feat: implement threaded message strategy for Slack notifier

.cursor/.gitignore

@@ -0,0 +1 @@
+plans/

.gitignore

@@ -5,3 +5,4 @@
 /amtool
 /data/
 /vendor
+.idea

config/config.go

@@ -426,6 +426,9 @@ func (c *Config) UnmarshalYAML(unmarshal func(any) error) error {
 			if sc.APIURL == nil && len(sc.APIURLFile) == 0 && sc.AppToken == "" && len(sc.AppTokenFile) == 0 {
 				return errors.New("no Slack API URL nor App token set either inline or in a file")
 			}
+			if err := sc.ValidateMessageStrategy(); err != nil {
+				return err
+			}
 			if sc.HTTPConfig == nil {
 				// we don't want to change the global http config when setting the receiver's http config, do we do a copy
 				httpconfig := *c.Global.HTTPConfig

config/config_test.go

@@ -1072,13 +1072,106 @@ func TestSlackBothAppTokenAndAPIURL(t *testing.T) {
 	}
 }
 
-func TestSlackUpdateMessageWebhookURL(t *testing.T) {
-	_, err := LoadFile("testdata/conf.slack-update-message-and-webhook.yml")
+func TestSlackMessageStrategyWithWrongAPIURL(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name     string
+		file     string
+		strategy string
+	}{
+		{
+			name:     "update strategy with webhook URL",
+			file:     "testdata/conf.slack-update-message-and-webhook.yml",
+			strategy: `update`,
+		},
+		{
+			name:     "update strategy with webhook URL (new field)",
+			file:     "testdata/conf.slack-update-message-and-webhook-with-new-field.yml",
+			strategy: `update`,
+		},
+		{
+			name:     "thread strategy with webhook URL",
+			file:     "testdata/conf.slack-thread-message-and-webhook.yml",
+			strategy: `thread`,
+		},
+	}
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			_, err := LoadFile(tt.file)
+			if err == nil {
+				t.Fatal("Expected an error")
+			}
+			expectedErrMsg := fmt.Sprintf("message_strategy %q requires a bot token; api_url must be https://slack.com/api/chat.postMessage", tt.strategy)
+			if err.Error() != expectedErrMsg {
+				t.Errorf("Expected: %s\nGot: %s", expectedErrMsg, err.Error())
+			}
+		})
+	}
+}
+
+func TestSlackMessageStrategyWithGlobalAPIURL(t *testing.T) {
+	t.Parallel()
+
+	_, err := LoadFile("testdata/conf.slack-thread-strategy-with-global-api-url.yml")
+	if err != nil {
+		t.Fatalf("Expected no error when message_strategy uses global slack_api_url, got: %s", err)
+	}
+}
+
+func TestSlackMessageStrategyWithoutAPIURL(t *testing.T) {
+	t.Parallel()
+
+	_, err := LoadFile("testdata/conf.slack-thread-strategy-without-api-url.yml")
 	if err == nil {
-		t.Fatalf("Expected an error parsing %s: %s", "testdata/conf.slack-update-message-and-webhook", err)
+		t.Fatal("Expected an error when message_strategy: thread has no api_url at any level")
+	}
+	expectedErrMsg := `no Slack API URL nor App token set either inline or in a file`
+	if err.Error() != expectedErrMsg {
+		t.Errorf("Expected: %s\nGot: %s", expectedErrMsg, err.Error())
+	}
+}
+
+func TestSlackThreadedOptionsValidation(t *testing.T) {
+	t.Parallel()
+
+	tests := []struct {
+		name           string
+		file           string
+		expectedErrMsg string
+	}{
+		{
+			name:           "threaded options without thread strategy (resolve emoji)",
+			file:           "testdata/conf.slack-thread-resolve-emoji-without-thread-message.yml",
+			expectedErrMsg: `threaded_options requires message_strategy to be "thread"`,
+		},
+		{
+			name:           "threaded options without thread strategy (update parent)",
+			file:           "testdata/conf.slack-thread-update-parent-without-thread-message.yml",
+			expectedErrMsg: `threaded_options requires message_strategy to be "thread"`,
+		},
+		{
+			name:           "resolve color without summary header",
+			file:           "testdata/conf.slack-resolve-color-without-summary-header.yml",
+			expectedErrMsg: `threaded_options.summary_header requires use_summary_header to be enabled`,
+		},
 	}
-	if err.Error() != "update_message can only be used with bot tokens. api_url must be set to https://slack.com/api/chat.postMessage" {
-		t.Errorf("Expected: %s\nGot: %s", "update_message can only be used with bot tokens. api_url must be set to https://slack.com/api/chat.postMessage", err.Error())
+
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			t.Parallel()
+
+			_, err := LoadFile(tt.file)
+			if err == nil {
+				t.Fatal("Expected an error")
+			}
+			if err.Error() != tt.expectedErrMsg {
+				t.Errorf("Expected: %s\nGot: %s", tt.expectedErrMsg, err.Error())
+			}
+		})
 	}
 }
 

