Fix channel_override not working with grouped Slack messages (#2030)

When channel_override uses labels/annotations (e.g. robusta-clusterrel-labels.severity),
grouped messages were broken in two ways:

1. The group key only used group_by attributes (e.g. cluster), so findings
   destined for different channels (sev1, sev2, sev3) were incorrectly merged
   into a single group.

2. The summary message resolved the channel with empty labels/annotations,
   falling back to the default channel instead of the override channel. Thread
   replies then went to a different channel than the summary, breaking threading.

Fix: resolve the channel per-finding and include it in the group key when
channel_override is configured, and pass the pre-resolved channel to the
summary message sender.

https://claude.ai/code/session_01L3rB5LufiC9W9hGDgYMTXP

Co-authored-by: Claude <noreply@anthropic.com>
Co-authored-by: Avi <97387909+Avi-Robusta@users.noreply.github.com>

Author: 3 people

src/robusta/core/sinks/slack/slack_sink.py

@@ -2,6 +2,7 @@
 
 from robusta.core.model.env_vars import ROBUSTA_UI_DOMAIN
 from robusta.core.reporting.base import Finding, FindingStatus
+from robusta.core.sinks.common.channel_transformer import ChannelTransformer
 from robusta.core.sinks.sink_base import NotificationGroup, NotificationSummary, SinkBase
 from robusta.core.sinks.slack.slack_sink_params import SlackSinkConfigWrapper, SlackSinkParams
 from robusta.integrations import slack as slack_module
@@ -47,10 +48,26 @@ def handle_notification_grouping(self, finding: Finding, platform_enabled: bool)
             finding_data["cluster"] = self.cluster_name
             resolved = finding.title.startswith("[RESOLVED]")
 
+            # Resolve the channel for this finding so we can include it in the group key.
+            # This ensures findings routed to different channels (via channel_override with
+            # labels/annotations) are grouped separately per channel.
+            resolved_channel = ChannelTransformer.template(
+                self.params.channel_override,
+                self.params.slack_channel,
+                self.cluster_name,
+                finding.subject.labels,
+                finding.subject.annotations,
+            )
+
             # 1. Notification accounting
             group_key, group_header = self.get_group_key_and_header(
                 finding_data, self.params.grouping.group_by
             )
+            # Include the resolved channel in the group key so that findings destined
+            # for different channels (e.g. due to label-based channel_override) get
+            # separate groups, summary messages, and threads.
+            if self.params.channel_override:
+                group_key = group_key + (resolved_channel,)
 
             if self.grouping_summary_mode:
                 summary_key, _ = self.get_group_key_and_header(
@@ -71,6 +88,7 @@ def handle_notification_grouping(self, finding: Finding, platform_enabled: bool)
                     msg_ts=notification_summary.message_id,
                     investigate_uri=investigate_uri,
                     grouping_interval=self.params.grouping.interval,
+                    channel=resolved_channel,
                 )
                 notification_summary.message_id = slack_thread_ts
                 should_send_notification = self.params.grouping.notification_mode.summary.threaded

src/robusta/integrations/slack/sender.py

@@ -861,6 +861,7 @@ def send_or_update_summary_message(
         msg_ts: str = None,  # message identifier (for updates)
         investigate_uri: str = None,
         grouping_interval: int = None,  # in seconds
+        channel: str = None,  # pre-resolved channel (when channel_override uses labels/annotations)
     ):
         """Create or update a summary message with tabular information about the amount of events
         fired/resolved and a header describing the event group that this information concerns."""
@@ -927,14 +928,15 @@ def send_or_update_summary_message(
         for block in blocks:
             output_blocks.extend(self.__to_slack(block, sink_params.name))
 
-        # For grouped notifications, channel override is supported only with the `cluster` attribute
-        channel = ChannelTransformer.template(
-            sink_params.channel_override,
-            sink_params.slack_channel,
-            self.cluster_name,
-            {},
-            {},
-        )
+        if not channel:
+            # Fallback: resolve channel without labels/annotations (only cluster_name will work)
+            channel = ChannelTransformer.template(
+                sink_params.channel_override,
+                sink_params.slack_channel,
+                self.cluster_name,
+                {},
+                {},
+            )
         if msg_ts is not None:
             method = self.slack_client.chat_update
             kwargs = {"ts": msg_ts}