fix(moderation): handle trailing Anthropic system messages

backend/internal/service/content_moderation_input.go

@@ -91,13 +91,19 @@ func (collector moderationTextCollector) collectLastAnthropicUserMessage(message
 	if len(array) == 0 {
 		return
 	}
-	last := array[len(array)-1]
-	if strings.ToLower(strings.TrimSpace(last.Get("role").String())) != "user" {
+	// Anthropic requests may append a system message after the user turn. It
+	// does not represent a new model/tool turn, so retain the latest user
+	// content while continuing to reject assistant/tool-ended loops.
+	lastUser := len(array) - 1
+	for lastUser >= 0 && strings.ToLower(strings.TrimSpace(array[lastUser].Get("role").String())) == "system" {
+		lastUser--
+	}
+	if lastUser < 0 || strings.ToLower(strings.TrimSpace(array[lastUser].Get("role").String())) != "user" {
 		return
 	}
 	var candidate []string
 	var candidateImages []string
-	collector.collectAnthropicUserContentValue(last.Get("content"), &candidate, &candidateImages)
+	collector.collectAnthropicUserContentValue(array[lastUser].Get("content"), &candidate, &candidateImages)
 	if normalizeContentModerationText(strings.Join(candidate, "\n")) == "" && len(candidateImages) == 0 {
 		return
 	}

backend/internal/service/content_moderation_input_test.go

@@ -50,6 +50,21 @@ func TestExtractContentModerationInput_AnthropicMultiTurnExtractsLatestUser(t *t
 	require.Equal(t, "Q2", input.Text)
 }
 
+func TestExtractContentModerationInput_AnthropicTrailingSystemExtractsLatestUser(t *testing.T) {
+	body := []byte(`{
+		"messages": [
+			{"role":"user","content":"Q1"},
+			{"role":"assistant","content":"A1"},
+			{"role":"user","content":"Q2"},
+			{"role":"system","content":"request metadata"}
+		]
+	}`)
+
+	input := ExtractContentModerationInput(ContentModerationProtocolAnthropicMessages, body)
+
+	require.Equal(t, "Q2", input.Text)
+}
+
 func TestExtractContentModerationInput_AnthropicStreamResendExtractsResend(t *testing.T) {
 	body := []byte(`{
 		"messages": [