diff --git a/skyrl/backends/skyrl_train/workers/megatron/megatron_worker.py b/skyrl/backends/skyrl_train/workers/megatron/megatron_worker.py
index 58061e9c3c..0c9544b9f8 100644
--- a/skyrl/backends/skyrl_train/workers/megatron/megatron_worker.py
+++ b/skyrl/backends/skyrl_train/workers/megatron/megatron_worker.py
@@ -331,7 +331,7 @@ def init_configs(
         Initialize the Megatron-Bridge bridge and provider objects + hf_config and tokenizer
         """
         tokenizer = get_tokenizer(model_path, trust_remote_code=True)
-        hf_config = AutoConfig.from_pretrained(model_path, trust_remote_code=True)
+        hf_config_original = AutoConfig.from_pretrained(model_path, trust_remote_code=True)
 
         override_config_kwargs = {
             "bos_token_id": tokenizer.bos_token_id,
@@ -339,7 +339,7 @@ def init_configs(
             "pad_token_id": tokenizer.pad_token_id,
         }
         override_config_kwargs.update(model_config_kwargs.get("model_config", {}))
-        update_model_config(hf_config, override_config_kwargs=override_config_kwargs)
+        hf_config = update_model_config(hf_config_original, override_config_kwargs=override_config_kwargs)
 
         transformer_config_kwargs = (
             transformer_config_kwargs
@@ -402,7 +402,10 @@ def init_configs(
         self.provider = provider
         self.bridge = bridge
 
-        self.strategy.hf_config = hf_config
+        # strategy.hf_config is the on-disk source-of-truth used by
+        # save_hf_configs and must NOT carry runtime overrides like
+        # mtp_num_layers=0; assign the un-mutated AutoConfig here.
+        self.strategy.hf_config = hf_config_original
         self.tokenizer = tokenizer
         self.enable_router_replay = megatron_config.moe_enable_routing_replay
 
diff --git a/skyrl/train/utils/utils.py b/skyrl/train/utils/utils.py
index 4ac09c3eb3..0811859bea 100644
--- a/skyrl/train/utils/utils.py
+++ b/skyrl/train/utils/utils.py
@@ -6,6 +6,7 @@
 import socket
 import sys
 import time
+from copy import deepcopy
 from datetime import datetime
 from pathlib import Path
 
@@ -976,15 +977,30 @@ def peer_access_supported(max_num_gpus_per_node: int):
 
 
 def update_model_config(module_config, override_config_kwargs):
-    """Update the module config with the override_config_kwargs.
+    """Return a copy of ``module_config`` with ``override_config_kwargs`` applied.
+
+    The returned config is a deep copy, so the caller's input is left
+    unmodified. Nested dict values in ``override_config_kwargs`` recurse into
+    the corresponding sub-config attribute (which is already part of the deep
+    copy, so the recursion mutates the copy in place).
 
     Args:
         module_config: The module config from Huggingface Transformers.
         override_config_kwargs: The kwargs to override the module config.
+
+    Returns:
+        A new module config with the overrides applied.
     """
+    new_config = deepcopy(module_config)
+    _apply_overrides_in_place(new_config, override_config_kwargs)
+    return new_config
+
+
+def _apply_overrides_in_place(module_config, override_config_kwargs):
+    """Apply override kwargs to ``module_config`` in place (used for sub-configs)."""
     for key, val in override_config_kwargs.items():
         if isinstance(val, dict):
-            update_model_config(getattr(module_config, key), val)
+            _apply_overrides_in_place(getattr(module_config, key), val)
         else:
             setattr(module_config, key, val)
 
diff --git a/tests/train/utils/test_update_model_config.py b/tests/train/utils/test_update_model_config.py
new file mode 100644
index 0000000000..4e8591d2db
--- /dev/null
+++ b/tests/train/utils/test_update_model_config.py
@@ -0,0 +1,38 @@
+"""
+uv run --isolated --extra dev pytest tests/train/utils/test_update_model_config.py
+"""
+
+from copy import deepcopy
+from types import SimpleNamespace
+
+from skyrl.train.utils.utils import update_model_config
+
+
+def _make_config():
+    """Build a config-like object with a nested sub-config attribute."""
+    sub = SimpleNamespace(mtp_num_layers=4, hidden_size=128)
+    return SimpleNamespace(num_nextn_predict_layers=4, sub_config=sub)
+
+
+class TestUpdateModelConfigNonMutating:
+    """Lock in the non-mutating contract of ``update_model_config``."""
+
+    def test_input_is_not_mutated_at_top_level(self):
+        cfg = _make_config()
+        before = deepcopy(cfg)
+        update_model_config(cfg, {"num_nextn_predict_layers": 0})
+        assert cfg.num_nextn_predict_layers == before.num_nextn_predict_layers
+        assert cfg.sub_config.mtp_num_layers == before.sub_config.mtp_num_layers
+
+    def test_returned_copy_carries_top_level_overrides(self):
+        cfg = _make_config()
+        new_cfg = update_model_config(cfg, {"num_nextn_predict_layers": 0})
+        assert new_cfg.num_nextn_predict_layers == 0
+        assert new_cfg is not cfg
+
+    def test_nested_overrides_do_not_leak_back_into_input(self):
+        cfg = _make_config()
+        new_cfg = update_model_config(cfg, {"sub_config": {"mtp_num_layers": 0}})
+        assert new_cfg.sub_config.mtp_num_layers == 0
+        assert cfg.sub_config.mtp_num_layers == 4
+        assert new_cfg.sub_config is not cfg.sub_config