diff --git a/litellm/litellm_core_utils/safe_json_dumps.py b/litellm/litellm_core_utils/safe_json_dumps.py
index 154306d01b8..81cd8e57798 100644
--- a/litellm/litellm_core_utils/safe_json_dumps.py
+++ b/litellm/litellm_core_utils/safe_json_dumps.py
@@ -24,7 +24,7 @@ def safe_dumps(data: Any, max_depth: int = DEFAULT_MAX_RECURSE_DEPTH) -> str:
             return "MaxDepthExceeded"
         # Base-case: if it is a primitive, simply return it.
         if isinstance(obj, str):
-            return strip_null_bytes(obj)
+            return obj.replace("\x00", "") if "\x00" in obj else obj
         if isinstance(obj, (int, float, bool, type(None))):
             return obj
         # Check for circular reference.
@@ -36,7 +36,8 @@ def safe_dumps(data: Any, max_depth: int = DEFAULT_MAX_RECURSE_DEPTH) -> str:
             result = {}
             for k, v in obj.items():
                 if isinstance(k, (str)):
-                    result[strip_null_bytes(k)] = _serialize(v, seen, depth + 1)
+                    clean_k = k.replace("\x00", "") if "\x00" in k else k
+                    result[clean_k] = _serialize(v, seen, depth + 1)
             seen.remove(id(obj))
             return result
         elif isinstance(obj, list):
diff --git a/tests/test_litellm/litellm_core_utils/test_safe_json_dumps.py b/tests/test_litellm/litellm_core_utils/test_safe_json_dumps.py
index 74574370e46..ad24105588d 100644
--- a/tests/test_litellm/litellm_core_utils/test_safe_json_dumps.py
+++ b/tests/test_litellm/litellm_core_utils/test_safe_json_dumps.py
@@ -174,6 +174,30 @@ def test_null_byte_stripped_in_fallback_str():
     assert json.loads(out)["obj"] == "objrepr"
 
 
+def test_clean_strings_are_not_run_through_replace():
+    """Regression for LIT-3910.
+
+    safe_dumps must not call ``str.replace`` on NUL-free strings. Running the
+    NUL strip unconditionally on every value and dict key (the v1.89.x behavior)
+    added per-request serialization overhead that scaled with payload size and
+    showed up under ``store_prompts_in_spend_logs``. Clean strings, which are the
+    overwhelming majority, must be returned untouched.
+    """
+
+    class ReplaceForbidden(str):
+        def replace(self, *args, **kwargs):
+            raise AssertionError("safe_dumps ran str.replace on a NUL-free string")
+
+    data = {
+        ReplaceForbidden("clean_key"): ReplaceForbidden("clean_value"),
+        "nested": [ReplaceForbidden("a"), {"deep": ReplaceForbidden("b")}],
+    }
+    result = json.loads(safe_dumps(data))
+    assert result["clean_key"] == "clean_value"
+    assert result["nested"][0] == "a"
+    assert result["nested"][1]["deep"] == "b"
+
+
 def test_pydantic_base_model():
     from pydantic import BaseModel