fix(router): resolve model_group_alias before pre-routing hook lookups

When a model_group_alias points to a complexity_router (or auto_router,
quality_router, adaptive_router) group, calling completion() with the
alias name skips the pre-routing hook because the alias is not found in
the router maps. The hook returns None, the raw auto_router deployment
is selected, and litellm.acompletion fails with "Unmapped LLM provider".

Resolve aliases at the top of async_pre_routing_hook using the existing
_get_model_from_alias() utility so the resolved group name is used for
all router map lookups.

Fixes #27473

litellm/router.py

@@ -10287,6 +10287,11 @@ class Router:
 
         Used for the litellm auto-router to modify the request before the routing decision is made.
         """
+        # Resolve model_group_alias so aliased names hit the correct router map
+        _resolved = self._get_model_from_alias(model)
+        if _resolved is not None:
+            model = _resolved
+
         #########################################################
         # Check if any auto-router should be used
         #########################################################

tests/test_litellm/router_utils/test_pre_routing_hook_alias_resolution.py

@@ -0,0 +1,210 @@
+"""
+Tests that Router.async_pre_routing_hook resolves model_group_alias
+before looking up router maps (complexity_routers, auto_routers, etc.).
+
+Without alias resolution, calling completion(model="my-alias") when
+"my-alias" maps to a complexity_router group causes an "Unmapped LLM
+provider" 400 error because the alias name is not found in the router
+maps, so the pre-routing hook returns None and the raw
+auto_router/complexity_router deployment is selected as-is.
+
+See: https://github.com/BerriAI/litellm/issues/27473
+"""
+
+import os
+import sys
+from unittest.mock import AsyncMock, MagicMock
+
+import pytest
+
+sys.path.insert(
+    0, os.path.abspath("../../..")
+)  # Adds the parent directory to the system path
+
+from litellm import Router
+from litellm.types.router import PreRoutingHookResponse
+
+
+def _make_router_with_alias(
+    alias_name: str,
+    target_group: str,
+    router_type: str = "complexity",
+) -> Router:
+    """Build a Router with a model_group_alias and a registered router."""
+    router = Router(
+        model_list=[
+            {
+                "model_name": "gpt-4o-mini",
+                "litellm_params": {"model": "openai/gpt-4o-mini"},
+            }
+        ],
+        model_group_alias={alias_name: target_group},
+    )
+
+    mock_sub_router = MagicMock()
+    mock_sub_router.async_pre_routing_hook = AsyncMock(
+        return_value=PreRoutingHookResponse(
+            model="gpt-4o-mini",
+            messages=[{"role": "user", "content": "test"}],
+        )
+    )
+
+    if router_type == "complexity":
+        router.complexity_routers[target_group] = mock_sub_router
+    elif router_type == "auto":
+        router.auto_routers[target_group] = mock_sub_router
+    elif router_type == "quality":
+        router.quality_routers[target_group] = mock_sub_router
+
+    return router
+
+
+class TestPreRoutingHookAliasResolution:
+    """Router.async_pre_routing_hook must resolve model_group_alias."""
+
+    @pytest.mark.asyncio
+    async def test_alias_resolves_to_complexity_router(self):
+        """An aliased model name should match the complexity_router for
+        the resolved group name."""
+        router = _make_router_with_alias(
+            alias_name="my-alias",
+            target_group="auto_router/complexity_router/my-router",
+            router_type="complexity",
+        )
+
+        result = await router.async_pre_routing_hook(
+            model="my-alias",
+            request_kwargs={},
+            messages=[{"role": "user", "content": "Hello"}],
+        )
+
+        assert result is not None
+        assert result.model == "gpt-4o-mini"
+        mock = router.complexity_routers["auto_router/complexity_router/my-router"]
+        mock.async_pre_routing_hook.assert_awaited_once()
+
+    @pytest.mark.asyncio
+    async def test_alias_resolves_to_auto_router(self):
+        """An aliased model name should match the auto_router for the
+        resolved group name."""
