fix(slack): preserve thread context for commands via channel_context

Prepending cold-start thread backfill directly onto the message text
moved a recognized command (e.g. a bang-normalized "!queue ...") away
from character zero, so downstream command routing misclassified it as
conversational text and the command silently didn't run.

Route the backfill through MessageEvent.channel_context instead —
gateway.run already prepends channel_context after command dispatch
("[New message]" framing), so commands keep their COMMAND type while
the recovered history stays available to the agent.

Supersedes #68020, which dropped the fetched context entirely for
commands instead of preserving it out-of-band.

Salvaged from #66069 by @LevSky22.

plugins/platforms/slack/adapter.py

@@ -3825,6 +3825,12 @@ class SlackAdapter(BasePlatformAdapter):
 
         # When entering a thread for the first time (no existing session),
         # fetch thread context so the agent understands the conversation.
+        #
+        # Keep recovered history separate from ``text``. Prepending it here
+        # moves a recognized command away from character zero, so downstream
+        # command routing can misclassify it as conversational text.
+        # ``channel_context`` is prepended only after command dispatch.
+        channel_context = None
         if is_thread_reply and not self._has_active_session_for_thread(
             channel_id=channel_id,
             thread_ts=event_thread_ts,
@@ -3838,7 +3844,7 @@ class SlackAdapter(BasePlatformAdapter):
                 team_id=team_id,
             )
             if thread_context:
-                text = thread_context + text
+                channel_context = thread_context
 
         # Determine message type
         msg_type = MessageType.TEXT
@@ -4176,6 +4182,7 @@ class SlackAdapter(BasePlatformAdapter):
             media_types=media_types,
             reply_to_message_id=thread_ts if thread_ts != ts else None,
             channel_prompt=_channel_prompt,
+            channel_context=channel_context,
             reply_to_text=reply_to_text,
             auto_skill=_auto_skill,
             metadata={

tests/gateway/test_slack.py

@@ -1483,6 +1483,55 @@ class TestBangPrefixCommands:
         # same thread.
         assert msg_event.source.thread_id == "1111111111.000001"
 
+    @pytest.mark.asyncio
+    async def test_bang_queue_survives_first_thread_context_backfill(self, adapter):
+        """Backfill stays out of command text while remaining available."""
+        adapter._has_active_session_for_thread = MagicMock(return_value=False)
+        adapter._fetch_thread_context = AsyncMock(
+            return_value=(
+                "[Slack thread context — earlier messages]\n"
+                "Alice: prior request\n"
+                "[End of thread context]\n\n"
+            )
+        )
+        adapter._fetch_thread_parent_text = AsyncMock(return_value="prior request")
+
+        evt = self._make_event(
+            "!queue follow up after the current task",
+            thread_ts="1111111111.000001",
+        )
+        await adapter._handle_slack_message(evt)
+
+        msg_event = adapter.handle_message.call_args[0][0]
+        assert msg_event.text == "/queue follow up after the current task"
+        assert msg_event.message_type == MessageType.COMMAND
+        assert msg_event.get_command() == "queue"
+        assert msg_event.get_command_args() == "follow up after the current task"
+        assert msg_event.channel_context.startswith("[Slack thread context")
+        assert "prior request" in msg_event.channel_context
+
+    @pytest.mark.asyncio
+    async def test_non_command_thread_backfill_uses_channel_context(self, adapter):
+        """Normal thread text remains separate without losing its backfill."""
+        adapter._has_active_session_for_thread = MagicMock(return_value=False)
+        adapter._fetch_thread_context = AsyncMock(
+            return_value="[Slack thread context]\nAlice: earlier note\n"
+        )
+        adapter._fetch_thread_parent_text = AsyncMock(return_value="earlier note")
+
+        evt = self._make_event(
+            "follow up",
+            thread_ts="1111111111.000001",
+        )
+        await adapter._handle_slack_message(evt)
+
+        msg_event = adapter.handle_message.call_args[0][0]
+        assert msg_event.text == "follow up"
+        assert msg_event.message_type == MessageType.TEXT
+        assert msg_event.channel_context == (
+            "[Slack thread context]\nAlice: earlier note\n"
+        )
+
     @pytest.mark.asyncio
     async def test_bang_unknown_token_passes_through_unchanged(self, adapter):
         """``!nice work`` is just a casual message — must NOT be rewritten."""