diff --git a/internal/im/mattermost_adapter.go b/internal/im/mattermost_adapter.go
index 005c4f369..bf68bf59f 100644
--- a/internal/im/mattermost_adapter.go
+++ b/internal/im/mattermost_adapter.go
@@ -404,6 +404,7 @@ func (a *mattermostAdapter) handleWSEvent(ctx context.Context, event map[string]
 		if !isFree && a.requireMention {
 			hasMention := a.hasMention(messageText)
 			if !hasMention {
+				debug.Log("mattermost", "adapter=%s drop: no mention of @%s in channel %s", a.name, a.botUsername, channelID)
 				return
 			}
 			messageText = a.stripMention(messageText)
@@ -469,12 +470,17 @@ func (a *mattermostAdapter) handleWSEvent(ctx context.Context, event map[string]
 
 func (a *mattermostAdapter) hasMention(text string) bool {
 	lower := strings.ToLower(text)
-	// Word-boundary match: bot name "al" must not fire on "@alex" (#963).
+	// Boundary match: bot name "al" must not fire on "@alex" (#963).
+	// The boundary is username-charset-aware, not ASCII \b: Mattermost
+	// usernames allow ^[a-z0-9.\-_]+$ and may legally end in "-" or ".",
+	// where RE2 \b can never match (both sides non-word chars) and real
+	// mentions were silently dropped (#4322). A mention ends only when the
+	// next char is NOT a username char.
 	for _, id := range []string{a.botUsername, a.botUserID} {
 		if id == "" {
 			continue
 		}
-		re := regexp.MustCompile(`(?i)@` + regexp.QuoteMeta(id) + `\b`)
+		re := regexp.MustCompile(`(?i)@` + regexp.QuoteMeta(id) + `(?:$|[^a-z0-9.\-_])`)
 		if re.MatchString(lower) {
 			return true
 		}
@@ -483,14 +489,16 @@ func (a *mattermostAdapter) hasMention(text string) bool {
 }
 
 func (a *mattermostAdapter) stripMention(text string) string {
-	// Strip @bot_username and @bot_user_id mentions, with word boundary so a
-	// short bot name ("al") doesn't carve text out of "@alex" (#963).
+	// Strip @bot_username and @bot_user_id mentions, with a username-charset
+	// boundary so a short bot name ("al") doesn't carve text out of "@alex"
+	// (#963) while names ending in "-"/"." still strip (#4322). The boundary
+	// char (when present) is captured and preserved in the replacement.
 	for _, pattern := range []string{a.botUsername, a.botUserID} {
 		if pattern == "" {
 			continue
 		}
-		re := regexp.MustCompile(`(?i)@` + regexp.QuoteMeta(pattern) + `\b`)
-		text = re.ReplaceAllString(text, "")
+		re := regexp.MustCompile(`(?i)@` + regexp.QuoteMeta(pattern) + `(?:$|([^a-z0-9.\-_]))`)
+		text = re.ReplaceAllString(text, "$1")
 	}
 	return strings.TrimSpace(text)
 }
diff --git a/internal/im/zz_mention_boundary_4322_test.go b/internal/im/zz_mention_boundary_4322_test.go
new file mode 100644
index 000000000..72ea90b21
--- /dev/null
+++ b/internal/im/zz_mention_boundary_4322_test.go
@@ -0,0 +1,43 @@
+package im
+
+import "testing"
+
+// #4322 regression: Mattermost usernames may end in "-" or "." - the old
+// ASCII \b boundary could never match there, silently dropping real mentions.
+func TestMentionBoundaryUsernameCharsetName(t *testing.T) {
+	cases := []struct {
+		bot     string
+		name    string
+		text    string
+		wantHit bool
+	}{
+		{"dev-", "trailing dash", "@dev- help with logs", true},
+		{"dev-", "trailing dash end-of-text", "ping @dev-", true},
+		{"ops1.", "trailing dot", "@ops1. check logs", true},
+		{"al", "plain name mention", "@al hello", true},
+		{"al", "substring no false hit", "@alex hello", false},
+		{"dev", "longer username no carve", "@dev-extra text", false},
+		{"dev-", "no mention", "hello world", false},
+	}
+	for _, c := range cases {
+		a := &mattermostAdapter{botUsername: c.bot}
+		if got := a.hasMention(c.text); got != c.wantHit {
+			t.Errorf("bot=%q case=%q hasMention(%q) = %v, want %v", c.bot, c.name, c.text, got, c.wantHit)
+		}
+	}
+}
+
+func TestStripMentionBoundaryPreservesSeparator(t *testing.T) {
+	a := &mattermostAdapter{botUsername: "dev-"}
+	if got := a.stripMention("@dev- help with logs"); got != "help with logs" {
+		t.Errorf("strip = %q, want %q", got, "help with logs")
+	}
+	b := &mattermostAdapter{botUsername: "al"}
+	if got := b.stripMention("@al hello there"); got != "hello there" {
+		t.Errorf("strip = %q, want %q", got, "hello there")
+	}
+	// "@alex" must not be carved by short bot "al".
+	if got := b.stripMention("@alex hello"); got != "@alex hello" {
+		t.Errorf("strip = %q, want unchanged %q", got, "@alex hello")
+	}
+}