refactor: reorder post-LLM pipeline to catch think-block repetition loops

Move repetition detection before think-block stripping so degenerate
loops inside <think> tags (e.g. 73K char incidents) are caught on raw
text. Reorder: token record → repetition detect → think strip → XML
tool call extract.

Extract StripThinkBlocks and DetectRepetitionLoop into pkg/utils/string
and remove the duplicate thinkBlockPattern from the Telegram channel.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

pkg/agent/loop.go

@@ -1547,6 +1547,44 @@ func (al *AgentLoop) runLLMIteration(
 			)
 		}
 
+		// Detect repetition loop on raw text (before stripping think
+		// blocks so loops inside <think> are caught).  Skip when the
+		// provider already returned native tool calls.
+		if len(response.ToolCalls) == 0 && utils.DetectRepetitionLoop(response.Content) {
+			logger.WarnCF("agent", "Repetition loop detected in LLM response, retrying",
+				map[string]any{
+					"agent_id":       agent.ID,
+					"iteration":      iteration,
+					"finish_reason":  response.FinishReason,
+					"content_length": len(response.Content),
+				})
+
+			// Retry once: inject nudge message and re-call
+			savedMsgs := messages
+			messages = append(append([]providers.Message(nil), messages...),
+				providers.Message{
+					Role:    "user",
+					Content: "[System] Your previous response contained degenerate repetition and was discarded. Please respond normally without repeating yourself.",
+				})
+			response, err = callLLM()
+			messages = savedMsgs // restore original messages
+
+			if err != nil {
+				return "", iteration, fmt.Errorf("LLM retry after repetition failed: %w", err)
+			}
+
+			// Re-check on raw text; if still repeating give up
+			if utils.DetectRepetitionLoop(response.Content) {
+				logger.ErrorCF("agent", "Repetition persists after retry, returning empty",
+					map[string]any{"agent_id": agent.ID})
+				response.Content = ""
+			}
+		}
+
+		// Strip think blocks before extracting XML tool calls so
+		// extraction operates on clean content.
+		response.Content = utils.StripThinkBlocks(response.Content)
+
 		// Recover XML tool calls emitted as plain text by some providers.
 		if len(response.ToolCalls) == 0 {
 			if xmlCalls := providers.ExtractXMLToolCalls(response.Content); len(xmlCalls) > 0 {

pkg/channels/telegram.go

@@ -51,8 +51,6 @@ const telegramMaxMessageChars = 3900
 const markdownTableMaxWidth = 42
 const markdownTableMinColWidth = 6
 
-var thinkBlockPattern = regexp.MustCompile(`(?is)<think>.*?</think>`)
-
 func NewTelegramChannel(cfg *config.Config, bus *bus.MessageBus) (*TelegramChannel, error) {
 	var opts []telego.BotOption
 	telegramCfg := cfg.Channels.Telegram
@@ -326,8 +324,7 @@ func (c *TelegramChannel) Send(ctx context.Context, msg bus.OutboundMessage) err
 }
 
 func sanitizeTelegramOutgoingContent(content string) string {
-	cleaned := thinkBlockPattern.ReplaceAllString(content, "")
-	cleaned = strings.TrimSpace(cleaned)
+	cleaned := strings.TrimSpace(content)
 	if cleaned == "" {
 		return "(empty response)"
 	}

pkg/channels/telegram_test.go

@@ -5,8 +5,8 @@ import (
 	"testing"
 )
 
-func TestSanitizeTelegramOutgoingContent_RemovesThinkBlock(t *testing.T) {
-	in := "<think>\nsecret reasoning\n</think>\n\nユーザー向け本文"
+func TestSanitizeTelegramOutgoingContent_PlainText(t *testing.T) {
+	in := "  ユーザー向け本文  "
 	got := sanitizeTelegramOutgoingContent(in)
 	want := "ユーザー向け本文"
 	if got != want {
@@ -14,9 +14,16 @@ func TestSanitizeTelegramOutgoingContent_RemovesThinkBlock(t *testing.T) {
 	}
 }
 
-func TestSanitizeTelegramOutgoingContent_EmptyAfterThink(t *testing.T) {
-	in := "<think>only reasoning</think>"
-	got := sanitizeTelegramOutgoingContent(in)
+func TestSanitizeTelegramOutgoingContent_Empty(t *testing.T) {
+	got := sanitizeTelegramOutgoingContent("")
+	want := "(empty response)"
+	if got != want {
+		t.Fatalf("sanitizeTelegramOutgoingContent() = %q, want %q", got, want)
+	}
+}
+
+func TestSanitizeTelegramOutgoingContent_WhitespaceOnly(t *testing.T) {
+	got := sanitizeTelegramOutgoingContent("   \n\t  ")
 	want := "(empty response)"
 	if got != want {
 		t.Fatalf("sanitizeTelegramOutgoingContent() = %q, want %q", got, want)

pkg/utils/string.go

@@ -1,5 +1,57 @@
 package utils
 
+import (
+	"regexp"
+	"strings"
+)
+
+// Repetition detection constants.
+const (
+	repetitionSampleSize      = 2000 // runes to sample from the tail
+	repetitionNgramSize       = 10   // sliding window length
+	repetitionUniqueThreshold = 0.1  // unique ratio below this → repetition
+)
+
+var (
+	thinkBlockClosedRe = regexp.MustCompile(`(?is)<think>.*?</think>`)
+	thinkBlockOpenRe   = regexp.MustCompile(`(?is)<think>.*$`)
+)
+
+// StripThinkBlocks removes <think>…</think> blocks (including unclosed ones)
+// from s and returns the trimmed result.
+func StripThinkBlocks(s string) string {
+	s = thinkBlockClosedRe.ReplaceAllString(s, "")
+	s = thinkBlockOpenRe.ReplaceAllString(s, "")
+	return strings.TrimSpace(s)
+}
+
+// DetectRepetitionLoop checks if text contains degenerate repetition
+// by computing the unique N-gram ratio on the last repetitionSampleSize runes.
+// Returns true if the ratio of unique N-grams to total N-grams
+// falls below repetitionUniqueThreshold (i.e., 90%+ are duplicates).
+func DetectRepetitionLoop(text string) bool {
+	runes := []rune(text)
+
+	// Sample the tail
+	if len(runes) > repetitionSampleSize {
+		runes = runes[len(runes)-repetitionSampleSize:]
+	}
+
+	total := len(runes) - repetitionNgramSize + 1
+	if total <= 0 {
+		return false
+	}
+
+	unique := make(map[string]struct{}, total/repetitionNgramSize)
+	for i := 0; i < total; i++ {
+		ng := string(runes[i : i+repetitionNgramSize])
+		unique[ng] = struct{}{}
+	}
+
+	ratio := float64(len(unique)) / float64(total)
+	return ratio < repetitionUniqueThreshold
+}
+
 // Truncate returns a truncated version of s with at most maxLen runes.
 // Handles multi-byte Unicode characters properly.
 // If the string is truncated, "..." is appended to indicate truncation.