fix(proxy): resolve router model aliases in /utils/supported_openai_params

Co-Authored-By: Devin AI <158243242+devin-ai-integration[bot]@users.noreply.github.com>

litellm/proxy/proxy_server.py

@@ -12478,11 +12478,14 @@ async def supported_openai_params(model: str):
         --header 'Authorization: Bearer sk-1234'
     ```
     """
+    global llm_router
     try:
-        model, custom_llm_provider, _, _ = litellm.get_llm_provider(model=model)
+        deployments: Final = llm_router.get_model_list(model_name=model) if llm_router is not None else None
+        model_to_map: Final = deployments[0]["litellm_params"]["model"] if deployments else model
+        litellm_model, custom_llm_provider, _, _ = litellm.get_llm_provider(model=model_to_map)
         return {
             "supported_openai_params": litellm.get_supported_openai_params(
-                model=model, custom_llm_provider=custom_llm_provider
+                model=litellm_model, custom_llm_provider=custom_llm_provider
             )
         }
     except Exception:

tests/test_litellm/proxy/proxy_server/test_routes_utils.py

@@ -9,7 +9,7 @@ Pins (PR2):
 from __future__ import annotations
 
 import asyncio
-from unittest.mock import AsyncMock, MagicMock
+from unittest.mock import MagicMock
 
 import pytest
 
@@ -124,6 +124,35 @@ def test_supported_openai_params_happy_path(client, auth_as, patched_supported_p
     }
 
 
+def test_supported_openai_params_resolves_router_alias(client, auth_as, monkeypatch):
+    """A router ``model_name`` alias unknown to the cost map (e.g. ``claude-opus-4-6-cached``) resolves via the router's underlying ``litellm_params.model`` instead of 400ing."""
+    router = MagicMock()
+    router.get_model_list.return_value = [
+        {"model_name": "claude-opus-4-6-cached", "litellm_params": {"model": "anthropic/claude-opus-4-6"}}
+    ]
+    monkeypatch.setattr(proxy_server, "llm_router", router)
+    seen = []
+
+    def _get_llm_provider(model):
+        seen.append(model)
+        return (model, "anthropic", None, None)
+
+    monkeypatch.setattr(litellm, "get_llm_provider", _get_llm_provider)
+    monkeypatch.setattr(
+        litellm,
+        "get_supported_openai_params",
+        lambda model, custom_llm_provider=None: ["max_tokens"],
+    )
+
+    with auth_as():
+        response = client.get("/utils/supported_openai_params", params={"model": "claude-opus-4-6-cached"})
+
+    assert response.status_code == 200
+    assert response.json() == {"supported_openai_params": ["max_tokens"]}
+    router.get_model_list.assert_called_once_with(model_name="claude-opus-4-6-cached")
+    assert seen == ["anthropic/claude-opus-4-6"]
+
+
 def test_supported_openai_params_invalid_model(client, auth_as, monkeypatch):
     """Pins ``GET /utils/supported_openai_params`` (error: unknown model)."""