From 2950677e5d4e7339f9484eead6c7fb6917f377f4 Mon Sep 17 00:00:00 2001
From: Sameer Kankute <sameer@berri.ai>
Date: Thu, 26 Mar 2026 12:28:25 +0530
Subject: [PATCH] fix(router): handle non-standard fallback formats with
 order-based fallback

When fallbacks use non-standard formats (e.g. ["claude-3-haiku"] or
[{"model": "...", "messages": [...]}]), detect them with
_check_non_standard_fallback_format and pass them through directly
instead of trying to parse with get_fallback_model_group which only
handles the standard dict-keyed format.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
---
 litellm/router.py                             | 19 +++++---
 .../test_router_order_fallback.py             | 47 +++++++++++++++++++
 2 files changed, 59 insertions(+), 7 deletions(-)

diff --git a/litellm/router.py b/litellm/router.py
index f63d08598a6..6b16a79ec85 100644
--- a/litellm/router.py
+++ b/litellm/router.py
@@ -5313,15 +5313,20 @@ class Router:
                 for o in order_values
                 if o > skip_up_to
             ]
-            # Get external fallbacks
+            # Get external fallbacks — handle both standard and non-standard formats
             external_fallback_group: Optional[List] = None
             if fallbacks is not None and model_group is not None:
-                external_fallback_group, generic_idx = get_fallback_model_group(
-                    fallbacks=fallbacks,
-                    model_group=cast(str, model_group),
-                )
-                if external_fallback_group is None and generic_idx is not None:
-                    external_fallback_group = fallbacks[generic_idx]["*"]
+                if _check_non_standard_fallback_format(fallbacks=fallbacks):
+                    # Non-standard formats (e.g. ["claude-3-haiku"] or
+                    # [{"model": "...", "messages": [...]}]) are passed through directly
+                    external_fallback_group = fallbacks
+                else:
+                    external_fallback_group, generic_idx = get_fallback_model_group(
+                        fallbacks=fallbacks,
+                        model_group=cast(str, model_group),
+                    )
+                    if external_fallback_group is None and generic_idx is not None:
+                        external_fallback_group = fallbacks[generic_idx]["*"]
 
             # Combined list: order fallbacks first, then external
             combined_fallbacks = order_fallback_entries + (
diff --git a/tests/test_litellm/test_router_order_fallback.py b/tests/test_litellm/test_router_order_fallback.py
index 51813fbc53b..760766a7461 100644
--- a/tests/test_litellm/test_router_order_fallback.py
+++ b/tests/test_litellm/test_router_order_fallback.py
@@ -282,3 +282,50 @@ async def test_router_order_fallback_then_external_fallback():
         messages=[{"role": "user", "content": "hi"}],
     )
     assert response._hidden_params["model_id"] == "fallback"
+
+
+@pytest.mark.asyncio
+async def test_router_order_fallback_with_non_standard_fallbacks():
+    """Non-standard fallback formats (e.g. fallbacks=["model-name"]) passed
+    per-request should still be tried after all order levels are exhausted."""
+    router = Router(
+        model_list=[
+            {
+                "model_name": "test-model",
+                "litellm_params": {
+                    "model": "gpt-4o",
+                    "api_key": "bad",
+                    "mock_response": Exception("fail order 1"),
+                    "order": 1,
+                },
+                "model_info": {"id": "1"},
+            },
+            {
+                "model_name": "test-model",
+                "litellm_params": {
+                    "model": "gpt-4o",
+                    "api_key": "bad",
+                    "mock_response": Exception("fail order 2"),
+                    "order": 2,
+                },
+                "model_info": {"id": "2"},
+            },
+            {
+                "model_name": "fallback-model",
+                "litellm_params": {
+                    "model": "gpt-4o",
+                    "api_key": "good",
+                    "mock_response": "success from non-standard fallback",
+                },
+                "model_info": {"id": "fallback"},
+            },
+        ],
+        num_retries=0,
+    )
+
+    response = await router.acompletion(
+        model="test-model",
+        messages=[{"role": "user", "content": "hi"}],
+        fallbacks=["fallback-model"],  # non-standard format, passed per-request
+    )
+    assert response._hidden_params["model_id"] == "fallback"