diff --git a/CONTRIBUTING.md b/CONTRIBUTING.md
index 3847963f..9e55d66a 100644
--- a/CONTRIBUTING.md
+++ b/CONTRIBUTING.md
@@ -210,7 +210,8 @@ implementation of a mathematical aggregator.
 
 > [!NOTE]
 > We also accept stateful aggregators, whose output depends both on the Jacobian and on some
-> internal state (which can be affected for example by previous Jacobians).
+> internal state (which can be affected for example by previous Jacobians). Such aggregators should
+> inherit from the `Stateful` mixin and implement a `reset` method.
 
 > [!NOTE]
 > Some aggregators may depend on something else than the Jacobian. To implement them, please add
diff --git a/docs/source/docs/aggregation/index.rst b/docs/source/docs/aggregation/index.rst
index 64ba6f63..0dbdab0c 100644
--- a/docs/source/docs/aggregation/index.rst
+++ b/docs/source/docs/aggregation/index.rst
@@ -22,6 +22,10 @@ Abstract base classes
     :undoc-members:
     :exclude-members: forward
 
+.. autoclass:: torchjd.aggregation.Stateful
+    :members:
+    :undoc-members:
+
 
 .. toctree::
     :hidden:
diff --git a/src/torchjd/aggregation/__init__.py b/src/torchjd/aggregation/__init__.py
index 93f824e3..400cfe27 100644
--- a/src/torchjd/aggregation/__init__.py
+++ b/src/torchjd/aggregation/__init__.py
@@ -71,6 +71,7 @@
 from ._krum import Krum, KrumWeighting
 from ._mean import Mean, MeanWeighting
 from ._mgda import MGDA, MGDAWeighting
+from ._mixins import Stateful
 from ._pcgrad import PCGrad, PCGradWeighting
 from ._random import Random, RandomWeighting
 from ._sum import Sum, SumWeighting
@@ -107,6 +108,7 @@
     "PCGradWeighting",
     "Random",
     "RandomWeighting",
+    "Stateful",
     "Sum",
     "SumWeighting",
     "TrimmedMean",
diff --git a/src/torchjd/aggregation/_gradvac.py b/src/torchjd/aggregation/_gradvac.py
index 57a08964..efb55f44 100644
--- a/src/torchjd/aggregation/_gradvac.py
+++ b/src/torchjd/aggregation/_gradvac.py
@@ -6,14 +6,16 @@
 from torch import Tensor
 
 from torchjd._linalg import PSDMatrix
+from torchjd.aggregation._mixins import Stateful
 
 from ._aggregator_bases import GramianWeightedAggregator
 from ._utils.non_differentiable import raise_non_differentiable_error
 from ._weighting_bases import Weighting
 
 
-class GradVac(GramianWeightedAggregator):
+class GradVac(GramianWeightedAggregator, Stateful):
     r"""
+    :class:`~torchjd.aggregation._mixins.Stateful`
     :class:`~torchjd.aggregation._aggregator_bases.Aggregator` implementing the aggregation step of
     Gradient Vaccine (GradVac) from `Gradient Vaccine: Investigating and Improving Multi-task
     Optimization in Massively Multilingual Models (ICLR 2021 Spotlight)
@@ -71,8 +73,9 @@ def __repr__(self) -> str:
         return f"GradVac(beta={self.beta!r}, eps={self.eps!r})"
 
 
-class GradVacWeighting(Weighting[PSDMatrix]):
+class GradVacWeighting(Weighting[PSDMatrix], Stateful):
     r"""
+    :class:`~torchjd.aggregation._mixins.Stateful`
     :class:`~torchjd.aggregation._weighting_bases.Weighting` giving the weights of
     :class:`~torchjd.aggregation.GradVac`.
 
diff --git a/src/torchjd/aggregation/_mixins.py b/src/torchjd/aggregation/_mixins.py
new file mode 100644
index 00000000..8481feab
--- /dev/null
+++ b/src/torchjd/aggregation/_mixins.py
@@ -0,0 +1,9 @@
+from abc import ABC, abstractmethod
+
+
+class Stateful(ABC):
+    """Mixin adding a reset method."""
+
+    @abstractmethod
+    def reset(self) -> None:
+        """Resets the internal state."""
diff --git a/src/torchjd/aggregation/_nash_mtl.py b/src/torchjd/aggregation/_nash_mtl.py
index 1edd5a0d..f64f5182 100644
--- a/src/torchjd/aggregation/_nash_mtl.py
+++ b/src/torchjd/aggregation/_nash_mtl.py
@@ -1,15 +1,16 @@
 # Partly adapted from https://github.com/AvivNavon/nash-mtl — MIT License, Copyright (c) 2022 Aviv Navon.
 # See NOTICES for the full license text.
 
-from typing import cast
-
 from torchjd._linalg import Matrix
+from torchjd.aggregation._mixins import Stateful
 
 from ._utils.check_dependencies import check_dependencies_are_installed
 from ._weighting_bases import Weighting
 
 check_dependencies_are_installed(["cvxpy", "ecos"])
 
+from typing import cast
+
 import cvxpy as cp
 import numpy as np
 import torch
@@ -20,8 +21,9 @@
 from ._utils.non_differentiable import raise_non_differentiable_error
 
 
-class NashMTL(WeightedAggregator):
+class NashMTL(WeightedAggregator, Stateful):
     """
+    :class:`~torchjd.aggregation._mixins.Stateful`
     :class:`~torchjd.aggregation._aggregator_bases.Aggregator` as proposed in Algorithm 1 of
     `Multi-Task Learning as a Bargaining Game <https://arxiv.org/pdf/2202.01017.pdf>`_.
 
@@ -83,11 +85,11 @@ def __repr__(self) -> str:
         )
 
 
-class _NashMTLWeighting(Weighting[Matrix]):
+class _NashMTLWeighting(Weighting[Matrix], Stateful):
     """
-    :class:`~torchjd.aggregation.Weighting` that extracts weights using the step decision
-    of Algorithm 1 of `Multi-Task Learning as a Bargaining Game
-    <https://arxiv.org/pdf/2202.01017.pdf>`_.
+    :class:`~torchjd.aggregation._mixins.Stateful` :class:`~torchjd.aggregation.Weighting` that
+    extracts weights using the step decision of Algorithm 1 of `Multi-Task Learning as a Bargaining
+    Game <https://arxiv.org/pdf/2202.01017.pdf>`_.
 
     :param n_tasks: The number of tasks, corresponding to the number of rows in the provided
         matrices.