diff --git a/models/activities/action.go b/models/activities/action.go
index 8e589eda88d90..942ddd1cd1dbd 100644
--- a/models/activities/action.go
+++ b/models/activities/action.go
@@ -372,6 +372,18 @@ func (a *Action) IsIssueEvent() bool {
 	return a.OpType.InActions("comment_issue", "approve_pull_request", "reject_pull_request", "comment_pull", "merge_pull_request")
 }
 
+// GetIssueContentBody returns the comment body from the action content.
+// Unlike GetIssueInfos which splits on "|" into 3 parts, this only splits on
+// the first "|" to preserve any "|" characters within the comment body
+// (e.g. Mermaid diagram syntax like "A -->|text| B").
+func (a *Action) GetIssueContentBody() string {
+	parts := strings.SplitN(a.Content, "|", 2)
+	if len(parts) < 2 {
+		return ""
+	}
+	return parts[1]
+}
+
 // GetIssueInfos returns a list of associated information with the action.
 func (a *Action) GetIssueInfos() []string {
 	// make sure it always returns 3 elements, because there are some access to the a[1] and a[2] without checking the length
diff --git a/models/activities/action_test.go b/models/activities/action_test.go
index 9447f39d62a1a..a91a81325be69 100644
--- a/models/activities/action_test.go
+++ b/models/activities/action_test.go
@@ -125,6 +125,24 @@ func TestConsistencyUpdateAction(t *testing.T) {
 	unittest.CheckConsistencyFor(t, &activities_model.Action{})
 }
 
+func TestGetIssueContentBody(t *testing.T) {
+	tests := []struct {
+		content  string
+		expected string
+	}{
+		{content: "1|simple body", expected: "simple body"},
+		{content: "1|A -->|text| B", expected: "A -->|text| B"},
+		{content: "1|first|second|third", expected: "first|second|third"},
+		{content: "1|", expected: ""},
+		{content: "no-delimiter", expected: ""},
+		{content: "", expected: ""},
+	}
+	for _, test := range tests {
+		action := &activities_model.Action{Content: test.content}
+		assert.Equal(t, test.expected, action.GetIssueContentBody(), "content: %q", test.content)
+	}
+}
+
 func TestDeleteIssueActions(t *testing.T) {
 	assert.NoError(t, unittest.PrepareTestDatabase())
 
diff --git a/services/feed/notifier.go b/services/feed/notifier.go
index 64aeccdfd227b..0acc0c59e10eb 100644
--- a/services/feed/notifier.go
+++ b/services/feed/notifier.go
@@ -25,6 +25,41 @@ type actionNotifier struct {
 	notify_service.NullNotifier
 }
 
+// trimUnclosedCodeBlock removes a trailing unclosed fenced code block from a
+// truncated string. When content is cut at an arbitrary character limit, a
+// fenced code block (``` ...) may be left open, producing invalid Markdown that
+// the renderer tries (and fails) to process. This function detects the
+// situation and strips the partial block.
+func trimUnclosedCodeBlock(s string) string {
+	inBlock := false
+	lastOpenIdx := -1
+	i := 0
+	for i < len(s) {
+		lineStart := i
+		lineEnd := strings.Index(s[i:], "\n")
+		var line string
+		if lineEnd == -1 {
+			line = s[i:]
+			i = len(s)
+		} else {
+			line = s[i : i+lineEnd]
+			i = i + lineEnd + 1
+		}
+		if strings.HasPrefix(strings.TrimLeft(line, " "), "```") {
+			if !inBlock {
+				lastOpenIdx = lineStart
+				inBlock = true
+			} else {
+				inBlock = false
+			}
+		}
+	}
+	if inBlock && lastOpenIdx >= 0 {
+		return strings.TrimRight(s[:lastOpenIdx], " \t\n")
+	}
+	return s
+}
+
 var _ notify_service.Notifier = &actionNotifier{}
 
 func Init() error {
@@ -117,6 +152,9 @@ func (a *actionNotifier) CreateIssueComment(ctx context.Context, doer *user_mode
 			truncatedContent = truncatedContent[:lastSpaceIdx] + "…"
 		}
 	}
+	if truncatedRight != "" {
+		truncatedContent = trimUnclosedCodeBlock(truncatedContent)
+	}
 	act.Content = fmt.Sprintf("%d|%s", issue.Index, truncatedContent)
 
 	if issue.IsPull {
diff --git a/services/feed/notifier_test.go b/services/feed/notifier_test.go
index ef70facd1e429..27a6501d6531e 100644
--- a/services/feed/notifier_test.go
+++ b/services/feed/notifier_test.go
@@ -22,6 +22,55 @@ func TestMain(m *testing.M) {
 	unittest.MainTest(m)
 }
 
+func TestTrimUnclosedCodeBlock(t *testing.T) {
+	tests := []struct {
+		name     string
+		input    string
+		expected string
+	}{
+		{
+			name:     "no code block",
+			input:    "hello world",
+			expected: "hello world",
+		},
+		{
+			name:     "closed code block",
+			input:    "before\n```go\nfmt.Println()\n```\nafter",
+			expected: "before\n```go\nfmt.Println()\n```\nafter",
+		},
+		{
+			name:     "unclosed code block",
+			input:    "before\n```mermaid\ngraph LR\nA --> B",
+			expected: "before",
+		},
+		{
+			name:     "unclosed code block with leading text",
+			input:    "some text here\n```\ncode line 1\ncode line 2",
+			expected: "some text here",
+		},
+		{
+			name:     "closed then unclosed",
+			input:    "```\nblock1\n```\ntext\n```\nunclosed",
+			expected: "```\nblock1\n```\ntext",
+		},
+		{
+			name:     "only unclosed fence",
+			input:    "```mermaid\ngraph LR",
+			expected: "",
+		},
+		{
+			name:     "empty string",
+			input:    "",
+			expected: "",
+		},
+	}
+	for _, tt := range tests {
+		t.Run(tt.name, func(t *testing.T) {
+			assert.Equal(t, tt.expected, trimUnclosedCodeBlock(tt.input))
+		})
+	}
+}
+
 func TestRenameRepoAction(t *testing.T) {
 	assert.NoError(t, unittest.PrepareTestDatabase())
 
diff --git a/templates/user/dashboard/feeds.tmpl b/templates/user/dashboard/feeds.tmpl
index c3c042dc38776..17e6a4cfb9d48 100644
--- a/templates/user/dashboard/feeds.tmpl
+++ b/templates/user/dashboard/feeds.tmpl
@@ -108,7 +108,7 @@
 					<span class="text truncate issue title">{{index .GetIssueInfos 1 | ctx.RenderUtils.RenderIssueSimpleTitle}}</span>
 				{{else if .GetOpType.InActions "comment_issue" "approve_pull_request" "reject_pull_request" "comment_pull"}}
 					<a href="{{.GetCommentLink ctx}}" class="text truncate issue title">{{(.GetIssueTitle ctx) | ctx.RenderUtils.RenderIssueSimpleTitle}}</a>
-					{{$comment := index .GetIssueInfos 1}}
+					{{$comment := .GetIssueContentBody}}
 					{{if $comment}}
 						<div class="render-content markup tw-text-14">{{ctx.RenderUtils.MarkdownToHtml $comment}}</div>
 					{{end}}