diff --git a/src/megatron/bridge/training/config.py b/src/megatron/bridge/training/config.py
index ab13fe0647..736a75cf65 100644
--- a/src/megatron/bridge/training/config.py
+++ b/src/megatron/bridge/training/config.py
@@ -20,9 +20,11 @@
 from pathlib import Path
 from typing import Any, Literal, Optional, Tuple, Union
 
+import torch
 from megatron.core.datasets.gpt_dataset import GPTDatasetConfig as MCoreGPTDatasetConfig
 from megatron.core.distributed import DistributedDataParallelConfig as MCoreDistributedDataParallelConfig
 from megatron.core.optimizer import OptimizerConfig as MCoreOptimizerConfig
+from megatron.core.transformer.enums import AttnBackend
 
 from megatron.bridge.data.datasets.packed_sequence import PackedSequenceSpecs
 from megatron.bridge.models import GPTModelProvider, T5ModelProvider
@@ -1084,6 +1086,32 @@ def set_data_parallel_size(self) -> None:
         if self.comm_overlap is not None:
             self.comm_overlap.data_parallel_size = self.data_parallel_size
 
+    def _validate_and_apply_deterministic_mode(self) -> None:
+        """Apply and validate deterministic mode requirements.
+
+        This enforces restrictions and settings that must hold when
+        the model is configured to run in deterministic mode.
+        """
+        if not getattr(self.model, "deterministic_mode", False):
+            return
+
+        # Disallow flash attention when running deterministically
+        if getattr(self.model, "attention_backend", None) == AttnBackend.flash:
+            raise AssertionError("Flash attention can not be used in deterministic mode.")
+
+        # Disallow cross-entropy loss fusion as it is not deterministic
+        assert not getattr(self.model, "cross_entropy_loss_fusion", False), (
+            "Cross Entropy Fusion is currently not deterministic."
+        )
+
+        all_reduce_choices = ("Tree", "Ring", "CollnetDirect", "CollnetChain", "^NVLS")
+        assert os.getenv("NCCL_ALGO", -1) != -1 and os.getenv("NCCL_ALGO") in all_reduce_choices, (
+            f"NCCL_ALGO must be one of {all_reduce_choices}."
+        )
+
+        # Enable deterministic algorithms in torch
+        torch.use_deterministic_algorithms(True)
+
     def _sync_and_validate_external_cuda_graph(self) -> None:
         """Sync necessary configs for external CUDA Graphs and and validates it."""
 
@@ -1149,6 +1177,9 @@ def validate(self) -> None:
             if self.comm_overlap is not None:
                 self.comm_overlap.data_parallel_size = self.data_parallel_size
 
+        # Deterministic mode validations and settings
+        self._validate_and_apply_deterministic_mode()
+
         # Run validations
         _validate_and_sync_distributed_optimizer_settings(self)
 
diff --git a/tests/unit_tests/training/test_config.py b/tests/unit_tests/training/test_config.py
index 83c4ee8c3c..6b77c670b8 100644
--- a/tests/unit_tests/training/test_config.py
+++ b/tests/unit_tests/training/test_config.py
@@ -383,7 +383,66 @@ def test_cannot_set_blend_fields(self):
 
 
 class TestConfigContainerValidation:
-    """Tests for the `validate` method of the `ConfigContainer` class."""
+    def test_deterministic_mode_disallows_flash_and_ce_fusion(self, monkeypatch):
+        """Test that deterministic mode disallows flash attention and cross-entropy loss fusion."""
+        from megatron.core.transformer.enums import AttnBackend
+
+        gpt_model_cfg = create_test_gpt_config(
+            deterministic_mode=True,
+            attention_backend=AttnBackend.flash,
+            cross_entropy_loss_fusion=True,
+        )
+
+        # Ensure NCCL_ALGO present but valid, so we fail earlier on flash/ce fusion
+        monkeypatch.setenv("NCCL_ALGO", "Tree")
+
+        container, og_ws, cfg_mod = create_test_config_container(world_size_override=1, model_config=gpt_model_cfg)
+
+        try:
+            with pytest.raises(AssertionError, match="Flash attention can not be used in deterministic mode"):
+                container.validate()
+
+            # Fix attention, still CE fusion should fail
+            container.model.attention_backend = AttnBackend.local
+            with pytest.raises(AssertionError, match="Cross Entropy Fusion is currently not deterministic"):
+                container.validate()
+        finally:
+            restore_get_world_size_safe(og_ws, cfg_mod)
+
+    def test_deterministic_mode_requires_nccl_algo_and_sets_torch(self, monkeypatch):
+        """Test that deterministic mode requires NCCL_ALGO and sets torch.use_deterministic_algorithms."""
+        gpt_model_cfg = create_test_gpt_config(
+            deterministic_mode=True,
+            cross_entropy_loss_fusion=False,
+            transformer_impl="transformer_engine",
+        )
+
+        container, og_ws, cfg_mod = create_test_config_container(world_size_override=1, model_config=gpt_model_cfg)
+
+        try:
+            # Missing NCCL_ALGO
+            monkeypatch.delenv("NCCL_ALGO", raising=False)
+            with pytest.raises(AssertionError, match="NCCL_ALGO must be one of"):
+                container.validate()
+
+            # Invalid NCCL_ALGO
+            monkeypatch.setenv("NCCL_ALGO", "AllReduce")
+            with pytest.raises(AssertionError, match="NCCL_ALGO must be one of"):
+                container.validate()
+
+            # Valid NCCL_ALGO -> should pass and call torch deterministic
+            monkeypatch.setenv("NCCL_ALGO", "Ring")
+
+            called = {"det": False}
+
+            def _mock_use_deterministic(flag):
+                called["det"] = flag
+
+            with patch.object(torch, "use_deterministic_algorithms", side_effect=_mock_use_deterministic):
+                container.validate()
+                assert called["det"] is True
+        finally:
+            restore_get_world_size_safe(og_ws, cfg_mod)
 
     @pytest.mark.parametrize(
         "world_size, expect_assertion_error",