diff --git a/atompack-py/python/atompack/__init__.pyi b/atompack-py/python/atompack/__init__.pyi
index 880939c..938f4df 100644
--- a/atompack-py/python/atompack/__init__.pyi
+++ b/atompack-py/python/atompack/__init__.pyi
@@ -327,7 +327,7 @@ class Molecule:
 
         Raises
         ------
-        ValueError
+        KeyError
             If property key does not exist
         """
         ...
diff --git a/atompack-py/python/atompack/_atompack_rs.pyi b/atompack-py/python/atompack/_atompack_rs.pyi
index 5235b99..99e8730 100644
--- a/atompack-py/python/atompack/_atompack_rs.pyi
+++ b/atompack-py/python/atompack/_atompack_rs.pyi
@@ -321,6 +321,11 @@ class PyMolecule:
         -------
         Any
             Property value
+
+        Raises
+        ------
+        KeyError
+            If property key does not exist
         """
         ...
 
diff --git a/atompack-py/src/molecule.rs b/atompack-py/src/molecule.rs
index f881e2f..78a83e9 100644
--- a/atompack-py/src/molecule.rs
+++ b/atompack-py/src/molecule.rs
@@ -667,10 +667,7 @@ impl PyMolecule {
         if let Some(inner) = molecule.as_owned() {
             return match inner.properties.get(key) {
                 Some(v) => property_value_to_pyobject(py, v),
-                None => Err(PyValueError::new_err(format!(
-                    "Property '{}' not found",
-                    key
-                ))),
+                None => Err(PyKeyError::new_err(format!("Property '{}' not found", key))),
             };
         }
         let view = molecule.as_view().ok_or_else(|| {
@@ -678,10 +675,7 @@ impl PyMolecule {
         })?;
         match view.find_custom_section(KIND_MOL_PROP, key)? {
             Some(section) => property_section_to_pyobject(py, view, section),
-            None => Err(PyValueError::new_err(format!(
-                "Property '{}' not found",
-                key
-            ))),
+            None => Err(PyKeyError::new_err(format!("Property '{}' not found", key))),
         }
     }
 
diff --git a/atompack-py/tests/test_atom_molecule.py b/atompack-py/tests/test_atom_molecule.py
index 0ff034c..50d4caf 100644
--- a/atompack-py/tests/test_atom_molecule.py
+++ b/atompack-py/tests/test_atom_molecule.py
@@ -225,10 +225,22 @@ def test_molecule_custom_properties() -> None:
         "vec3_f64",
     }
 
-    with pytest.raises(ValueError, match=r"not found"):
+    with pytest.raises(KeyError, match=r"not found"):
         mol.get_property("stress")
 
-    with pytest.raises(ValueError, match=r"not found"):
+    with pytest.raises(KeyError, match=r"not found"):
+        mol.get_property("does_not_exist")
+
+
+def test_missing_property_raises_keyerror_consistently() -> None:
+    # Lock in symmetry: both indexing patterns must raise the same exception
+    # type for missing custom properties. They previously disagreed.
+    mol = _make_molecule()
+    mol.set_property("method", "DFT")
+
+    with pytest.raises(KeyError, match=r"not found"):
+        _ = mol["does_not_exist"]
+    with pytest.raises(KeyError, match=r"not found"):
         mol.get_property("does_not_exist")
 
 
diff --git a/atompack-py/tests/test_database.py b/atompack-py/tests/test_database.py
index 7fbb075..52c9946 100644
--- a/atompack-py/tests/test_database.py
+++ b/atompack-py/tests/test_database.py
@@ -387,7 +387,7 @@ def test_database_single_item_mutation_is_copy_on_write(
     assert fresh.energy == pytest.approx(-5.0)
     np.testing.assert_allclose(fresh.forces, original.forces)
     assert fresh.get_property("tag") == "train"
-    with pytest.raises(ValueError, match="Property 'new_ids' not found"):
+    with pytest.raises(KeyError, match="Property 'new_ids' not found"):
         fresh.get_property("new_ids")
 
 
diff --git a/atompack-py/tests/test_from_ase.py b/atompack-py/tests/test_from_ase.py
index 542cdd4..705a405 100644
--- a/atompack-py/tests/test_from_ase.py
+++ b/atompack-py/tests/test_from_ase.py
@@ -130,7 +130,7 @@ def test_from_ase_extracts_core_fields() -> None:
         mol.stress,
         np.array([[1.1, 1.2, 1.3], [2.1, 2.2, 2.3], [3.1, 3.2, 3.3]], dtype=np.float64),
     )
-    with pytest.raises(ValueError, match=r"not found"):
+    with pytest.raises(KeyError, match=r"not found"):
         mol.get_property("stress")
 
     assert mol.has_property("unsupported") is False
@@ -188,7 +188,7 @@ def test_from_ase_extracts_arrays_and_calc_results() -> None:
         mol.get_property("magmoms"), np.array([0.3, 0.4], dtype=np.float64)
     )
     np.testing.assert_allclose(mol.stress, np.eye(3, dtype=np.float64) * 3.0)
-    with pytest.raises(ValueError, match=r"not found"):
+    with pytest.raises(KeyError, match=r"not found"):
         mol.get_property("forces")