Fix save_last behavior in absence of validation (#20960)

* Implement test to reproduce the issue
* Implement fix
* chlog

---------

Co-authored-by: pre-commit-ci[bot] <66853113+pre-commit-ci[bot]@users.noreply.github.com>
Co-authored-by: Jirka Borovec <[email protected]>
Co-authored-by: Jirka B <[email protected]>

(cherry picked from commit 72bb751)

Author: baskrahmer

src/lightning/pytorch/CHANGELOG.md

@@ -41,6 +41,9 @@ The format is based on [Keep a Changelog](http://keepachangelog.com/en/1.0.0/).
 - Fixed metrics in `RichProgressBar` being updated according to user provided `refresh_rate` ([#21032](https://github.com/Lightning-AI/pytorch-lightning/pull/21032))
 
 
+- Fix `save_last` behavior in the absence of validation ([#20960](https://github.com/Lightning-AI/pytorch-lightning/pull/20960))
+
+
 ---
 
 ## [2.5.2] - 2025-06-20

src/lightning/pytorch/callbacks/model_checkpoint.py

@@ -344,6 +344,13 @@ def on_validation_end(self, trainer: "pl.Trainer", pl_module: "pl.LightningModul
                 self._save_topk_checkpoint(trainer, monitor_candidates)
             self._save_last_checkpoint(trainer, monitor_candidates)
 
+    @override
+    def on_train_end(self, trainer: "pl.Trainer", pl_module: "pl.LightningModule") -> None:
+        """Ensure save_last=True is applied when training ends."""
+        if self.save_last and not self._last_checkpoint_saved:
+            monitor_candidates = self._monitor_candidates(trainer)
+            self._save_last_checkpoint(trainer, monitor_candidates)
+
     @override
     def state_dict(self) -> dict[str, Any]:
         return {

tests/tests_pytorch/checkpointing/test_model_checkpoint.py

@@ -1666,3 +1666,30 @@ def val_dataloader(self) -> DataLoader:
     trainer_kwargs["max_epochs"] = 4
     trainer = Trainer(**trainer_kwargs, callbacks=ModelCheckpoint(**mc_kwargs))
     trainer.fit(model, ckpt_path=checkpoint_path)
+
+
+def test_save_last_without_save_on_train_epoch_and_without_val(tmp_path):
+    """Test that save_last=True works correctly when save_on_train_epoch_end=False in a model without validation."""
+
+    # Remove validation methods to test the edge case
+    model = BoringModel()
+    model.validation_step = None
+    model.val_dataloader = None
+
+    checkpoint_callback = ModelCheckpoint(
+        dirpath=tmp_path,
+        save_last=True,
+        save_on_train_epoch_end=False,
+    )
+
+    trainer = Trainer(
+        max_epochs=2,
+        callbacks=[checkpoint_callback],
+        logger=False,
+        enable_progress_bar=False,
+    )
+
+    trainer.fit(model)
+
+    # save_last=True should always save last.ckpt
+    assert (tmp_path / "last.ckpt").exists()