fix(router): resolve realtime session model to routed deployment

litellm/router.py

@@ -30,7 +30,7 @@
 import httpx
 import openai
 from openai import AsyncOpenAI
-from pydantic import BaseModel
+from pydantic import BaseModel, TypeAdapter, ValidationError
 from typing_extensions import overload
 
 import litellm
@@ -383,6 +383,28 @@ def _stream_chunks_have_generated_content(chunks: Sequence[ModelResponseStream])
     return False
 
 
+_NO_SESSION_KWARGS: Final[Mapping[str, Mapping[str, object]]] = MappingProxyType({})
+_SESSION_ADAPTER: Final = TypeAdapter(Mapping[str, object])
+
+
+def _with_router_resolved_session_model(session: object, model_name: str) -> Mapping[str, Mapping[str, object]]:
+    """
+    Realtime client-secret requests carry the model inside ``session`` as well, and the caller's copy of it still
+    holds the pre-routing model group name, so it has to follow the deployment the router just picked.
+
+    Returns kwargs to merge into the downstream call, empty when there is no session model to resolve.
+    """
+    try:
+        typed_session: Final = _SESSION_ADAPTER.validate_python(session)
+    except ValidationError:
+        return _NO_SESSION_KWARGS
+    if "model" not in typed_session:
+        return _NO_SESSION_KWARGS
+    return MappingProxyType(
+        {"session": {**typed_session, "model": model_name}}  # mutable-ok: callees deepcopy and JSON-dump session
+    )
+
+
 # Router._aanthropic_messages_streaming_iterator buffers lifecycle chunks
 # until real content commits the primary stream; a hostile or slow-starting
 # upstream that never emits content or an error could otherwise grow that
@@ -4930,6 +4952,7 @@ async def _ageneric_api_call_with_fallbacks_helper(self, model: str, original_ge
                 "caching": self.cache_responses,
                 **kwargs,
                 "model": model_name,
+                **_with_router_resolved_session_model(kwargs.get("session"), model_name),
             }
             # Only set custom_llm_provider if it's not None
             if custom_llm_provider is not None:

tests/test_litellm/test_router.py

@@ -6,6 +6,7 @@
 import os
 import threading
 from types import SimpleNamespace
+from typing import Final
 from unittest.mock import AsyncMock, MagicMock, patch
 
 import httpx
@@ -1537,6 +1538,91 @@ def inject_alias_into_kwargs(deployment, kwargs, function_name=None):
     ), f"Expected deployment model 'vertex_ai/gemini-2.5-flash', got '{captured['model']}'"
 
 
+@pytest.mark.asyncio
+async def test_ageneric_api_call_resolves_realtime_session_model():
+    """
+    Regression for #36742: realtime client secret requests carry the model inside `session` too, and the proxy
+    fills it with the pre-routing model group name. The underlying litellm function reads session.model first,
+    so it must see the resolved deployment, while a caller's nested transcription model stays untouched.
+    """
+    routed: Final = AsyncMock(return_value={"result": "ok"})
+
+    router = litellm.Router(
+        model_list=[
+            {
+                "model_name": "my-realtime-group",
+                "litellm_params": {
+                    "model": "openai/gpt-realtime-2.1-mini",
+                    "api_key": "fake-key",
+                },
+                "model_info": {"mode": "realtime"},
+            }
+        ]
+    )
+
+    await router._ageneric_api_call_with_fallbacks(
+        model="my-realtime-group",
+        original_function=routed,
+        session={
+            "type": "realtime",
+            "model": "my-realtime-group",
+            "audio": {"input": {"transcription": {"model": "gpt-4o-transcribe"}}},
+        },
+    )
+
+    sent: Final = routed.call_args.kwargs
+    assert sent["model"] == "openai/gpt-realtime-2.1-mini"
+    assert sent["session"]["model"] == "openai/gpt-realtime-2.1-mini"
+    assert sent["session"]["audio"]["input"]["transcription"]["model"] == "gpt-4o-transcribe"
+
+
+@pytest.mark.asyncio
+async def test_ageneric_api_call_does_not_add_session_model():
+    """
+    A session that never carried a model must not gain one from routing: the underlying function then falls back
+    to the resolved `model` kwarg itself, and the outgoing session body keeps the caller's shape.
+    """
+    routed: Final = AsyncMock(return_value={"result": "ok"})
+
+    router = litellm.Router(
+        model_list=[
+            {
+                "model_name": "my-realtime-group",
+                "litellm_params": {
+                    "model": "openai/gpt-realtime-2.1-mini",
+                    "api_key": "fake-key",
+                },
+                "model_info": {"mode": "realtime"},
+            }
+        ]
+    )
+
+    await router._ageneric_api_call_with_fallbacks(
+        model="my-realtime-group",
+        original_function=routed,
+        session={"type": "realtime"},
+    )
+
+    sent: Final = routed.call_args.kwargs
+    assert sent["model"] == "openai/gpt-realtime-2.1-mini"
+    assert sent["session"] == {"type": "realtime"}
+
+
+@pytest.mark.parametrize(
+    "session, expected",
+    [
+        ({"type": "realtime", "model": "my-realtime-group"}, {"session": {"type": "realtime", "model": "resolved"}}),
+        ({"type": "realtime"}, {}),
+        (None, {}),
+        ("not-a-session", {}),
+    ],
+)
+def test_with_router_resolved_session_model(session, expected):
+    from litellm.router import _with_router_resolved_session_model
+
+    assert dict(_with_router_resolved_session_model(session, "resolved")) == expected
+
+
 def test_router_get_model_access_groups_team_only_models():
     """
     Test that Router.get_model_access_groups returns the correct response for team-only models