config/notifiers.go

@@ -470,6 +470,45 @@ func (c *SlackField) UnmarshalYAML(unmarshal func(any) error) error {
 	return nil
 }
 
+// SlackMessageStrategy controls how subsequent notifications for the same
+// alert group are delivered to Slack.
+type SlackMessageStrategy string
+
+const (
+	// SlackMessageStrategyNew sends each notification as a separate message (default).
+	SlackMessageStrategyNew SlackMessageStrategy = "new"
+	// SlackMessageStrategyUpdate updates the original message in-place.
+	SlackMessageStrategyUpdate SlackMessageStrategy = "update"
+	// SlackMessageStrategyThread sends subsequent notifications as threaded replies.
+	SlackMessageStrategyThread SlackMessageStrategy = "thread"
+)
+
+// SlackThreadedOptions configures thread-specific behavior when message_strategy is "thread".
+type SlackThreadedOptions struct {
+	// ResolveEmoji is the emoji name (without colons) to react with on the
+	// original thread message when all alerts in the group are resolved.
+	// Requires the bot token to have reactions:write scope.
+	ResolveEmoji string `yaml:"resolve_emoji,omitempty" json:"resolve_emoji,omitempty"`
+
+	// UseSummaryHeader controls whether the thread parent is a lightweight
+	// auto-updated summary (true, default) or the first actual alert message
+	// (false). When true, all alert content is posted as replies and the parent
+	// is continuously updated with the transition title and color.
+	UseSummaryHeader *bool `yaml:"use_summary_header,omitempty" json:"use_summary_header,omitempty"`
+
+	// SummaryHeader holds options for summary-header mode only (see UseSummaryHeader).
+	SummaryHeader *SlackThreadSummaryHeaderOptions `yaml:"summary_header,omitempty" json:"summary_header,omitempty"`
+}
+
+// SlackThreadSummaryHeaderOptions configures fields that only apply when
+// message_strategy is "thread" and use_summary_header is true (the lightweight
+// parent summary mode).
+type SlackThreadSummaryHeaderOptions struct {
+	// ResolveColor overrides the parent summary attachment color when the alert
+	// group resolves. Supports Go templates.
+	ResolveColor string `yaml:"resolve_color,omitempty" json:"resolve_color,omitempty"`
+}
+
 // SlackConfig configures notifications via Slack.
 type SlackConfig struct {
 	amcommoncfg.NotifierConfig `yaml:",inline" json:",inline"`
@@ -505,10 +544,19 @@ type SlackConfig struct {
 	MrkdwnIn    []string       `yaml:"mrkdwn_in,omitempty" json:"mrkdwn_in,omitempty"`
 	Actions     []*SlackAction `yaml:"actions,omitempty" json:"actions,omitempty"`
 
+	// MessageStrategy controls how subsequent notifications for the same alert
+	// group are delivered: "new" (default), "update" (edit in-place), or "thread"
+	// (threaded replies). "update" and "thread" require a bot token.
+	MessageStrategy SlackMessageStrategy `yaml:"message_strategy,omitempty" json:"message_strategy,omitempty"`
+
 	// UpdateMessage enables updating existing Slack messages instead of creating new ones.
-	// Requires bot token with chat:write scope. Webhook URLs do not support updates.
+	// Deprecated: use message_strategy: update instead. If true, message_strategy must
+	// be unset or "update"; when message_strategy is unset it is treated as "update".
+	UpdateMessage bool `yaml:"update_message,omitempty" json:"update_message,omitempty"`
 
