diff --git a/python/packages/declarative/agent_framework_declarative/_workflows/_declarative_base.py b/python/packages/declarative/agent_framework_declarative/_workflows/_declarative_base.py
index 42aec8c49bb..b586b086692 100644
--- a/python/packages/declarative/agent_framework_declarative/_workflows/_declarative_base.py
+++ b/python/packages/declarative/agent_framework_declarative/_workflows/_declarative_base.py
@@ -734,14 +734,14 @@ def _eval_custom_function(self, formula: str) -> Any | None:
             evaluated_args: list[str] = []
             for arg in args:
                 arg = arg.strip()
-                if arg.startswith('"') and arg.endswith('"'):
+                if re.fullmatch(r'"(?:[^"]|"")*"', arg):
                     # String literal
-                    evaluated_args.append(arg[1:-1])
+                    evaluated_args.append(arg[1:-1].replace('""', '"'))
                 elif arg.startswith("'") and arg.endswith("'"):
                     # Single-quoted string literal
                     evaluated_args.append(arg[1:-1])
                 else:
-                    # Variable reference - evaluate it
+                    # Variable reference or expression - evaluate it
                     result = self.eval(f"={arg}")
                     evaluated_args.append(str(result) if result is not None else "")
             return "".join(evaluated_args)
diff --git a/python/packages/declarative/tests/test_graph_executors.py b/python/packages/declarative/tests/test_graph_executors.py
index c6b451e6f5a..f7e0edd252c 100644
--- a/python/packages/declarative/tests/test_graph_executors.py
+++ b/python/packages/declarative/tests/test_graph_executors.py
@@ -1810,6 +1810,29 @@ def test_recovered_outer_array_wins_over_nested_object(self):
 class TestPowerFxConditionalImport:
     """The _declarative_base module should be importable without dotnet/powerfx."""
 
+    @pytest.mark.parametrize("function", ["Concat", "Concatenate"])
+    @pytest.mark.parametrize(
+        ("arguments", "expected"),
+        [
+            ('"hello", " world"', "hello world"),
+            ('"", ""', ""),
+            ('"a""b", "c"', 'a"bc'),
+            ('"a,b(c)", "d"', "a,b(c)d"),
+            ("'legacy', ' strings'", "legacy strings"),
+        ],
+    )
+    def test_concatenation_literals_without_engine(
+        self, monkeypatch: pytest.MonkeyPatch, function: str, arguments: str, expected: str
+    ) -> None:
+        """String-only concatenation retains its engine-independent fast path."""
+        import agent_framework_declarative._workflows._declarative_base as base_mod
+
+        state = DeclarativeWorkflowState(State())
+        state.initialize()
+        monkeypatch.setattr(base_mod, "Engine", None)
+
+        assert state.eval(f"={function}({arguments})") == expected
+
     def test_import_guard_exists(self):
         """The powerfx import must be wrapped in try/except."""
         import agent_framework_declarative._workflows._declarative_base as base_mod
diff --git a/python/packages/declarative/tests/test_graph_workflow_integration.py b/python/packages/declarative/tests/test_graph_workflow_integration.py
index 830570ae0c9..87246b01fc7 100644
--- a/python/packages/declarative/tests/test_graph_workflow_integration.py
+++ b/python/packages/declarative/tests/test_graph_workflow_integration.py
@@ -197,6 +197,47 @@ async def test_workflow_with_condition_group(self):
 class TestWorkflowFactory:
     """Tests for WorkflowFactory."""
 
+    @pytest.mark.parametrize("function", ["Concat", "Concatenate"])
+    @pytest.mark.parametrize(
+        ("arguments", "expected"),
+        [
+            ('"hello", " world"', "hello world"),
+            ('"", ""', ""),
+            ('"a""b", "c"', 'a"bc'),
+            ('"""", """"', '""'),
+            ('"a" & "b", "c"', "abc"),
+            ('"a" & Local.word & "b", "c"', "aWORDbc"),
+            ('"a,b(c)", "d"', "a,b(c)d"),
+            ('"a"",b(c)", "d"', 'a",b(c)d'),
+            ('"路径", "与文本"', "路径与文本"),
+            ('"a\\b", "c"', "a\\bc"),
+            ('Local.word, "!"', "WORD!"),
+            ('Concatenate("a""b", "c"), "d"', 'a"bcd'),
+            ('MessageText(Local.messages), "!"', "hello!"),
+            ('Local.missing, "!"', "!"),
+        ],
+    )
+    async def test_concatenation_arguments(self, function: str, arguments: str, expected: str) -> None:
+        """Render literals and expressions through factory-created workflow state."""
+        workflow = WorkflowFactory().create_workflow_from_definition({
+            "name": "concatenation_arguments",
+            "actions": [
+                {"kind": "SetValue", "id": "set_word", "path": "Local.word", "value": "WORD"},
+                {
+                    "kind": "SetValue",
+                    "id": "set_messages",
+                    "path": "Local.messages",
+                    "value": [{"role": "user", "text": "hello"}],
+                },
+                {"kind": "SendActivity", "id": "send", "activity": {"text": f"={function}({arguments})"}},
+            ],
+        })
+
+        events = await workflow.run({})
+
+        # SendActivity omits empty rendered activities.
+        assert events.get_outputs() == ([expected] if expected else [])
+
     def test_factory_creates_workflow(self):
         """Test creating workflow."""
         factory = WorkflowFactory()