Fix InputParameter serialisation (#5289)

* fix `InputParameter` serialisation

* Update CHANGELOG.md

Author: MarcBerliner

CHANGELOG.md

@@ -4,6 +4,8 @@
 
 ## Bug fixes
 
+- Fix a bug with serialising `InputParameter`s. ([#5289](https://github.com/pybamm-team/PyBaMM/pull/5289))
+
 # [v25.10.1](https://github.com/pybamm-team/PyBaMM/tree/v25.10.1) - 2025-11-14
 
 ## Features

src/pybamm/expression_tree/operations/serialise.py

@@ -1625,6 +1625,8 @@ def convert_symbol_from_json(json_data):
     elif json_data["type"] == "Parameter":
         # Convert stored parameters back to PyBaMM Parameter objects
         return pybamm.Parameter(json_data["name"])
+    elif json_data["type"] == "InputParameter":
+        return pybamm.InputParameter(json_data["name"])
     elif json_data["type"] == "Scalar":
         # Convert stored numerical values back to PyBaMM Scalar objects
         return pybamm.Scalar(json_data["value"])

tests/unit/test_serialisation/test_serialisation.py

@@ -617,6 +617,14 @@ def test_serialise_time(self):
         t2 = convert_symbol_from_json(j)
         assert isinstance(t2, pybamm.Time)
 
+    def test_serialise_input_parameter(self):
+        """Test InputParameter serialization and deserialization."""
+        ip = pybamm.InputParameter("test_param")
+        j = convert_symbol_to_json(ip)
+        ip_restored = convert_symbol_from_json(j)
+        assert isinstance(ip_restored, pybamm.InputParameter)
+        assert ip_restored.name == "test_param"
+
     def test_convert_symbol_to_json_with_number_and_list(self):
         for val in (0, 3.14, -7, True):
             out = convert_symbol_to_json(val)