diff --git a/pymc_marketing/mmm/additive_effect.py b/pymc_marketing/mmm/additive_effect.py
index 79ca27156..a9b10d45f 100644
--- a/pymc_marketing/mmm/additive_effect.py
+++ b/pymc_marketing/mmm/additive_effect.py
@@ -104,12 +104,13 @@ def set_data(self, mmm, model, X):
   - In `set_data`, update the data variables when dates/dims change.
 """
 
-from typing import Any, Protocol
+from abc import ABC, abstractmethod
+from typing import Annotated, Any, Protocol
 
 import pandas as pd
 import pymc as pm
 import xarray as xr
-from pydantic import BaseModel, InstanceOf
+from pydantic import BaseModel, Field, InstanceOf, PlainValidator, WithJsonSchema
 from pymc_extras.prior import create_dim_handler
 from pytensor import tensor as pt
 
@@ -131,35 +132,31 @@ def model(self) -> pm.Model:
         """The PyMC model."""
 
 
-class MuEffect(Protocol):
-    """Protocol for arbitrary additive mu effect."""
+class MuEffect(ABC, BaseModel):
+    """Abstract base class for arbitrary additive mu effects.
 
+    All mu_effects must inherit from this Pydantic BaseModel to ensure proper
+    serialization and deserialization when saving/loading MMM models.
+    """
+
+    @abstractmethod
     def create_data(self, mmm: Model) -> None:
         """Create the required data in the model."""
 
+    @abstractmethod
     def create_effect(self, mmm: Model) -> pt.TensorVariable:
         """Create the additive effect in the model."""
 
+    @abstractmethod
     def set_data(self, mmm: Model, model: pm.Model, X: xr.Dataset) -> None:
         """Set the data for new predictions."""
 
 
-class FourierEffect:
+class FourierEffect(MuEffect):
     """Fourier seasonality additive effect for MMM."""
 
-    def __init__(self, fourier: FourierBase, date_dim_name: str = "date"):
-        """Initialize the Fourier effect.
-
-        Parameters
-        ----------
-        fourier : FourierBase
-            The FourierBase instance to use for the effect.
-        date_dim_name : str, optional
-            The name of the date dimension in the model, by default "date".
-
-        """
-        self.fourier = fourier
-        self.date_dim_name: str = date_dim_name
+    fourier: InstanceOf[FourierBase]
+    date_dim_name: str = Field("date")
 
     def create_data(self, mmm: Model) -> None:
         """Create the required data in the model.
@@ -256,7 +253,14 @@ def set_data(self, mmm: Model, model: pm.Model, X: xr.Dataset) -> None:
         pm.set_data(new_data=new_data, model=model)
 
 
-class LinearTrendEffect:
+_Timestamp = Annotated[
+    pd.Timestamp,
+    PlainValidator(lambda x: pd.Timestamp(x)),
+    WithJsonSchema({"type": "date-time"}),
+]
+
+
+class LinearTrendEffect(MuEffect):
     """Wrapper for LinearTrend to use with MMM's MuEffect protocol.
 
     This class adapts the LinearTrend component to be used as an additive effect
@@ -268,6 +272,8 @@ class LinearTrendEffect:
         The LinearTrend instance to wrap.
     prefix : str
         The prefix to use for variables in the model.
+    date_dim_name : str
+        The name of the date dimension in the model.
 
     Examples
     --------
@@ -357,11 +363,10 @@ class MockMMM:
 
     """
 
-    def __init__(self, trend: LinearTrend, prefix: str, date_dim_name: str = "date"):
-        self.trend = trend
-        self.prefix = prefix
-        self.linear_trend_first_date: pd.Timestamp
-        self.date_dim_name: str = date_dim_name
+    trend: InstanceOf[LinearTrend]
+    prefix: str
+    date_dim_name: str = Field("date")
+    linear_trend_first_date: _Timestamp | None = Field(None, init=False)
 
     def create_data(self, mmm: Model) -> None:
         """Create the required data in the model.
@@ -439,7 +444,7 @@ def set_data(self, mmm: Model, model: pm.Model, X: xr.Dataset) -> None:
         pm.set_data({f"{self.prefix}_t": t}, model=model)
 
 
-class EventAdditiveEffect(BaseModel):
+class EventAdditiveEffect(MuEffect):
     """Event effect class for the MMM.
 
     Parameters
diff --git a/tests/mmm/test_additive_effect.py b/tests/mmm/test_additive_effect.py
index 901920582..b7efd5651 100644
--- a/tests/mmm/test_additive_effect.py
+++ b/tests/mmm/test_additive_effect.py
@@ -99,7 +99,7 @@ def test_fourier_effect(
     dims,
     coords,
 ) -> None:
-    effect = FourierEffect(fourier)
+    effect = FourierEffect(fourier=fourier)
 
     mmm = create_mock_mmm(
         dims=dims,
@@ -168,7 +168,7 @@ def test_fourier_effect_multidimensional(
     prefix = "weekly"
     prior = Prior("Laplace", mu=0, b=0.1, dims=prior_dims)
     fourier = WeeklyFourier(n_order=10, prefix=prefix, prior=prior)
-    fourier_effect = FourierEffect(fourier)
+    fourier_effect = FourierEffect(fourier=fourier)
 
     with mmm.model:
         fourier_effect.create_data(mmm)
@@ -252,7 +252,7 @@ def test_linear_trend_effect(
 ) -> None:
     prefix = "linear_trend"
     effect = LinearTrendEffect(
-        LinearTrend(priors=priors, dims=linear_trend_dims),
+        trend=LinearTrend(priors=priors, dims=linear_trend_dims),
         prefix=prefix,
     )