diff --git a/src/cleveragents/tui/_tui_events.py b/src/cleveragents/tui/_tui_events.py
index 0f205bcd6..81c459e3a 100644
--- a/src/cleveragents/tui/_tui_events.py
+++ b/src/cleveragents/tui/_tui_events.py
@@ -86,7 +86,30 @@ class TuiWidgetEvent:
         )
 
 
+# ---------------------------------------------------------------------------
+# UnknownElementType exception
+# ---------------------------------------------------------------------------
+
+
+class UnknownElementType(ValueError):
+    """Raised when an element type is not recognised by the TUI renderer.
+
+    This exception is raised instead of falling back to ``str(element)``
+    in :func:`render_element_for_tui` so that callers get a meaningful
+    diagnostic rather than silently rendering garbage into the TUI output.
+    """
+
+    __slots__ = ("element",)
+
+    def __init__(self, element: object) -> None:
+        super().__init__(
+            f"Unknown element type {type(element).__name__!r} value={element!r}"
+        )
+        self.element = element
+
+
 __all__ = [
     "TuiWidgetEvent",
     "TuiWidgetEventType",
+    "UnknownElementType",
 ]
diff --git a/src/cleveragents/tui/_tui_renderers.py b/src/cleveragents/tui/_tui_renderers.py
index 72416313d..c39787164 100644
--- a/src/cleveragents/tui/_tui_renderers.py
+++ b/src/cleveragents/tui/_tui_renderers.py
@@ -18,9 +18,10 @@ from cleveragents.cli.output.handles._models import (
     StatusMessage,
     Table,
     TextBlock,
-    TreeNode,
     Tree,
+    TreeNode,
 )
+from cleveragents.tui._tui_events import UnknownElementType
 
 # ---------------------------------------------------------------------------
 # Rendering helpers
@@ -196,7 +197,7 @@ def render_element_for_tui(element: ElementSnapshot) -> str:
         return _render_action_hint(element)
     if isinstance(element, TextBlock):
         return _render_text(element)
-    return str(element)  # pragma: no cover
+    raise UnknownElementType(element)  # pragma: no cover
 
 
 __all__ = [
diff --git a/src/cleveragents/tui/materializer.py b/src/cleveragents/tui/materializer.py
index 674bf58f7..98a50ace6 100644
--- a/src/cleveragents/tui/materializer.py
+++ b/src/cleveragents/tui/materializer.py
@@ -41,6 +41,7 @@ from typing import TYPE_CHECKING
 from cleveragents.tui._tui_events import (
     TuiWidgetEvent,
     TuiWidgetEventType,
+    UnknownElementType,
 )
 from cleveragents.tui._tui_renderers import render_element_for_tui
 
@@ -61,6 +62,7 @@ __all__ = [
     "TuiMaterializer",
     "TuiWidgetEvent",
     "TuiWidgetEventType",
+    "UnknownElementType",
     "render_element_for_tui",
 ]