diff --git a/src/conductor/engine/workflow.py b/src/conductor/engine/workflow.py
index cd7e11cb..e035c876 100644
--- a/src/conductor/engine/workflow.py
+++ b/src/conductor/engine/workflow.py
@@ -3998,8 +3998,8 @@ async def _execute_loop(self, current_agent_name: str) -> dict[str, Any]:
             raise
 
     # Type-appropriate zero values for optional inputs with no declared default.
-    # Using None causes templates to render "None" instead of empty string,
-    # and | default() won't catch None without the boolean=true flag.
+    # Using None causes templates to render the literal "None" instead of a clean
+    # value, so we substitute a type-appropriate zero value up front.
     # Note: mutable types (array, object) return fresh copies via the method below.
     _TYPE_ZERO_VALUES: dict[str, Any] = {
         "string": "",
diff --git a/src/conductor/executor/template.py b/src/conductor/executor/template.py
index f2bba67b..8254bccd 100644
--- a/src/conductor/executor/template.py
+++ b/src/conductor/executor/template.py
@@ -16,6 +16,7 @@
     StrictUndefined,
     TemplateNotFound,
     TemplateSyntaxError,
+    Undefined,
 )
 from jinja2 import UndefinedError as Jinja2UndefinedError
 
@@ -116,16 +117,23 @@ def _json_filter(value: Any, indent: int = 2) -> str:
         return json.dumps(value, indent=indent, default=str)
 
     @staticmethod
-    def _default_filter(value: Any, default: Any = "") -> Any:
+    def _default_filter(value: Any, default: Any = "", boolean: bool = False) -> Any:
         """Return default if value is None or undefined.
 
         Args:
             value: The value to check.
-            default: Default value to return if value is None.
+            default: Value returned for None, Undefined, or for any falsey value
+                when boolean=True.
+            boolean: If True, return default for any falsey value.
 
         Returns:
-            The value if not None, otherwise the default.
+            The value if not None/Undefined (or not falsey if boolean=True),
+            otherwise the default.
         """
+        if isinstance(value, Undefined):
+            return default
+        if boolean and not value:
+            return default
         if value is None:
             return default
         return value
diff --git a/tests/test_executor/test_template.py b/tests/test_executor/test_template.py
index d20311f3..4dc430ce 100644
--- a/tests/test_executor/test_template.py
+++ b/tests/test_executor/test_template.py
@@ -120,6 +120,198 @@ def test_default_filter_empty_string(self) -> None:
         )
         assert result == "Value: "
 
+    def test_default_filter_boolean_true_falsey(self) -> None:
+        """Test that default filter with boolean=True returns default for falsey values.
+
+        Requirements:
+        - When boolean=True, falsey values like None, empty string, 0, False,
+          empty list, and empty dict must return the fallback default value.
+        """
+        renderer = TemplateRenderer()
+        falsey_values = [None, "", 0, False, [], {}]
+        for val in falsey_values:
+            result = renderer.render(
+                "Value: {{ value | default('fallback', boolean=True) }}",
+                {"value": val},
+            )
+            assert result == "Value: fallback"
+
+            result_shorthand = renderer.render(
+                "Value: {{ value | default('fallback', true) }}",
+                {"value": val},
+            )
+            assert result_shorthand == "Value: fallback"
+
+    def test_default_filter_boolean_true_truthy(self) -> None:
+        """Test that default filter with boolean=True returns the truthy value.
+
+        Requirements:
+        - When boolean=True, truthy values must be returned unchanged.
+        """
+        renderer = TemplateRenderer()
+        result = renderer.render(
+            "Value: {{ value | default('fallback', boolean=True) }}",
+            {"value": "actual"},
+        )
+        assert result == "Value: actual"
+
+    def test_default_filter_two_arg_none(self) -> None:
+        """Test default filter with two arguments returns default when value is None."""
+        renderer = TemplateRenderer()
+        result = renderer.render(
+            "Value: {{ value | default('fallback') }}",
+            {"value": None},
+        )
+        assert result == "Value: fallback"
+
+    def test_default_filter_two_arg_empty_string_preserved(self) -> None:
+        """Test default filter with two arguments preserves empty string."""
+        renderer = TemplateRenderer()
+        result = renderer.render(
+            "Value: {{ value | default('fallback') }}",
+            {"value": ""},
+        )
+        assert result == "Value: "
+
+    def test_default_filter_two_arg_zero_preserved(self) -> None:
+        """Test default filter with two arguments preserves 0 and returns '0'."""
+        renderer = TemplateRenderer()
+        result = renderer.render(
+            "Value: {{ value | default('fallback') }}",
+            {"value": 0},
+        )
+        assert result == "Value: 0"
+
+    def test_default_filter_two_arg_false_preserved(self) -> None:
+        """Test default filter with two arguments preserves False and returns 'False'."""
+        renderer = TemplateRenderer()
+        result = renderer.render(
+            "Value: {{ value | default('fallback') }}",
+            {"value": False},
+        )
+        assert result == "Value: False"
+
+    def test_default_filter_two_arg_empty_list_preserved(self) -> None:
+        """Test default filter with two arguments preserves empty list and returns '[]'."""
+        renderer = TemplateRenderer()
+        result = renderer.render(
+            "Value: {{ value | default('fallback') }}",
+            {"value": []},
+        )
+        assert result == "Value: []"
+
+    def test_default_filter_two_arg_empty_dict_preserved(self) -> None:
+        """Test default filter with two arguments preserves empty dict and returns '{}'."""
+        renderer = TemplateRenderer()
+        result = renderer.render(
+            "Value: {{ value | default('fallback') }}",
+            {"value": {}},
+        )
+        assert result == "Value: {}"
+
+    def test_default_filter_boolean_false_none(self) -> None:
+        """Test default filter with boolean=False returns default when value is None."""
+        renderer = TemplateRenderer()
+        result = renderer.render(
+            "Value: {{ value | default('fallback', boolean=False) }}",
+            {"value": None},
+        )
+        assert result == "Value: fallback"
+
+    def test_default_filter_boolean_false_empty_string_preserved(self) -> None:
+        """Test default filter with boolean=False preserves empty string."""
+        renderer = TemplateRenderer()
+        result = renderer.render(
+            "Value: {{ value | default('fallback', boolean=False) }}",
+            {"value": ""},
+        )
+        assert result == "Value: "
+
+    def test_default_filter_boolean_false_zero_preserved(self) -> None:
+        """Test default filter with boolean=False preserves 0 and returns '0'."""
+        renderer = TemplateRenderer()
+        result = renderer.render(
+            "Value: {{ value | default('fallback', boolean=False) }}",
+            {"value": 0},
+        )
+        assert result == "Value: 0"
+
+    def test_default_filter_boolean_false_false_preserved(self) -> None:
+        """Test default filter with boolean=False preserves False and returns 'False'."""
+        renderer = TemplateRenderer()
+        result = renderer.render(
+            "Value: {{ value | default('fallback', boolean=False) }}",
+            {"value": False},
+        )
+        assert result == "Value: False"
+
+    def test_default_filter_boolean_false_empty_list_preserved(self) -> None:
+        """Test default filter with boolean=False preserves empty list and returns '[]'."""
+        renderer = TemplateRenderer()
+        result = renderer.render(
+            "Value: {{ value | default('fallback', boolean=False) }}",
+            {"value": []},
+        )
+        assert result == "Value: []"
+
+    def test_default_filter_boolean_false_empty_dict_preserved(self) -> None:
+        """Test default filter with boolean=False preserves empty dict and returns '{}'."""
+        renderer = TemplateRenderer()
+        result = renderer.render(
+            "Value: {{ value | default('fallback', boolean=False) }}",
+            {"value": {}},
+        )
+        assert result == "Value: {}"
+
+    def test_default_filter_missing_variable_returns_default(self) -> None:
+        """Test default filter returns fallback for an undefined variable.
+
+        Requirements:
+        - A missing variable guarded by default() must render the fallback instead
+          of raising TemplateError under StrictUndefined.
+        """
+        renderer = TemplateRenderer()
+        result = renderer.render(
+            "Value: {{ missing | default('fallback') }}",
+            {},
+        )
+        assert result == "Value: fallback"
+
+    def test_default_filter_boolean_true_missing_variable_returns_default(self) -> None:
+        """Test default filter with boolean=True returns fallback for undefined values.
+
+        Requirements:
+        - A missing variable guarded by default(..., boolean=True) or default(..., true)
+          must render the fallback instead of evaluating StrictUndefined truthiness.
+        """
+        renderer = TemplateRenderer()
+
+        result = renderer.render(
+            "Value: {{ missing | default('fallback', boolean=True) }}",
+            {},
+        )
+        assert result == "Value: fallback"
+
+        shorthand_result = renderer.render(
+            "Value: {{ missing | default('fallback', true) }}",
+            {},
+        )
+        assert shorthand_result == "Value: fallback"
+
+    def test_default_filter_missing_nested_key_returns_default(self) -> None:
+        """Test default filter returns fallback for a missing nested dict key.
+
+        Requirements:
+        - A missing key on an existing dict guarded by default() must render the
+          fallback instead of raising TemplateError.
+        """
+        renderer = TemplateRenderer()
+        result = renderer.render(
+            "Value: {{ data.missing | default('fallback') }}",
+            {"data": {}},
+        )
+        assert result == "Value: fallback"
+
 
 class TestTemplateRendererConditionals:
     """Tests for conditional expressions in templates."""