diff --git a/frappyman.py b/frappyman.py
index 104f3dd..4287ed7 100644
--- a/frappyman.py
+++ b/frappyman.py
@@ -377,16 +377,29 @@ class FrappyManager(ServiceManager):
                     proposed_cfg[service] = proposed
                     self.state[f'sea {service}'] = seacfg
 
-        given_addons = givencfgs.get('addons', '')
-        running_addons = self.frappy_cfgs.get('addons', '')
-        if running_addons:
-            for cfg in running_addons.split(','):
-                seacfg = self.frappy2sea.get(cfg.strip())
-                if seacfg is not None and seacfg.endswith('.addon') and seacfg[:-6] not in sealist[2:]:
-                    proposed_cfg['addons'] = {running_addons}
-                    break
-        elif given_addons:
-            proposed_cfg['addons'] = {given_addons}
+        running_addons = self.frappy_cfgs.get('addons')
+        running_addons = set(','.split(running_addons)) if running_addons else ()
+        proposed_addons = set()
+        running_sea_addons = set()
+        for cfg in running_addons:
+            seacfg = self.frappy2sea.get(cfg.strip())
+            if seacfg is None:
+                proposed_addons.add(cfg)  # addons with no sea cfg should be kept
+            elif seacfg.endswith('.addon') and seacfg[:-6] in sealist[2:]:
+                proposed_addons.add(cfg)
+                running_sea_addons.add(seacfg)
+
+        for scfg in sealist[2:]:
+            seacfg = scfg + '.addon'
+            if seacfg not in running_sea_addons:
+                proposed = self.sea2frappy.get(seacfg) or set()
+                if proposed:
+                    if len(proposed) > 1:
+                        self.error = f'ambiguous frappy cfg for {seacfg}.addon: {proposed}'
+                    else:
+                        proposed_addons |= proposed
+        if proposed_addons and set(proposed_addons) != set(running_addons):
+            proposed_cfg['addons'] = {','.join(sorted(proposed_addons))}
 
         for service in FrappyManager.services:
             given = givencfgs.get(service)
@@ -412,7 +425,7 @@ class FrappyManager(ServiceManager):
                         proposed = running
                 if proposed is None:
                     self.remarks[service] = 'ambiguous frappy cfg'
-                    self.error = True
+                    self.error = f'ambiguous frappy cfg for {seacfg}: {available}'
                 else:
                     self.remarks[service] = ''
                     if running == proposed: