fix: handle None return from error handler in no-output background callbacks

dash/_callback.py

@@ -635,6 +635,7 @@ def _update_background_callback(
     kwargs,
     background,
     multi,
+    output_spec,
     cache_key=None,
     job_id=None,
 ):
@@ -663,6 +664,7 @@ def _update_background_callback(
         error_handler,
         callback_ctx,
         multi,
+        output_spec,
         cache_key=cache_key,
         job_id=job_id,
     )
@@ -675,6 +677,7 @@ def _handle_rest_background_callback(
     error_handler,
     callback_ctx,
     multi,
+    output_spec,
     has_update=False,
     cache_key=None,
     job_id=None,
@@ -693,7 +696,7 @@ def _handle_rest_background_callback(
         if error_handler:
             output_value = error_handler(exc)
 
-            if output_value is None:
+            if output_value is None and output_spec:
                 output_value = NoUpdate()
             # set_props from the error handler uses the original ctx
             # instead of manager.get_updated_props since it runs in the
@@ -927,7 +930,13 @@ def add_context(*args, **kwargs):
                         )
 
                     output_value, has_update, skip = _update_background_callback(
-                        error_handler, callback_ctx, response, kwargs, background, multi
+                        error_handler,
+                        callback_ctx,
+                        response,
+                        kwargs,
+                        background,
+                        multi,
+                        output_spec,
                     )
                     if skip:
                         return output_value
@@ -998,7 +1007,13 @@ async def async_add_context(*args, **kwargs):
                             callback_ctx,
                         )
                     output_value, has_update, skip = _update_background_callback(
-                        error_handler, callback_ctx, response, kwargs, background, multi
+                        error_handler,
+                        callback_ctx,
+                        response,
+                        kwargs,
+                        background,
+                        multi,
+                        output_spec,
                     )
                     if skip:
                         return output_value

dash/mcp/tasks/tasks.py

@@ -162,6 +162,7 @@ def get_task_result(task_id: str) -> Any:
         kwargs={"background_callback_manager": manager},
         background=background,
         multi=multi,
+        output_spec=output_spec,
         cache_key=cache_key,
         job_id=job_id,
     )

tests/background_callback/app_bg_on_error.py

@@ -20,6 +20,8 @@ def global_error_handler(err):
     html.Div(id="cb-output"),
     html.Button("global on_error", id="start-global-onerror"),
     html.Div(id="global-output"),
+    html.Button("no-output callback on_error", id="start-no-output-cb-onerror"),
+    html.Div(id="no-output-cb-onerror-output"),
 ]
 
 
@@ -48,5 +50,19 @@ def on_click_global(_):
     raise Exception("global error")
 
 
+def no_output_callback_on_error(err):
+    set_props("no-output-cb-onerror-output", {"children": f"callback: {err}"})
+
+
+@app.callback(
+    Input("start-no-output-cb-onerror", "n_clicks"),
+    prevent_initial_call=True,
+    background=True,
+    on_error=no_output_callback_on_error,
+)
+def on_click_no_output(_):
+    raise Exception("no output callback error")
+
+
 if __name__ == "__main__":
     app.run(debug=True)

tests/background_callback/test_basic_long_callback019.py

@@ -0,0 +1,12 @@
+from tests.background_callback.utils import setup_background_callback_app
+
+
+def test_lcbc019_background_callback_on_error_without_output(dash_duo, manager):
+    with setup_background_callback_app(manager, "app_bg_on_error") as app:
+        dash_duo.start_server(app)
+
+        dash_duo.find_element("#start-no-output-cb-onerror").click()
+        dash_duo.wait_for_contains_text(
+            "#no-output-cb-onerror-output",
+            "callback: An error occurred inside a background callback: no output callback error",
+        )

tests/unit/mcp/tools/test_mcp_background_callbacks.py

@@ -10,6 +10,7 @@
 
 import json
 import time
+from types import SimpleNamespace
 
 import diskcache
 from dash import Dash, Input, Output, html, _callback_signing
@@ -298,3 +299,53 @@ def test_mcpbg011_task_id_encodes_tool_name_job_id_cache_key():
     assert job_id.isdigit()
     assert len(cache_key) == 64  # SHA256 hex
     assert created_epoch.isdigit()
+
+
+def test_mcpbg012_tasks_result_passes_output_spec(monkeypatch):
+    from dash.mcp.tasks import tasks
+
+    output_spec = [{"id": "output", "property": "children"}]
+    adapter = SimpleNamespace(
+        _cb_info={"background": object(), "no_output": False},
+        output_id="output.children",
+        as_callback_body=lambda _params: {"outputs": output_spec},
+    )
+    app = SimpleNamespace(
+        mcp_callback_map=SimpleNamespace(find_by_tool_name=lambda _name: adapter)
+    )
+    manager = object()
+    captured = {}
+
+    def update_background_callback(
+        error_handler,
+        callback_ctx,
+        response,
+        kwargs,
+        background,
+        multi,
+        output_spec,
+        cache_key=None,
+        job_id=None,
+    ):
+        captured["output_spec"] = output_spec
+        return None, False, False
+
+    monkeypatch.setattr(
+        tasks,
+        "parse_task_id",
+        lambda _task_id: ("tool", "job", "cache", None),
+    )
+    monkeypatch.setattr(tasks, "get_app", lambda: app)
+    monkeypatch.setattr(tasks, "_get_callback_manager", lambda _tool_name: manager)
+    monkeypatch.setattr(
+        tasks, "_update_background_callback", update_background_callback
+    )
+    monkeypatch.setattr(tasks, "_prepare_response", lambda *args, **kwargs: None)
+    monkeypatch.setattr(
+        tasks,
+        "format_callback_response",
+        lambda _response, _adapter: "formatted",
+    )
+
+    assert tasks.get_task_result("task-id") == "formatted"
+    assert captured["output_spec"] is output_spec