fix(fallback_utils): handle dictionary fallback configurations correctly in async_completion_with_fallbacks

litellm/litellm_core_utils/fallback_utils.py

@@ -47,8 +47,9 @@ async def async_completion_with_fallbacks(**kwargs):
             completion_kwargs = safe_deep_copy(base_kwargs)
             # Handle dictionary fallback configurations
             if isinstance(fallback, dict):
-                model = fallback.pop("model", original_model)
-                completion_kwargs.update(fallback)
+                fallback_config = safe_deep_copy(dict(fallback))
+                model = fallback_config.pop("model", original_model)
+                completion_kwargs.update(fallback_config)
             else:
                 model = fallback
 

tests/test_litellm/litellm_core_utils/test_fallback_utils.py

@@ -0,0 +1,43 @@
+import pytest
+
+import litellm
+from litellm.litellm_core_utils.fallback_utils import async_completion_with_fallbacks
+
+
+@pytest.mark.asyncio
+async def test_fallback_dict_not_mutated(monkeypatch):
+    fallback_dict = {"model": "fallback-model", "temperature": 0.2}
+    original_fallback_dict = dict(fallback_dict)
+
+    attempted_models: list[str] = []
+
+    async def _fake_acompletion(*, model: str, **kwargs):
+        attempted_models.append(model)
+        if model == "primary-model":
+            raise Exception("primary failed")
+        return {"model": model, "temperature": kwargs.get("temperature")}
+
+    monkeypatch.setattr(litellm, "acompletion", _fake_acompletion)
+
+    # Call 1: primary fails, fallback dict succeeds
+    response_1 = await async_completion_with_fallbacks(
+        model="primary-model",
+        kwargs={"fallbacks": [fallback_dict]},
+    )
+    assert response_1["model"] == "fallback-model"
+    assert fallback_dict == original_fallback_dict
+
+    # Call 2: re-use the same dict object; it should still work and remain unchanged
+    response_2 = await async_completion_with_fallbacks(
+        model="primary-model",
+        kwargs={"fallbacks": [fallback_dict]},
+    )
+    assert response_2["model"] == "fallback-model"
+    assert fallback_dict == original_fallback_dict
+
+    assert attempted_models == [
+        "primary-model",
+        "fallback-model",
+        "primary-model",
+        "fallback-model",
+    ]