diff --git a/litellm/router.py b/litellm/router.py
index 99bb10717e8..c7cff4ba412 100644
--- a/litellm/router.py
+++ b/litellm/router.py
@@ -1493,31 +1493,30 @@ class Router:
         """
         Prepare kwargs for a silent experiment by ensuring isolation from the primary call.
 
-        IMPORTANT: We avoid calling safe_deep_copy(kwargs) because it temporarily
-        mutates the original dict (pops litellm_parent_otel_span, replaces with
-        "placeholder", then restores). Since this runs in a background thread while
-        the primary request's async callbacks may still be reading the same dict,
-        that mutation causes a race condition that breaks otel/prometheus callbacks
-        for the primary request.
+        Guarantee metadata isolation: safe_deep_copy falls back to the original
+        reference when deepcopy fails (e.g. metadata contains UserAPIKeyAuth with
+        parent_otel_span — an OTel Span that is not deepcopy-able). Force a shallow
+        copy of the metadata dict so mutations (model_group, is_silent_experiment)
+        never corrupt the main call's metadata.
         """
-        import copy
+        from litellm.litellm_core_utils.core_helpers import safe_deep_copy
 
-        # Shallow copy top-level kwargs — does NOT mutate the original
-        silent_kwargs = dict(kwargs)
+        silent_kwargs = safe_deep_copy(kwargs)
 
-        # Deep-copy metadata so we don't share state with the primary request.
-        # Remove the OTEL span BEFORE deep-copying (it's not picklable and is
-        # thread-bound anyway).
-        original_metadata = kwargs.get("metadata") or {}
-        metadata_copy = {
-            k: v
-            for k, v in original_metadata.items()
-            if k != "litellm_parent_otel_span"
-        }
-        try:
-            silent_kwargs["metadata"] = copy.deepcopy(metadata_copy)
-        except Exception:
-            silent_kwargs["metadata"] = dict(metadata_copy)
+        # safe_deep_copy may fall back to the original metadata reference when
+        # deepcopy fails (UserAPIKeyAuth.parent_otel_span is not deepcopy-able).
+        # Detect this via identity check and force a shallow copy so that setting
+        # model_group / is_silent_experiment on the silent dict doesn't corrupt
+        # the primary call's metadata.
+        original_metadata = kwargs.get("metadata")
+        if (
+            original_metadata is not None
+            and silent_kwargs.get("metadata") is original_metadata
+        ):
+            silent_kwargs["metadata"] = dict(original_metadata)
+
+        if "metadata" not in silent_kwargs:
+            silent_kwargs["metadata"] = {}
 
         silent_kwargs["metadata"]["is_silent_experiment"] = True
 
diff --git a/tests/test_litellm/test_router_silent_experiment.py b/tests/test_litellm/test_router_silent_experiment.py
index 8bced04ab8e..4d6a775c446 100644
--- a/tests/test_litellm/test_router_silent_experiment.py
+++ b/tests/test_litellm/test_router_silent_experiment.py
@@ -35,10 +35,12 @@ def test_get_silent_experiment_kwargs():
     assert result["stream"] is False
     # proxy_server_request must be preserved for spend log metadata
     assert "proxy_server_request" in result
-    # parent OTEL span must be removed — it's thread-bound and invalid in the
-    # background thread's new event loop
-    assert "litellm_parent_otel_span" not in result["metadata"]
-    # CRITICAL: original kwargs must NOT be mutated (race condition with primary callbacks)
+    # CRITICAL: metadata must be a DIFFERENT dict object than the original,
+    # so that setting model_group / is_silent_experiment on the silent dict
+    # doesn't corrupt the primary call's metadata.
+    assert result["metadata"] is not kwargs["metadata"]
+    # Original metadata must NOT be mutated
+    assert "is_silent_experiment" not in kwargs["metadata"]
     assert kwargs["metadata"]["litellm_parent_otel_span"] is mock_span