Add note about startup on windows

app/fanout/mqtt_base.py

@@ -12,6 +12,7 @@ from __future__ import annotations
 import asyncio
 import json
 import logging
+import sys
 import time
 from abc import ABC, abstractmethod
 from typing import Any
@@ -252,6 +253,34 @@ class BaseMqttPublisher(ABC):
                 self._client = None
                 self._last_error = _format_error_detail(e)
 
+                # Windows ProactorEventLoop does not implement add_reader /
+                # add_writer, which paho-mqtt requires.  The failure can
+                # surface as a direct NotImplementedError (add_writer in
+                # __aenter__) or as a generic timeout (add_reader fails
+                # inside an event-loop callback, so paho never hears back).
+                # Either way, if we're on Windows with Proactor the root
+                # cause is the same and retrying won't help.
+                _on_proactor = (
+                    sys.platform == "win32"
+                    and type(asyncio.get_event_loop()).__name__ == "ProactorEventLoop"
+                )
+                if _on_proactor:
+                    broadcast_error(
+                        "MQTT unavailable — Windows event loop incompatible",
+                        "The default Windows event loop (ProactorEventLoop) does "
+                        "not support MQTT. Add --loop none to your uvicorn "
+                        "command and restart. See README.md for details.",
+                    )
+                    _broadcast_health()
+                    logger.error(
+                        "%s cannot run: Windows ProactorEventLoop does not "
+                        "implement add_reader/add_writer required by paho-mqtt. "
+                        "Restart uvicorn with '--loop none' to use "
+                        "SelectorEventLoop instead. Giving up (will not retry).",
+                        self._integration_label(),
+                    )
+                    return
+
                 title, detail = self._on_error()
                 broadcast_error(title, detail)
                 _broadcast_health()

app/main.py

@@ -1,5 +1,41 @@
-import asyncio
 import logging
+import sys
+
+# ---------------------------------------------------------------------------
+# Windows event-loop advisory for MQTT fanout
+# ---------------------------------------------------------------------------
+# On Windows, uvicorn's default event loop (ProactorEventLoop) does not
+# implement add_reader()/add_writer(), which paho-mqtt (via aiomqtt) requires.
+# We cannot fix this from inside the app — the loop is already created by the
+# time this module is imported.  Log a prominent warning so Windows operators
+# who want MQTT know to add ``--loop none`` to their uvicorn command.
+# ---------------------------------------------------------------------------
+if sys.platform == "win32":
+    import asyncio as _asyncio
+
+    _loop = _asyncio.get_event_loop()
+    _is_proactor = type(_loop).__name__ == "ProactorEventLoop"
+    if _is_proactor:
+        print(
+            "\n" + "!" * 78 + "\n"
+            "  NOTE FOR WINDOWS USERS\n" + "!" * 78 + "\n"
+            "\n"
+            "  The running event loop is ProactorEventLoop, which is not\n"
+            "  compatible with MQTT fanout (aiomqtt / paho-mqtt).\n"
+            "\n"
+            "  If you use MQTT integrations, restart with --loop none:\n"
+            "\n"
+            "    uv run uvicorn app.main:app \033[1m--loop none\033[0m"
+            " [... other options ...]\n"
+            "\n"
+            "  Everything else works fine as-is.\n"
+            "\n" + "!" * 78 + "\n",
+            file=sys.stderr,
+            flush=True,
+        )
+    del _loop, _is_proactor
+
+import asyncio
 from contextlib import asynccontextmanager
 from pathlib import Path