fix(router): stop fallback lookups from mutating the router fallbacks config

get_fallback_model_group resolved a bare-string fallback by popping it out
of the fallbacks list it was handed. That list is frequently the live
router.fallbacks config, so a single lookup permanently removed the entry and
the configured fallback stopped applying to later requests until restart. The
pop also ran inside enumerate(), shifting indices and skipping an adjacent
string fallback. Read the item instead of popping it, and add a regression
test that fails on the old mutating behavior

litellm/router_utils/fallback_event_handlers.py

@@ -72,7 +72,7 @@ def get_fallback_model_group(
             elif list(item.keys())[0] == "*":  # check generic fallback
                 generic_fallback_idx = idx
         elif isinstance(item, str):
-            fallback_model_group = [fallbacks.pop(idx)]  # returns single-item list
+            fallback_model_group = [item]
     ## if none, check for generic fallback
     if fallback_model_group is None:
         if stripped_model_fallback is not None:

tests/test_litellm/router_utils/test_fallback_event_handlers.py

@@ -2,7 +2,10 @@ import json
 
 import pytest
 
-from litellm.router_utils.fallback_event_handlers import run_async_fallback
+from litellm.router_utils.fallback_event_handlers import (
+    get_fallback_model_group,
+    run_async_fallback,
+)
 
 
 class StreamingWrapper:
@@ -137,3 +140,16 @@ async def test_run_async_fallback_skips_original_model_group():
     )
 
     assert response._hidden_params["additional_headers"]["x-litellm-attempted-fallbacks"] == 1
+
+
+def test_get_fallback_model_group_does_not_mutate_fallbacks():
+    """A string fallback must be resolved without mutating the caller's
+    fallbacks list, which is the live router config shared across requests."""
+    fallbacks = [{"gpt-3.5-turbo": ["claude-3-haiku"]}, "gpt-4o-mini"]
+
+    fallback_model_group, _ = get_fallback_model_group(
+        fallbacks=fallbacks, model_group="unmatched-model"
+    )
+
+    assert fallback_model_group == ["gpt-4o-mini"]
+    assert fallbacks == [{"gpt-3.5-turbo": ["claude-3-haiku"]}, "gpt-4o-mini"]