fix: address Copilot review round 4

- runReport splits results by mode suffix into token/llm for PrintComparison
- backward compat fallback (ValidF1Count=0 -> TotalQuestions) only for
  non-LLM modes; LLM modes keep ValidF1Count=0 when all scores sentinel
- MaxRetries==0 means no retry; only negative falls back to default 3
- truncateStr uses []rune to avoid cutting multi-byte UTF-8 characters
- Complete() returns error on empty LLM response (vs silent empty string)

cmd/membench/eval.go

@@ -305,9 +305,10 @@ func SaveAggregated(results []EvalResult, outDir string) error {
 func computeModeAgg(results []EvalResult) AggMetrics {
 	agg := AggMetrics{ByCategory: map[int]*CatMetrics{}}
 	for _, r := range results {
-		// Backward compat: old eval JSON without ValidF1Count → use TotalQuestions.
+		// Backward compat: old eval JSON (token mode) without ValidF1Count → use TotalQuestions.
+		// LLM modes may legitimately have ValidF1Count==0 (all failures).
 		vf1 := r.Agg.ValidF1Count
-		if vf1 == 0 && r.Agg.TotalQuestions > 0 {
+		if vf1 == 0 && r.Agg.TotalQuestions > 0 && !strings.HasSuffix(r.Mode, "-llm") {
 			vf1 = r.Agg.TotalQuestions
 		}
 		agg.OverallF1 += r.Agg.OverallF1 * float64(vf1)
@@ -321,7 +322,7 @@ func computeModeAgg(results []EvalResult) AggMetrics {
 				agg.ByCategory[cat] = existing
 			}
 			cvf1 := cm.ValidF1Count
-			if cvf1 == 0 && cm.QuestionCount > 0 {
+			if cvf1 == 0 && cm.QuestionCount > 0 && !strings.HasSuffix(r.Mode, "-llm") {
 				cvf1 = cm.QuestionCount
 			}
 			existing.F1 += cm.F1 * float64(cvf1)

cmd/membench/eval_llm.go

@@ -276,8 +276,9 @@ func countTotalQA(samples []LocomoSample) int {
 
 func truncateStr(s string, maxLen int) string {
 	s = strings.ReplaceAll(s, "\n", " ")
-	if len(s) > maxLen {
-		return s[:maxLen] + "..."
+	runes := []rune(s)
+	if len(runes) > maxLen {
+		return string(runes[:maxLen]) + "..."
 	}
 	return s
 }

cmd/membench/llm_client.go

@@ -38,7 +38,7 @@ func NewLLMClient(opts LLMClientOptions) *LLMClient {
 		opts.Timeout = 120 * time.Second
 	}
 	maxRetries := opts.MaxRetries
-	if maxRetries <= 0 {
+	if maxRetries < 0 {
 		maxRetries = 3
 	}
 	return &LLMClient{
@@ -170,5 +170,8 @@ func (c *LLMClient) Complete(ctx context.Context, systemPrompt, userPrompt strin
 	if idx := strings.Index(content, "</think>"); idx >= 0 {
 		content = strings.TrimSpace(content[idx+len("</think>"):])
 	}
+	if content == "" {
+		return "", fmt.Errorf("empty LLM response")
+	}
 	return content, nil
 }

cmd/membench/main.go

@@ -272,7 +272,15 @@ func runReport(cmd *cobra.Command, args []string) error {
 		return fmt.Errorf("no eval results found in %s", flagOut)
 	}
 
-	PrintComparison(allResults, nil)
+	var tokenResults, llmResults []EvalResult
+	for _, r := range allResults {
+		if strings.HasSuffix(r.Mode, "-llm") {
+			llmResults = append(llmResults, r)
+		} else {
+			tokenResults = append(tokenResults, r)
+		}
+	}
+	PrintComparison(tokenResults, llmResults)
 	return nil
 }