+        router = _make_router_with_alias(
+            alias_name="smart",
+            target_group="auto_router/my-auto-router",
+            router_type="auto",
+        )
+
+        result = await router.async_pre_routing_hook(
+            model="smart",
+            request_kwargs={},
+            messages=[{"role": "user", "content": "Hello"}],
+        )
+
+        assert result is not None
+        mock = router.auto_routers["auto_router/my-auto-router"]
+        mock.async_pre_routing_hook.assert_awaited_once()
+
+    @pytest.mark.asyncio
+    async def test_alias_resolves_to_quality_router(self):
+        """An aliased model name should match the quality_router for the
+        resolved group name."""
+        router = _make_router_with_alias(
+            alias_name="best",
+            target_group="quality-router-group",
+            router_type="quality",
+        )
+
+        result = await router.async_pre_routing_hook(
+            model="best",
+            request_kwargs={},
+            messages=[{"role": "user", "content": "Hello"}],
+        )
+
+        assert result is not None
+        mock = router.quality_routers["quality-router-group"]
+        mock.async_pre_routing_hook.assert_awaited_once()
+
+    @pytest.mark.asyncio
+    async def test_non_alias_model_still_works(self):
+        """A model name that is NOT an alias should still match the router
+        map directly (no regression)."""
+        router = Router(
+            model_list=[
+                {
+                    "model_name": "gpt-4o-mini",
+                    "litellm_params": {"model": "openai/gpt-4o-mini"},
+                }
+            ],
+        )
+        mock_sub_router = MagicMock()
+        mock_sub_router.async_pre_routing_hook = AsyncMock(
+            return_value=PreRoutingHookResponse(
+                model="gpt-4o-mini",
+                messages=[{"role": "user", "content": "test"}],
+            )
+        )
+        router.complexity_routers["my-router"] = mock_sub_router
+
+        result = await router.async_pre_routing_hook(
+            model="my-router",
+            request_kwargs={},
+            messages=[{"role": "user", "content": "Hello"}],
+        )
+
+        assert result is not None
+        mock_sub_router.async_pre_routing_hook.assert_awaited_once()
+
+    @pytest.mark.asyncio
+    async def test_no_alias_no_router_returns_none(self):
+        """When the model is not an alias and not in any router map,
+        async_pre_routing_hook should return None."""
+        router = Router(
+            model_list=[
+                {
+                    "model_name": "gpt-4o-mini",
+                    "litellm_params": {"model": "openai/gpt-4o-mini"},
+                }
+            ],
+        )
+
+        result = await router.async_pre_routing_hook(
+            model="some-unknown-model",
+            request_kwargs={},
+            messages=[{"role": "user", "content": "Hello"}],
+        )
+
+        assert result is None
+
+    @pytest.mark.asyncio
+    async def test_alias_dict_format_resolves(self):
+        """model_group_alias supports dict format with a 'model' key."""
+        router = Router(
+            model_list=[
+                {
+                    "model_name": "gpt-4o-mini",
+                    "litellm_params": {"model": "openai/gpt-4o-mini"},
+                }
+            ],
+            model_group_alias={
+                "my-alias": {
+                    "model": "auto_router/complexity_router/cr",
+                    "hidden": False,
+                }
+            },
+        )
+        mock_sub_router = MagicMock()
+        mock_sub_router.async_pre_routing_hook = AsyncMock(
+            return_value=PreRoutingHookResponse(
+                model="gpt-4o-mini",
+                messages=[{"role": "user", "content": "test"}],
+            )
+        )
+        router.complexity_routers["auto_router/complexity_router/cr"] = mock_sub_router
+
+        result = await router.async_pre_routing_hook(
+            model="my-alias",
+            request_kwargs={},
+            messages=[{"role": "user", "content": "Hello"}],
+        )
+
+        assert result is not None
+        mock_sub_router.async_pre_routing_hook.assert_awaited_once()