1D+ sensitivities error with IDAKLU output_variables (#5118)

* Adds a test checking for sensitivitiy equality between output_variables=True/False

* Test with 0D, 1D and 2D variables which should all have non-zero sensitivities

* Update changelog

---------

Co-authored-by: Marc Berliner <[email protected]>

Author: pipliggins

CHANGELOG.md

@@ -12,6 +12,7 @@
 - Fix a bug in setting initial stoichiometries where the reference temperature was used instead of the initial temperature. ([#5189](https://github.com/pybamm-team/PyBaMM/pull/5189))
 - Fix a bug in the calculation of "Bulk" OCP terms in hysteresis models ([#5169](https://github.com/pybamm-team/PyBaMM/pull/5169))
 - Fixed a bug where the final duration of a drive cycle would not be inferred correctly. ([#5153](https://github.com/pybamm-team/PyBaMM/pull/5153))
+- Fixes a bug where sensitivities for 1D+ variables calculated using the `output_variables` options were incorrect ([#5118](https://github.com/pybamm-team/PyBaMM/pull/5118))
 
 # [v25.8.0](https://github.com/pybamm-team/PyBaMM/tree/v25.8.0) - 2025-08-04
 

tests/unit/test_solvers/test_idaklu_solver.py

@@ -942,9 +942,10 @@ def test_with_output_variables_and_sensitivities(self):
 
         # Use a selection of variables of different types
         output_variables = [
-            "Voltage [V]",
-            "Time [min]",
-            "x [m]",
+            "Voltage [V]",  # 0D
+            "x [m]",  # 1D, empty sensitivities
+            "Negative electrode potential [V]",  # 1D
+            "Negative particle concentration [mol.m-3]",  # 2D
             "Throughput capacity [A.h]",  # ExplicitTimeIntegral
         ]
 
@@ -993,6 +994,17 @@ def test_with_output_variables_and_sensitivities(self):
                 == sol_all[varname].sensitivities["all"].shape
             )
 
+        # test each of the sensitivity calculations match
+        for varname in output_variables:
+            for key in input_parameters:
+                np.testing.assert_allclose(
+                    sol[varname].sensitivities[key],
+                    sol_all[varname].sensitivities[key],
+                    rtol=tol,
+                    atol=tol,
+                    err_msg=f"Failed for '{varname}', sensitivity '{key}'",
+                )
+
     def test_with_output_variables_and_event_termination(self):
         model = pybamm.lithium_ion.DFN()
         parameter_values = pybamm.ParameterValues("Chen2020")