From db1028c73ed770fdd6eac777030aaf725fa7f5fc Mon Sep 17 00:00:00 2001
From: RAHUL KUMAR <hellorahul@google.com>
Date: Thu, 5 Mar 2026 15:49:17 +0530
Subject: [PATCH 1/2] Fix EarlyStopping and ReduceLROnPlateau not resetting
 best metric between runs

When reusing an EarlyStopping callback across multiple model.fit() calls
(e.g., in a for loop training different models), self.best was not reset
in on_train_begin(). This caused subsequent runs to compare against the
best metric from a previous run, triggering premature early stopping.

Reset self.best to None in EarlyStopping.on_train_begin() and
ReduceLROnPlateau._reset() so the first epoch of each new training run
is always accepted as an improvement, matching the behavior of a fresh
callback instance.

Fixes https://github.com/keras-team/keras/issues/20256
---
 keras/src/callbacks/early_stopping.py       |  1 +
 keras/src/callbacks/early_stopping_test.py  | 27 +++++++++++++++++++++
 keras/src/callbacks/reduce_lr_on_plateau.py |  1 +
 3 files changed, 29 insertions(+)

diff --git a/keras/src/callbacks/early_stopping.py b/keras/src/callbacks/early_stopping.py
index 30fef26b8d9e..0e13613164b2 100644
--- a/keras/src/callbacks/early_stopping.py
+++ b/keras/src/callbacks/early_stopping.py
@@ -88,6 +88,7 @@ def on_train_begin(self, logs=None):
         # Allow instances to be re-used
         self.wait = 0
         self.stopped_epoch = 0
+        self.best = None
         self.best_weights = None
         self.best_epoch = 0
 
diff --git a/keras/src/callbacks/early_stopping_test.py b/keras/src/callbacks/early_stopping_test.py
index d4b127675e7b..7a6853e0ad5b 100644
--- a/keras/src/callbacks/early_stopping_test.py
+++ b/keras/src/callbacks/early_stopping_test.py
@@ -126,6 +126,33 @@ def test_early_stopping_reuse(self):
         )
         self.assertGreaterEqual(len(history2.epoch), patience)
 
+    @pytest.mark.requires_trainable_backend
+    def test_early_stopping_reuse_across_models(self):
+        # Regression test for https://github.com/keras-team/keras/issues/20256
+        # When reusing EarlyStopping across multiple model.fit() calls with
+        # different models, self.best must be reset so that the new model
+        # isn't compared against a stale best value from a previous run.
+        patience = 5
+        data = np.random.random((100, 1))
+        labels = np.where(data > 0.5, 1, 0)
+
+        stopper = callbacks.EarlyStopping(
+            monitor="loss", patience=patience
+        )
+
+        for _ in range(3):
+            model = models.Sequential(
+                [
+                    layers.Dense(10, activation="relu"),
+                    layers.Dense(1, activation="sigmoid"),
+                ]
+            )
+            model.compile(optimizer="sgd", loss="binary_crossentropy")
+            history = model.fit(
+                data, labels, callbacks=[stopper], verbose=0, epochs=50
+            )
+            self.assertGreater(len(history.epoch), patience)
+
     @pytest.mark.requires_trainable_backend
     def test_early_stopping_with_baseline(self):
         baseline = 0.6
diff --git a/keras/src/callbacks/reduce_lr_on_plateau.py b/keras/src/callbacks/reduce_lr_on_plateau.py
index b9c40afc4e92..391f4d3902ac 100644
--- a/keras/src/callbacks/reduce_lr_on_plateau.py
+++ b/keras/src/callbacks/reduce_lr_on_plateau.py
@@ -74,6 +74,7 @@ def __init__(
 
     def _reset(self):
         """Resets wait counter and cooldown counter."""
+        self.best = None
         self.cooldown_counter = 0
         self.wait = 0
 

From 4f614c122ceb80c1d32d9303ae182a6f3f13667c Mon Sep 17 00:00:00 2001
From: RAHUL KUMAR <hellorahul@google.com>
Date: Thu, 5 Mar 2026 17:25:33 +0530
Subject: [PATCH 2/2] style: run pre-commit hooks (ruff formatting)

---
 keras/src/callbacks/early_stopping_test.py | 4 +---
 1 file changed, 1 insertion(+), 3 deletions(-)

diff --git a/keras/src/callbacks/early_stopping_test.py b/keras/src/callbacks/early_stopping_test.py
index 7a6853e0ad5b..5054567bf345 100644
--- a/keras/src/callbacks/early_stopping_test.py
+++ b/keras/src/callbacks/early_stopping_test.py
@@ -136,9 +136,7 @@ def test_early_stopping_reuse_across_models(self):
         data = np.random.random((100, 1))
         labels = np.where(data > 0.5, 1, 0)
 
-        stopper = callbacks.EarlyStopping(
-            monitor="loss", patience=patience
-        )
+        stopper = callbacks.EarlyStopping(monitor="loss", patience=patience)
 
         for _ in range(3):
             model = models.Sequential(