diff --git a/src/megatron/bridge/training/optim.py b/src/megatron/bridge/training/optim.py
index 38426990ba..658bf85b54 100644
--- a/src/megatron/bridge/training/optim.py
+++ b/src/megatron/bridge/training/optim.py
@@ -19,8 +19,20 @@
     MegatronOptimizer,
     OptimizerConfig,
     get_megatron_optimizer,
-    get_mup_config_overrides,
 )
+
+
+# TODO: Remove try/except once `get_mup_config_overrides` lands in mcore main.
+#       This guard exists because the symbol lives in mcore dev but not yet in
+#       the main branch that the submodule tracks.
+#
+#       We assign None (not a bool flag) so the module attribute always exists
+#       and tests can patch it without AttributeError.
+try:
+    from megatron.core.optimizer import get_mup_config_overrides
+except ImportError:
+    get_mup_config_overrides = None  # type: ignore[assignment]
+
 from megatron.core.optimizer.muon import get_megatron_muon_optimizer
 from megatron.core.optimizer_param_scheduler import OptimizerParamScheduler
 from megatron.core.process_groups_config import ProcessGroupCollection
@@ -65,10 +77,12 @@ def setup_optimizer(
         OptimizerConfigOverrideProviderContext(scheduler_config, optimizer_config, model)
     )
 
-    # Apply μP optimizer scaling if enabled on the model config
+    # Apply μP optimizer scaling if enabled on the model config.
+    # Guard on the callable itself (None when mcore main lacks the symbol) so
+    # unit tests can patch the module attribute without hitting AttributeError.
     model_chunks = model if isinstance(model, list) else [model]
     model_config = get_model_config(model_chunks[0])
-    if getattr(model_config, "use_mup", False):
+    if get_mup_config_overrides is not None and getattr(model_config, "use_mup", False):
         mup_overrides = get_mup_config_overrides(
             config=optimizer_config,
             mup_width_mult=model_config.mup_width_mult,