From eac63695d55b24444288b2159d8fe9602946ad93 Mon Sep 17 00:00:00 2001
From: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
Date: Sun, 9 Aug 2026 00:29:34 +0000
Subject: [PATCH] fix(routing_groups): build every group selector before
 swapping router state

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>
---
 litellm/router.py                             | 28 ++++++++++++-------
 .../test_router_routing_groups.py             | 27 ++++++++++++++++++
 2 files changed, 45 insertions(+), 10 deletions(-)

diff --git a/litellm/router.py b/litellm/router.py
index a00b919cf19..8f23b67d5c4 100644
--- a/litellm/router.py
+++ b/litellm/router.py
@@ -28,6 +28,7 @@ import anyio
 import httpx
 import openai
 from openai import AsyncOpenAI
+from pydantic import ValidationError
 from typing_extensions import overload
 
 import litellm
@@ -1030,6 +1031,12 @@ class Router:
             known_model_names=known_model_names,
         )
 
+        built: Final = tuple((group, self._try_build_group_selector(group)) for group in groups)
+        failures: Final = tuple(outcome for _, outcome in built if isinstance(outcome, Exception))
+        if failures:
+            self._unregister_router_selectors([outcome for _, outcome in built if not isinstance(outcome, Exception)])
+            raise failures[0]
+
         self._unregister_router_selectors(
             [sel for selectors in getattr(self, "_group_selectors", {}).values() for sel in selectors.values()]
         )
@@ -1040,19 +1047,20 @@ class Router:
         }
         self._group_selectors: dict[str, dict[str, Any]] = {
             group.group_name: (
-                {}
-                if (
-                    selector := self._build_strategy_selector(
-                        strategy=group.routing_strategy,
-                        routing_strategy_args=group.routing_strategy_args or {},
-                    )
-                )
-                is None
-                else {self._normalize_strategy(group.routing_strategy) or "": selector}
+                {} if selector is None else {self._normalize_strategy(group.routing_strategy) or "": selector}
             )
-            for group in groups
+            for group, selector in built
         }
 
+    def _try_build_group_selector(self, group: RoutingGroup) -> object | None:
+        try:
+            return self._build_strategy_selector(
+                strategy=group.routing_strategy,
+                routing_strategy_args=group.routing_strategy_args or {},
+            )
+        except ValidationError as build_error:
+            return build_error
+
     _OVERRIDABLE_ROUTING_STRATEGIES: frozenset[str] = frozenset({"simple-shuffle", *_DEFAULT_SELECTOR_ATTR_BY_STRATEGY})
 
     def _get_request_routing_strategy_override(self, request_kwargs: dict | None) -> str | None:
diff --git a/tests/test_litellm/router_strategy/test_router_routing_groups.py b/tests/test_litellm/router_strategy/test_router_routing_groups.py
index 8865bdf0172..c4d667a6af9 100644
--- a/tests/test_litellm/router_strategy/test_router_routing_groups.py
+++ b/tests/test_litellm/router_strategy/test_router_routing_groups.py
@@ -788,3 +788,30 @@ def test_invalid_group_strategy_does_not_leak_a_selector():
 
     assert list(router._routing_groups) == ["g1"]
     assert any(id(cb) == id(selector) for cb in litellm.callbacks)
+
+
+def test_unbuildable_group_selector_keeps_previous_groups():
+    router = _build_router(
+        routing_groups=[
+            {"group_name": "g1", "models": ["filtered-model"], "routing_strategy": "latency-based-routing"},
+        ],
+    )
+    selector = router._group_selectors["g1"]["latency-based-routing"]
+
+    with pytest.raises(Exception):
+        router.update_settings(
+            routing_groups=[
+                {"group_name": "g1", "models": ["filtered-model"], "routing_strategy": "latency-based-routing"},
+                {
+                    "group_name": "g2",
+                    "models": ["other-model"],
+                    "routing_strategy": "latency-based-routing",
+                    "routing_strategy_args": {"ttl": "not-a-number"},
+                },
+            ],
+        )
+
+    assert list(router._routing_groups) == ["g1"]
+    assert router._model_to_group == {"filtered-model": "g1"}
+    assert router._group_selectors["g1"]["latency-based-routing"] is selector
+    assert sum(1 for cb in litellm.callbacks if type(cb) is type(selector)) == 1