diff --git a/packages/prime-tunnel/src/prime_tunnel/tunnel.py b/packages/prime-tunnel/src/prime_tunnel/tunnel.py
index 1535800f..27823984 100644
--- a/packages/prime-tunnel/src/prime_tunnel/tunnel.py
+++ b/packages/prime-tunnel/src/prime_tunnel/tunnel.py
@@ -1,4 +1,5 @@
 import asyncio
+import atexit
 import fcntl
 import os
 import re
@@ -29,6 +30,22 @@
 )
 _ANSI_RE = re.compile(r"\x1b\[[0-9;]*m")
 
+# Started-but-not-stopped tunnels, so an atexit backstop can delete their backend
+# registrations if the caller never stops them (forgotten cleanup, unhandled
+# exception)
+_active_tunnels: "set[Tunnel]" = set()
+
+
+def _stop_active_tunnels() -> None:
+    for tunnel in list(_active_tunnels):
+        try:
+            tunnel.sync_stop()
+        except Exception:
+            pass
+
+
+atexit.register(_stop_active_tunnels)
+
 
 def _parse_frpc_error(
     output_lines: list[str],
@@ -205,6 +222,8 @@ async def start(self) -> str:
 
         self._started = True
 
+        _active_tunnels.add(self)
+
         return self.url
 
     async def stop(self) -> None:
@@ -220,6 +239,8 @@ def sync_stop(self) -> None:
         if not self._started:
             return
 
+        _active_tunnels.discard(self)
+
         if self._process is not None:
             try:
                 self._process.terminate()
@@ -258,6 +279,8 @@ def sync_stop(self) -> None:
 
     async def _cleanup(self) -> None:
         """Clean up tunnel resources."""
+        _active_tunnels.discard(self)
+
         # Stop frpc process (this will cause drain threads to exit via EOF)
         if self._process is not None:
             try: