fix(router): don't wipe model_group_alias when updating other router settings

model_group_alias was the only UpdateRouterConfig field defaulting to {}
instead of None. /config/update merges the request's dict(exclude_none=True)
over the stored router_settings row, and {} is not None so an unset alias
survived that filter and overwrote the configured map. Saving retry settings
from the Admin UI (which sends only retry fields) therefore wiped every
model group alias. Default it to None like every sibling field so an unset
alias is dropped and the merge preserves the stored value.

litellm/types/router.py

@@ -121,7 +121,7 @@ class UpdateRouterConfig(BaseModel):
     retry_after: float | None = None
     fallbacks: list[dict] | None = None
     context_window_fallbacks: list[dict] | None = None
-    model_group_alias: dict[str, str | dict] | None = {}
+    model_group_alias: dict[str, str | dict] | None = None
     enable_tag_filtering: bool | None = None
     tag_routing_prefix: str | None = None
 

tests/test_litellm/test_router_retry_policy_update.py

@@ -27,6 +27,7 @@ from unittest.mock import AsyncMock, MagicMock
 
 import pytest
 from pydantic import ValidationError
+from pydantic_core import PydanticUndefined
 
 sys.path.insert(0, os.path.abspath("../.."))
 
@@ -83,6 +84,29 @@ def test_update_router_config_rejects_malformed_model_group_retry_policy():
         )
 
 
+def test_update_router_config_omits_unset_model_group_alias():
+    """A payload that touches only retry fields must not resurrect
+    ``model_group_alias`` in its ``exclude_none`` dump. ``/config/update``
+    merges that dump over the stored ``router_settings`` row, so a non-None
+    default here (the old ``= {}``) would overwrite a configured alias map
+    with an empty dict on every unrelated save."""
+    dumped = UpdateRouterConfig(retry_policy={"BadRequestErrorRetries": 5}).model_dump(exclude_none=True)
+    assert "model_group_alias" not in dumped
+
+
+def test_update_router_config_has_no_non_none_field_defaults():
+    """Recurrence guard for the model_group_alias overwrite bug: every field
+    must default to None (or be required) so ``/config/update``'s
+    ``dict(exclude_none=True)`` merge carries only keys the caller actually
+    sent. A field added later with ``= {}`` / ``= []`` / a ``default_factory``
+    would silently clobber that key in the stored router_settings row."""
+    for name, field in UpdateRouterConfig.model_fields.items():
+        assert field.default is None or field.default is PydanticUndefined, (
+            f"{name} has non-None default {field.default!r}"
+        )
+        assert field.default_factory is None, f"{name} uses a default_factory"
+
+
 # ---------------------------------------------------------------------------
 # Router.update_settings retry_policy path (LIT-3152 part 2)
 # ---------------------------------------------------------------------------
@@ -283,3 +307,48 @@ async def test_config_update_persists_and_reads_back_retry_policy(monkeypatch):
     assert read_back.BadRequestErrorRetries == 5
     assert read_back.TimeoutErrorRetries == 3
     assert read_back.RateLimitErrorRetries == 7
+
+
+@pytest.mark.asyncio
+async def test_config_update_retry_policy_preserves_model_group_alias(monkeypatch):
+    """Saving only retry_policy through /config/update must not wipe a
+    previously configured model_group_alias. Both live in the single
+    ``router_settings`` LiteLLM_Config JSON row, and the merge writes the
+    ``UpdateRouterConfig`` ``exclude_none`` dump over it. A non-None
+    ``model_group_alias`` default used to leak ``{}`` into that dump and
+    overwrite the stored map; the poisoned row then flows through
+    ``update_settings`` and wipes the live router too, so both are asserted."""
+    from litellm.proxy import proxy_server
+    from litellm.proxy._types import ConfigYAML, LitellmUserRoles, UserAPIKeyAuth
+
+    router = _build_router()
+
+    fake_table = _FakeConfigTable()
+    fake_table.rows["router_settings"] = _FakeConfigRow(
+        "router_settings", {"model_group_alias": {"gpt-4": "azure-gpt-4"}}
+    )
+    prisma_client = MagicMock()
+    prisma_client.db.litellm_config = fake_table
+
+    async def _apply_router_settings(*args, **kwargs):
+        await proxy_server.proxy_config._add_router_settings_from_db_config(
+            config_data={}, llm_router=router, prisma_client=prisma_client
+        )
+
+    monkeypatch.setattr(proxy_server, "prisma_client", prisma_client)
+    monkeypatch.setattr(proxy_server, "llm_router", router)
+    monkeypatch.setattr(proxy_server.proxy_config, "add_deployment", _apply_router_settings)
+    monkeypatch.setattr(proxy_server.proxy_config, "get_config", AsyncMock(return_value={}))
+
+    await proxy_server.update_config(
+        config_info=ConfigYAML(router_settings=UpdateRouterConfig(retry_policy=RetryPolicy(RateLimitErrorRetries=7))),
+        user_api_key_dict=UserAPIKeyAuth(user_role=LitellmUserRoles.PROXY_ADMIN, api_key="sk-1234"),
+    )
+
+    persisted = fake_table.rows["router_settings"].param_value
+    assert persisted["model_group_alias"] == {"gpt-4": "azure-gpt-4"}
+    assert persisted["retry_policy"]["RateLimitErrorRetries"] == 7
+
+    assert router.model_group_alias == {"gpt-4": "azure-gpt-4"}
+    assert isinstance(router.retry_policy, RetryPolicy)
+    assert router.retry_policy.RateLimitErrorRetries == 7