diff --git a/cron/scheduler.py b/cron/scheduler.py
index 548f83423..520b0f463 100644
--- a/cron/scheduler.py
+++ b/cron/scheduler.py
@@ -1411,11 +1411,13 @@ def _deliver_result(job: dict, content: str, adapters=None, loop=None) -> Option
         # the origin DM session (handled after delivery). Cf. _process_handoff.
         #
         # in_channel surface (D2): SKIP thread creation entirely — leave
-        # thread_id=None so the delivery posts flat, and let the existing
-        # origin-mirror (below) seed the shared-channel session (F5: for a
-        # channel-origin job with thread_id=None, _target_matches_origin matches
-        # and _maybe_mirror_cron_delivery seeds (platform, chat_id, None)). No
-        # new seed call is needed.
+        # thread_id=None so the delivery posts flat, then
+        # ``_seed_cron_channel_session`` (below) CREATES the shared-channel
+        # session and mirrors the brief into it. The shipped mirror alone is
+        # NOT enough here: ``mirror_to_session`` only APPENDS to an existing
+        # session and a flat ``(platform, chat_id, None)`` row is otherwise
+        # absent for a ``chat_postMessage`` delivery, so the seed must create
+        # the row first (F5).
         thread_seeded = False
         opened_thread_id: Optional[str] = None
         if (
diff --git a/tests/cron/test_scheduler.py b/tests/cron/test_scheduler.py
index d53209fd3..01bae7ed2 100644
--- a/tests/cron/test_scheduler.py
+++ b/tests/cron/test_scheduler.py
@@ -4153,8 +4153,10 @@ class TestCronContinuableSurfaceInChannel:
     generically from pconfig.extra; the in_channel branch is gated on the
     adapter capability flag ``supports_inchannel_continuable`` (Slack=True,
     others fail SAFE to thread). In in_channel mode the thread-open branch is
-    SKIPPED (thread_id stays None), so the existing origin-mirror seeds the
-    shared-channel session — no new seed code (G6).
+    SKIPPED (thread_id stays None), then ``_seed_cron_channel_session`` CREATES
+    the flat shared-channel session and mirrors the brief into it (the shipped
+    mirror only APPENDS to an existing session, and the flat channel row is
+    otherwise absent for a chat_postMessage delivery).
     """
 
     def _slack_cfg(self, extra):