Fix handling of empty steering responses in tool call middleware

src/core/services/tool_call_reactor_middleware.py

@@ -195,7 +195,7 @@ async def process(
                     )
 
                     # Create a new response with the replacement content
-                    if result.replacement_response:
+                    if result.replacement_response is not None:
                         replacement_response = self._create_replacement_response(
                             response,
                             result.replacement_response,

tests/unit/core/services/test_tool_call_reactor_middleware.py

@@ -306,6 +306,50 @@ async def test_process_with_tool_calls_swallowed(self, middleware, mock_reactor)
         assert result.metadata["original"] == "metadata"
         assert result.metadata["tool_call_reactor"]["handler"] == "test_handler"
 
+    @pytest.mark.asyncio
+    async def test_process_with_empty_replacement(self, middleware, mock_reactor):
+        """Handlers should be able to swallow calls with empty steering content."""
+
+        tool_call_response = {
+            "choices": [
+                {
+                    "message": {
+                        "tool_calls": [
+                            {
+                                "id": "call_321",
+                                "type": "function",
+                                "function": {
+                                    "name": "test_tool",
+                                    "arguments": '{"arg": "value"}',
+                                },
+                            }
+                        ]
+                    }
+                }
+            ]
+        }
+
+        response = ProcessedResponse(content=json.dumps(tool_call_response))
+
+        swallow_result = ToolCallReactionResult(
+            should_swallow=True,
+            replacement_response="",
+            metadata={"handler": "test_handler"},
+        )
+
+        mock_reactor.process_tool_call.return_value = swallow_result
+
+        result = await middleware.process(
+            response=response,
+            session_id="test_session",
+            context={"backend_name": "test", "model_name": "test"},
+        )
+
+        assert isinstance(result, ProcessedResponse)
+        assert result is not response
+        assert result.content == ""
+        assert result.metadata["tool_call_swallowed"] is True
+
     @pytest.mark.asyncio
     async def test_process_with_tool_calls_swallowed_merges_metadata(
         self, middleware, mock_reactor