specifier
     | invalid_scenic_specifier
+# Split into non-operator-like vs operator-like instance specifiers.
 scenic_valid_specifier:
-    | 'with' p=NAME v=expression { s.WithSpecifier(prop=p.string, value=v, LOCATIONS) }
+    | scenic_instance_specifier_nonoperator
+    | scenic_instance_specifier_operator_like
+scenic_instance_specifier_operator_like:
     | 'at' position=expression { s.AtSpecifier(position=position, LOCATIONS) }
     | "offset" 'by' o=expression { s.OffsetBySpecifier(offset=o, LOCATIONS) }
     | "offset" "along" d=expression 'by' o=expression { s.OffsetAlongSpecifier(direction=d, offset=o, LOCATIONS) }
+    | 'in' r=expression { s.InSpecifier(region=r, LOCATIONS) }
+scenic_instance_specifier_nonoperator:
+    | 'with' p=NAME v=expression { s.WithSpecifier(prop=p.string, value=v, LOCATIONS) }
     | direction=scenic_specifier_position_direction position=expression distance=['by' e=expression { e }] {
         s.DirectionOfSpecifier(direction=direction, position=position, distance=distance, LOCATIONS)
      }
     | "beyond" v=expression 'by' o=expression b=['from' a=expression {a}] { s.BeyondSpecifier(position=v, offset=o, base=b) }
     | "visible" b=['from' r=expression { r }] { s.VisibleSpecifier(base=b, LOCATIONS) }
     | 'not' "visible" b=['from' r=expression { r }] { s.NotVisibleSpecifier(base=b, LOCATIONS) }
-    | 'in' r=expression { s.InSpecifier(region=r, LOCATIONS) }
     | 'on' r=expression { s.OnSpecifier(region=r, LOCATIONS) }
     | "contained" 'in' r=expression { s.ContainedInSpecifier(region=r, LOCATIONS) }
     | "following" f=expression b=['from' e=expression {e}] 'for' d=expression {
@@ -2492,7 +2497,7 @@ invalid_kwarg[NoReturn]:
      }
 
 invalid_scenic_instance_creation[NoReturn]:
-    | n=NAME s=scenic_valid_specifier {
+    | n=NAME s=scenic_instance_specifier_nonoperator {
         self.raise_syntax_error_known_range("invalid syntax. Perhaps you forgot 'new'?", n, s)
     }
 invalid_scenic_specifier[NoReturn]:
diff --git a/tests/syntax/test_parser.py b/tests/syntax/test_parser.py
index 797f7f403..b997c8613 100644
--- a/tests/syntax/test_parser.py
+++ b/tests/syntax/test_parser.py
@@ -1518,6 +1518,44 @@ def test_missing_new(self):
             parse_string_helper("Object facing x")
         assert "forgot 'new'" in e.value.msg
 
+    def test_in_operator_not_missing_new(self):
+        # Regression test for issue #356: "in" is also a specifier keyword, but
+        # `x in [0]` must not trigger the "forgot 'new'" error message.
+        with pytest.raises(ScenicSyntaxError) as e:
+            parse_string_helper(
+                """
+                x = 0
+                x in [0]
+                =
+                """
+            )
+        err = e.value
+        assert "forgot 'new'" not in err.msg
+        assert "invalid syntax" in err.msg
+        assert err.lineno == 3
+
+    def test_invalid_specifier_line_number(self):
+        # Regression test for issue #345: a malformed position specifier like
+        # `offset left` should not trigger the "forgot 'new'" error or be attributed
+        # to an earlier valid `... until self in ...` expression.
+        with pytest.raises(ScenicSyntaxError) as e:
+            parse_string_helper(
+                """
+                behavior AdvBehavior():
+                    do CrossingBehavior(ego) until self in ego.lane
+                    while True:
+                        take SetWalkingSpeedAction(0)
+
+                SHIFT = Vector(1, 2)
+                AdvAgent = new Pedestrian at Truck offset left Truck.heading by SHIFT,
+                    with heading Truck.heading
+                """
+            )
+        err = e.value
+        assert "forgot 'new'" not in err.msg
+        assert "invalid syntax" in err.msg
+        assert err.lineno == 7
+
     def test_invalid_specifier(self):
         with pytest.raises(ScenicSyntaxError) as e:
             parse_string_helper("new Object blobbing")