diff --git a/osbuild/api.py b/osbuild/api.py
index e2fee3ed..d4ece715 100644
--- a/osbuild/api.py
+++ b/osbuild/api.py
@@ -91,9 +91,13 @@ class BaseAPI(abc.ABC):
             self.event_loop.run_forever()
             self.event_loop.remove_reader(server)
 
+    @property
+    def running(self):
+        return self.event_loop is not None
+
     def __enter__(self):
         # We are not re-entrant, so complain if re-entered.
-        assert self.event_loop is None
+        assert not self.running
 
         if not self.socket_address:
             self._socketdir = self._make_socket_dir()
@@ -141,6 +145,13 @@ class API(BaseAPI):
 
     @property
     def output(self):
+        # Only once the event-loop was stopped, you are guaranteed that the
+        # api-thread scheduled all outstanding events. Therefore, we disallow
+        # asking for the output-data from a running api context. If we happen
+        # to need live streaming access to the output in the future, we need
+        # to redesign the output-handling, anyway.
+        assert not self.running
+
         return self._output_data.getvalue()
 
     def _prepare_input(self):
diff --git a/osbuild/pipeline.py b/osbuild/pipeline.py
index 0cf1746f..1b63b3f1 100644
--- a/osbuild/pipeline.py
+++ b/osbuild/pipeline.py
@@ -91,7 +91,7 @@ class Stage:
                                binds=[os.fspath(tree) + ":/run/osbuild/tree"],
                                readonly_binds=ro_binds)
 
-            return BuildResult(self, r.returncode, api.output, api.metadata)
+        return BuildResult(self, r.returncode, api.output, api.metadata)
 
 
 class Assembler:
@@ -149,7 +149,7 @@ class Assembler:
                                binds=binds,
                                readonly_binds=ro_binds)
 
-            return BuildResult(self, r.returncode, api.output, api.metadata)
+        return BuildResult(self, r.returncode, api.output, api.metadata)
 
 
 class Pipeline:
diff --git a/test/mod/test_monitor.py b/test/mod/test_monitor.py
index 8004a76b..01d84e77 100644
--- a/test/mod/test_monitor.py
+++ b/test/mod/test_monitor.py
@@ -73,12 +73,13 @@ class TestMonitor(unittest.TestCase):
             path = os.path.join(tmpdir, "osbuild-api")
             logfile = os.path.join(tmpdir, "log.txt")
 
-            with open(logfile, "w") as log, \
-                 API(args, LogMonitor(log.fileno()), socket_address=path) as api:
-                p = mp.Process(target=echo, args=(path, ))
-                p.start()
-                p.join()
-                self.assertEqual(p.exitcode, 0)
+            with open(logfile, "w") as log:
+                api = API(args, LogMonitor(log.fileno()), socket_address=path)
+                with api as api:
+                    p = mp.Process(target=echo, args=(path, ))
+                    p.start()
+                    p.join()
+                    self.assertEqual(p.exitcode, 0)
                 output = api.output  # pylint: disable=no-member
                 assert output