diff --git a/nanobot/channels/slack.py b/nanobot/channels/slack.py
index afd1d2d..a4e7324 100644
--- a/nanobot/channels/slack.py
+++ b/nanobot/channels/slack.py
@@ -82,13 +82,14 @@ class SlackChannel(BaseChannel):
             thread_ts = slack_meta.get("thread_ts")
             channel_type = slack_meta.get("channel_type")
             # Only reply in thread for channel/group messages; DMs don't use threads
-            use_thread = thread_ts and channel_type != "im"
             thread_ts_param = thread_ts if use_thread else None
 
-            if msg.content:
+            # Slack rejects empty text payloads. Keep media-only messages media-only,
+            # but send a single blank message when the bot has no text or files to send.
+            if msg.content or not (msg.media or []):
                 await self._web_client.chat_postMessage(
                     channel=msg.chat_id,
-                    text=self._to_mrkdwn(msg.content),
+                    text=self._to_mrkdwn(msg.content) if msg.content else " ",
                     thread_ts=thread_ts_param,
                 )