diff --git a/litellm/router.py b/litellm/router.py
index c934b1e9a8c..92174adee39 100644
--- a/litellm/router.py
+++ b/litellm/router.py
@@ -339,9 +339,9 @@ class Router:
         )  # names of models under litellm_params. ex. azure/chatgpt-v-2
         self.deployment_latency_map = {}
         ### CACHING ###
-        cache_type: Literal[
-            "local", "redis", "redis-semantic", "s3", "disk"
-        ] = "local"  # default to an in-memory cache
+        cache_type: Literal["local", "redis", "redis-semantic", "s3", "disk"] = (
+            "local"  # default to an in-memory cache
+        )
         redis_cache = None
         cache_config: Dict[str, Any] = {}
 
@@ -562,9 +562,9 @@ class Router:
                 )
             )
 
-        self.model_group_retry_policy: Optional[
-            Dict[str, RetryPolicy]
-        ] = model_group_retry_policy
+        self.model_group_retry_policy: Optional[Dict[str, RetryPolicy]] = (
+            model_group_retry_policy
+        )
 
         self.allowed_fails_policy: Optional[AllowedFailsPolicy] = None
         if allowed_fails_policy is not None:
@@ -1104,17 +1104,21 @@ class Router:
     ) -> None:
         """
         Adds default litellm params to kwargs, if set.
+
+        Handles inserting this as either "metadata" or "litellm_metadata" depending on the metadata_variable_name
         """
-        self.default_litellm_params[
-            metadata_variable_name
-        ] = self.default_litellm_params.pop("metadata", {})
-        for k, v in self.default_litellm_params.items():
-            if (
-                k not in kwargs and v is not None
-            ):  # prioritize model-specific params > default router params
-                kwargs[k] = v
-            elif k == metadata_variable_name:
-                kwargs[metadata_variable_name].update(v)
+        # 1) copy your defaults and pull out metadata
+        defaults = self.default_litellm_params.copy()
+        metadata_defaults = defaults.pop("metadata", {}) or {}
+
+        # 2) add any non-metadata defaults that aren't already in kwargs
+        for key, value in defaults.items():
+            if value is None:
+                continue
+            kwargs.setdefault(key, value)
+
+        # 3) merge in metadata, this handles inserting this as either "metadata" or "litellm_metadata"
+        kwargs.setdefault(metadata_variable_name, {}).update(metadata_defaults)
 
     def _handle_clientside_credential(
         self, deployment: dict, kwargs: dict
@@ -3243,11 +3247,11 @@ class Router:
 
                 if isinstance(e, litellm.ContextWindowExceededError):
                     if context_window_fallbacks is not None:
-                        fallback_model_group: Optional[
-                            List[str]
-                        ] = self._get_fallback_model_group_from_fallbacks(
-                            fallbacks=context_window_fallbacks,
-                            model_group=model_group,
+                        fallback_model_group: Optional[List[str]] = (
+                            self._get_fallback_model_group_from_fallbacks(
+                                fallbacks=context_window_fallbacks,
+                                model_group=model_group,
+                            )
                         )
                         if fallback_model_group is None:
                             raise original_exception
@@ -3279,11 +3283,11 @@ class Router:
                         e.message += "\n{}".format(error_message)
                 elif isinstance(e, litellm.ContentPolicyViolationError):
                     if content_policy_fallbacks is not None:
-                        fallback_model_group: Optional[
-                            List[str]
-                        ] = self._get_fallback_model_group_from_fallbacks(
-                            fallbacks=content_policy_fallbacks,
-                            model_group=model_group,
+                        fallback_model_group: Optional[List[str]] = (
+                            self._get_fallback_model_group_from_fallbacks(
+                                fallbacks=content_policy_fallbacks,
+                                model_group=model_group,
+                            )
                         )
                         if fallback_model_group is None:
                             raise original_exception
diff --git a/tests/litellm/test_router.py b/tests/litellm/test_router.py
new file mode 100644
index 00000000000..66a420e79d4
--- /dev/null
+++ b/tests/litellm/test_router.py
@@ -0,0 +1,54 @@
+import copy
+import json
+import os
+import sys
+
+import pytest
+from fastapi.testclient import TestClient
+
+sys.path.insert(
+    0, os.path.abspath("../../..")
+)  # Adds the parent directory to the system path
+
+
+import litellm
+
+
+def test_update_kwargs_does_not_mutate_defaults_and_merges_metadata():
+    # initialize a real Router (env‑vars can be empty)
+    router = litellm.Router(
+        model_list=[
+            {
+                "model_name": "gpt-3.5-turbo",
+                "litellm_params": {
+                    "model": "azure/chatgpt-v-3",
+                    "api_key": os.getenv("AZURE_API_KEY"),
+                    "api_version": os.getenv("AZURE_API_VERSION"),
+                    "api_base": os.getenv("AZURE_API_BASE"),
+                },
+            }
+        ],
+    )
+
+    # override to known defaults for the test
+    router.default_litellm_params = {
+        "foo": "bar",
+        "metadata": {"baz": 123},
+    }
+    original = copy.deepcopy(router.default_litellm_params)
+    kwargs = {}
+
+    # invoke the helper
+    router._update_kwargs_with_default_litellm_params(
+        kwargs=kwargs,
+        metadata_variable_name="litellm_metadata",
+    )
+
+    # 1) router.defaults must be unchanged
+    assert router.default_litellm_params == original
+
+    # 2) non‑metadata keys get merged
+    assert kwargs["foo"] == "bar"
+
+    # 3) metadata lands under "metadata"
+    assert kwargs["litellm_metadata"] == {"baz": 123}