diff --git a/nodes/_base.py b/nodes/_base.py
index f407f38..7e79fa7 100644
--- a/nodes/_base.py
+++ b/nodes/_base.py
@@ -76,3 +76,15 @@ class OpRegistry:
 
     def all_op_ids(self) -> list[str]:
         return sorted(self.ops.keys())
+
+
+class OutputNotProducedError(RuntimeError):
+    """Raised when a workflow wires an opaque output (MODEL/CLIP/VAE) that the active mode does not produce."""
+
+    def __init__(self, *, slot_name: str, op_id: str) -> None:
+        self.slot_name = slot_name
+        self.op_id = op_id
+        super().__init__(
+            f"Operation '{op_id}' does not produce a '{slot_name}' output — "
+            f"disconnect that slot or pick a different mode."
+        )
diff --git a/tests/unit/test_base_errors.py b/tests/unit/test_base_errors.py
new file mode 100644
index 0000000..8022174
--- /dev/null
+++ b/tests/unit/test_base_errors.py
@@ -0,0 +1,19 @@
+"""Tests for OutputNotProducedError — raised when opaque-type slots are wired but unfilled."""
+
+from nodes._base import OutputNotProducedError
+
+
+def test_error_is_runtime_error_subclass():
+    assert issubclass(OutputNotProducedError, RuntimeError)
+
+
+def test_error_message_includes_slot_name_and_op_id():
+    err = OutputNotProducedError(slot_name="model", op_id="for_loop")
+    assert "model" in str(err)
+    assert "for_loop" in str(err)
+
+
+def test_error_attributes_are_accessible():
+    err = OutputNotProducedError(slot_name="clip", op_id="echo")
+    assert err.slot_name == "clip"
+    assert err.op_id == "echo"