fix(router): preserve fusion tool choice semantics

litellm/fusion_router.py

@@ -982,8 +982,9 @@ class FusionRouter:
         kwargs: Final = _outer_kwargs(request_kwargs)
         kwargs.pop("litellm_metadata", None)
         kwargs["metadata"] = _fusion_call_metadata(request_kwargs, FUSION_INITIAL_CALL_ORIGIN)
+        client_tools: Final = _client_tools(request_kwargs.get("tools"))
         kwargs["tools"] = [  # mutable-ok: local provider payload
-            *_client_tools(request_kwargs.get("tools")),
+            *client_tools,
             _fusion_tool(),
         ]  # mutable-ok: local provider payload
         if self.config.invocation == "required":
@@ -993,6 +994,13 @@ class FusionRouter:
                     "name": FUSION_TOOL_NAME
                 },  # mutable-ok: local provider payload
             }  # mutable-ok: local provider payload
+        elif kwargs.get("tool_choice") == "none":
+            # The caller's tool policy applies to executable client tools, not
+            # to Fusion's private reasoning step. Let the outer model deliberate
+            # while keeping client tools unavailable until the final call, where
+            # the original `tool_choice="none"` is preserved.
+            kwargs["tools"] = [_fusion_tool()]  # mutable-ok: local provider payload
+            kwargs["tool_choice"] = "auto"
         elif kwargs.get("tool_choice") is None:
             kwargs["tool_choice"] = "auto"
         response: Final = await self._completion(
@@ -1198,14 +1206,11 @@ class FusionRouter:
         final_kwargs: Final = _outer_kwargs(request_kwargs)
         final_kwargs.pop("litellm_logging_obj", None)
         final_kwargs.pop("litellm_call_id", None)
-        # `required` has already been satisfied by the private Fusion call. Do
-        # not force the continuation into another tool call (or an impossible
-        # tool call when the caller supplied no client tools).
-        if request_kwargs.get("tool_choice") == "required":
-            if _client_tools(request_kwargs.get("tools")):
-                final_kwargs["tool_choice"] = "auto"
-            else:
-                final_kwargs.pop("tool_choice", None)
+        # A private Fusion call does not satisfy the caller's requirement for
+        # an executable tool call. Preserve `required` when client tools exist;
+        # only remove the impossible no-tools combination.
+        if request_kwargs.get("tool_choice") == "required" and not _client_tools(request_kwargs.get("tools")):
+            final_kwargs.pop("tool_choice", None)
         final_metadata: Final = _fusion_call_metadata(request_kwargs, FUSION_CONTINUATION_CALL_ORIGIN)
         final_kwargs.pop("litellm_metadata", None)
         final_kwargs["metadata"] = final_metadata

tests/test_litellm/test_fusion_router.py

@@ -168,6 +168,74 @@ async def test_outer_client_tool_call_is_returned_without_running_panel_or_secon
     assert [call["model"] for call in completion.calls] == ["outer"]
 
 
+@pytest.mark.asyncio
+async def test_tool_choice_none_allows_private_deliberation_but_never_client_tools() -> None:
+    completion = RecordingCompletion(
+        {
+            "outer": [_fusion_call(), _response("Final answer without a tool call")],
+            "panel-a": [_response("Panel A")],
+            "panel-b": [_response("Panel B")],
+            "analyst": [_response(_analysis())],
+        }
+    )
+    client_tool = {
+        "type": "function",
+        "function": {"name": "send_email", "parameters": {"type": "object"}},
+    }
+
+    response = await _router(completion).acompletion(
+        messages=[{"role": "user", "content": "Research this but do not send anything"}],
+        stream=False,
+        request_kwargs={"tools": [client_tool], "tool_choice": "none"},
+    )
+
+    assert isinstance(response, ModelResponse)
+    assert response.choices[0].message.content == "Final answer without a tool call"
+    assert [call["model"] for call in completion.calls] == [
+        "outer",
+        "panel-a",
+        "panel-b",
+        "analyst",
+        "outer",
+    ]
+    initial = completion.calls[0]
+    assert [tool["function"]["name"] for tool in initial["tools"]] == [FUSION_TOOL_NAME]
+    assert initial["tool_choice"] == "auto"
+    final = completion.calls[-1]
+    assert final["tools"] == [client_tool]
+    assert final["tool_choice"] == "none"
+
+
+@pytest.mark.asyncio
+async def test_named_client_tool_choice_is_preserved_and_bypasses_private_deliberation() -> None:
+    client_call = {
+        "id": "email-1",
+        "type": "function",
+        "function": {"name": "send_email", "arguments": '{"to":"user@example.com"}'},
+    }
+    completion = RecordingCompletion({"outer": [_response(None, [client_call])]})
+    client_tool = {
+        "type": "function",
+        "function": {"name": "send_email", "parameters": {"type": "object"}},
+    }
+    named_choice = {"type": "function", "function": {"name": "send_email"}}
+
+    response = await _router(completion).acompletion(
+        messages=[{"role": "user", "content": "Send the update"}],
+        stream=False,
+        request_kwargs={"tools": [client_tool], "tool_choice": named_choice},
+    )
+
+    assert isinstance(response, ModelResponse)
+    assert response.choices[0].message.tool_calls[0].function.name == "send_email"
+    assert [call["model"] for call in completion.calls] == ["outer"]
+    assert completion.calls[0]["tool_choice"] == named_choice
+    assert [tool["function"]["name"] for tool in completion.calls[0]["tools"]] == [
+        "send_email",
+        FUSION_TOOL_NAME,
+    ]
+
+
 @pytest.mark.asyncio
 async def test_mixed_fusion_and_client_tool_calls_return_only_executable_client_calls() -> None:
     client_call = {
@@ -321,7 +389,7 @@ async def test_forced_fusion_runs_parallel_panel_then_analyst_then_outer() -> No
     assert analyst["metadata"]["user_api_key_budget_reservation"] is reservation
     final = completion.calls[4]
     assert final["tools"] == [client_tool]
-    assert final["tool_choice"] == "auto"
+    assert final["tool_choice"] == "required"
     continuation = final["messages"]
     assert continuation[0] == messages[0]
     assert continuation[1]["role"] == "developer"