-	UpdateMessage bool `yaml:"update_message" json:"update_message,omitempty"`
+	// ThreadedOptions configures thread-specific behavior.
+	// Only valid when message_strategy is "thread".
+	ThreadedOptions *SlackThreadedOptions `yaml:"threaded_options,omitempty" json:"threaded_options,omitempty"`
 	// Timeout is the maximum time allowed to invoke the slack. Setting this to 0
 	// does not impose a timeout.
 	Timeout time.Duration `yaml:"timeout" json:"timeout"`
@@ -532,13 +580,77 @@ func (c *SlackConfig) UnmarshalYAML(unmarshal func(any) error) error {
 		return errors.New("at most one of api_url/api_url_file & app_token/app_token_file must be configured")
 	}
 
-	if c.UpdateMessage && c.APIURL.String() != "https://slack.com/api/chat.postMessage" {
-		return errors.New("update_message can only be used with bot tokens. api_url must be set to https://slack.com/api/chat.postMessage")
+	// Deprecated: remove this block when update_message is deleted.
+	if c.UpdateMessage {
+		if c.MessageStrategy != "" && c.MessageStrategy != SlackMessageStrategyUpdate {
+			return fmt.Errorf("update_message: true is incompatible with message_strategy %q; omit message_strategy or set message_strategy: \"update\"", c.MessageStrategy)
+		}
+		if c.MessageStrategy == "" {
+			c.MessageStrategy = SlackMessageStrategyUpdate
+		}
+	}
+	if c.MessageStrategy == "" {
+		c.MessageStrategy = SlackMessageStrategyNew
+	}
+
+	if c.MessageStrategy != SlackMessageStrategyNew &&
+		c.MessageStrategy != SlackMessageStrategyUpdate &&
+		c.MessageStrategy != SlackMessageStrategyThread {
+		return fmt.Errorf("unknown message_strategy %q; must be \"new\", \"update\", or \"thread\"", c.MessageStrategy)
+	}
+
+	if c.ThreadedOptions != nil {
+		if c.MessageStrategy != SlackMessageStrategyThread {
+			return errors.New("threaded_options requires message_strategy to be \"thread\"")
+		}
+		if c.ThreadedOptions.UseSummaryHeader != nil && !*c.ThreadedOptions.UseSummaryHeader && c.ThreadedOptions.SummaryHeader != nil {
+			return errors.New("threaded_options.summary_header requires use_summary_header to be enabled")
+		}
 	}
 
 	return nil
 }
 
+// ValidateMessageStrategy checks that the resolved api_url (after global defaults
+// have been applied) satisfies the requirements for update/thread strategies.
+func (c *SlackConfig) ValidateMessageStrategy() error {
+	switch c.MessageStrategy {
+	case SlackMessageStrategyUpdate, SlackMessageStrategyThread:
+		if c.APIURL == nil && c.APIURLFile == "" {
+			return fmt.Errorf("message_strategy %q requires api_url or api_url_file", c.MessageStrategy)
+		}
+		if c.APIURL != nil && c.APIURL.String() != "https://slack.com/api/chat.postMessage" {
+			return fmt.Errorf("message_strategy %q requires a bot token; api_url must be https://slack.com/api/chat.postMessage", c.MessageStrategy)
+		}
+	}
+	return nil
+}
+
+// UseSummaryHeaderInThread returns true when the thread parent should be a lightweight
+// auto-updated summary. Returns true by default (nil or explicit true).
+func (c *SlackConfig) UseSummaryHeaderInThread() bool {
+	if c.ThreadedOptions == nil || c.ThreadedOptions.UseSummaryHeader == nil {
+		return true
+	}
+	return *c.ThreadedOptions.UseSummaryHeader
+}
+
+// HasStrategyThatUpdatesParent reports whether message_strategy uses nflog to tie
+// multiple notifications to one Slack message or thread.
+func (c *SlackConfig) HasStrategyThatUpdatesParent() bool {
+	return c.HasUpdateStrategy() || c.HasThreadStrategy()
+}
+
+// HasUpdateStrategy is true when message_strategy is "update" (chat.update in place).
+func (c *SlackConfig) HasUpdateStrategy() bool {
+	return c.MessageStrategy == SlackMessageStrategyUpdate
+}
+
+// HasThreadStrategy is true when message_strategy is "thread" (threaded replies).
+func (c *SlackConfig) HasThreadStrategy() bool {
+	return c.MessageStrategy == SlackMessageStrategyThread
+}
+
 // IncidentioConfig configures notifications via incident.io.
 type IncidentioConfig struct {
 	amcommoncfg.NotifierConfig `yaml:",inline" json:",inline"`

