fix(decisions): honor base_url alias for custom backends (#48505)

Co-authored-by: Azmi Muwahid <azmimuwahid@gmail.com>

litellm/decisions/call.py

@@ -456,10 +456,14 @@ def prepare_call(
     extra_headers: Mapping[str, str] | None,
     kwargs: Mapping[str, object],
 ) -> DecisionsCall:
+    base_url_alias: Final = kwargs.get("base_url")
+    call_api_base: Final = (
+        base_url_alias if api_base is None and isinstance(base_url_alias, str) and base_url_alias else api_base
+    )
     upstream_model, provider, dynamic_api_key, dynamic_api_base = litellm.get_llm_provider(
         model=model,
         custom_llm_provider=custom_llm_provider,
-        api_base=api_base,
+        api_base=call_api_base,
         api_key=api_key,
     )
     provider_config: Final = _provider_config(upstream_model, provider)
@@ -488,7 +492,7 @@ def prepare_call(
 
     try:
         resolved_api_base, static_api_key = provider_config.resolve_connection(
-            dynamic_api_base or api_base, dynamic_api_key or api_key
+            dynamic_api_base or call_api_base, dynamic_api_key or api_key
         )
     except ValueError as error:
         raise litellm.BadRequestError(message=str(error), model=model, llm_provider=provider) from error

tests/unit/decisions/test_main.py

@@ -400,6 +400,68 @@ async def test_systemone_uses_evaluation_pricing(respx_mock: respx.MockRouter, d
     )
 
 
+@pytest.mark.asyncio
+@pytest.mark.parametrize(
+    ("base_url", "expected_api_base"),
+    [
+        ("https://onprem.example/v1", "https://onprem.example/v1"),
+        ("", "https://fallback.example/v1"),
+        (None, "https://fallback.example/v1"),
+        (123, "https://fallback.example/v1"),
+    ],
+)
+async def test_openai_decisions_honors_base_url_alias(
+    respx_mock: respx.MockRouter,
+    monkeypatch: pytest.MonkeyPatch,
+    base_url: str | int | None,
+    expected_api_base: str,
+) -> None:
+    monkeypatch.setenv("OPENAI_BASE_URL", "https://fallback.example/v1")
+    route: Final = respx_mock.post(f"{expected_api_base}/decisions").respond(json=_OPENAI_RESPONSE)
+    response: Final = await litellm.adecisions(
+        model="openai/gpt-6-luna",
+        input="The package arrived broken.",
+        questions=[{"type": "predicate", "name": "is_defect", "instructions": "Is this a defect?"}],
+        api_key="backend-secret-key",
+        base_url=base_url,
+    )
+
+    assert route.called
+    assert route.calls[0].request.headers["authorization"] == "Bearer backend-secret-key"
+    assert response._hidden_params["model"] == "openai/gpt-6-luna"
+
+
+@pytest.mark.asyncio
+async def test_openai_systemone_honors_base_url_alias(respx_mock: respx.MockRouter) -> None:
+    route: Final = respx_mock.post("https://onprem.example/v1/decisions").respond(json=_OPENAI_RESPONSE)
+    response: Final = await litellm.asystemone(
+        model="openai/gpt-6-luna",
+        state="The package arrived broken.",
+        questions=_QUESTIONS,
+        api_key="backend-secret-key",
+        base_url="https://onprem.example/v1",
+    )
+
+    assert route.called
+    assert route.calls[0].request.headers["authorization"] == "Bearer backend-secret-key"
+    assert response._hidden_params["model"] == "openai/gpt-6-luna"
+
+
+@pytest.mark.asyncio
+async def test_openai_decisions_api_base_wins_over_base_url_alias(respx_mock: respx.MockRouter) -> None:
+    route: Final = respx_mock.post("https://api.openai.com/v1/decisions").respond(json=_OPENAI_RESPONSE)
+    await litellm.adecisions(
+        model="openai/gpt-6-luna",
+        input="The package arrived broken.",
+        questions=[{"type": "predicate", "name": "is_defect", "instructions": "Is this a defect?"}],
+        api_key="test-key",
+        api_base="https://api.openai.com/v1",
+        base_url="https://onprem.example/v1",
+    )
+
+    assert route.called
+
+
 @pytest.mark.asyncio
 async def test_systemone_falls_back_to_plain_pricing_without_decisions_row(
     respx_mock: respx.MockRouter,