config/testdata/conf.slack-resolve-color-without-summary-header.yml

@@ -0,0 +1,18 @@
+route:
+  receiver: 'slack-notifications'
+  group_by: [alertname]
+receivers:
+  - name: 'slack-notifications'
+    slack_configs:
+      - channel: '#alerts1'
+        text: 'test'
+        send_resolved: true
+        api_url: 'https://slack.com/api/chat.postMessage'
+        http_config:
+          authorization:
+            credentials: 'xoxb-XXXXXXXXXXXX-XXXXXXXXXXXX-XXXXXXXXXXXXXXXXXXXXXXXX'
+        message_strategy: thread
+        threaded_options:
+          use_summary_header: false
+          summary_header:
+            resolve_color: '#AAAAAA'

config/testdata/conf.slack-thread-message-and-webhook.yml

@@ -0,0 +1,11 @@
+route:
+  receiver: 'slack-notifications'
+  group_by: [alertname]
+receivers:
+  - name: 'slack-notifications'
+    slack_configs:
+      - channel: '#alerts1'
+        text: 'test'
+        send_resolved: true
+        api_url: 'https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX'
+        message_strategy: thread

config/testdata/conf.slack-thread-resolve-emoji-without-thread-message.yml

@@ -0,0 +1,15 @@
+route:
+  receiver: 'slack-notifications'
+  group_by: [alertname]
+receivers:
+  - name: 'slack-notifications'
+    slack_configs:
+      - channel: '#alerts1'
+        text: 'test'
+        send_resolved: true
+        api_url: 'https://slack.com/api/chat.postMessage'
+        http_config:
+          authorization:
+            credentials: 'xoxb-XXXXXXXXXXXX-XXXXXXXXXXXX-XXXXXXXXXXXXXXXXXXXXXXXX'
+        threaded_options:
+          resolve_emoji: 'white_check_mark'

config/testdata/conf.slack-thread-strategy-with-global-api-url.yml

@@ -0,0 +1,14 @@
+global:
+  slack_api_url: 'https://slack.com/api/chat.postMessage'
+route:
+  receiver: 'slack-notifications'
+  group_by: [alertname]
+receivers:
+  - name: 'slack-notifications'
+    slack_configs:
+      - channel: '#alerts'
+        text: 'test'
+        http_config:
+          authorization:
+            credentials: 'xoxb-test-token'
+        message_strategy: thread

config/testdata/conf.slack-thread-strategy-without-api-url.yml

@@ -0,0 +1,9 @@
+route:
+  receiver: 'slack-notifications'
+  group_by: [alertname]
+receivers:
+  - name: 'slack-notifications'
+    slack_configs:
+      - channel: '#alerts'
+        text: 'test'
+        message_strategy: thread

config/testdata/conf.slack-thread-update-parent-without-thread-message.yml

@@ -0,0 +1,15 @@
+route:
+  receiver: 'slack-notifications'
+  group_by: [alertname]
+receivers:
+  - name: 'slack-notifications'
+    slack_configs:
+      - channel: '#alerts1'
+        text: 'test'
+        send_resolved: true
+        api_url: 'https://slack.com/api/chat.postMessage'
+        http_config:
+          authorization:
+            credentials: 'xoxb-XXXXXXXXXXXX-XXXXXXXXXXXX-XXXXXXXXXXXXXXXXXXXXXXXX'
+        threaded_options:
+          resolve_emoji: 'white_check_mark'

config/testdata/conf.slack-update-message-and-webhook-with-new-field.yml

@@ -0,0 +1,13 @@
+route:
+  receiver: 'slack-notifications'
+  group_by: [alertname]
+receivers:
+  - name: 'slack-notifications'
+    slack_configs:
+      # use global
+      - channel: '#alerts1'
+        text: 'test'
+        send_resolved: true
+        # trying to use webhook urls with update_message
+        api_url: 'https://hooks.slack.com/services/T00000000/B00000000/XXXXXXXXXXXXXXXXXXXXXXXX'
+        message_strategy: update