Fix ambiguous empty log cursor handling

internal/api/handlers/management/logs.go

@@ -31,6 +31,12 @@ const (
 )
 
 // GetLogs returns log lines with optional incremental loading.
+//
+// The legacy timestamp path keeps line-count as the total scanned line count for
+// compatibility. Cursor and tail reads avoid scanning older files, so line-count
+// is the number of returned lines there. A cursor emitted by the legacy path
+// points at the latest complete log boundary; combining after with limit is
+// therefore tail semantics and does not replay lines trimmed by limit.
 func (h *Handler) GetLogs(c *gin.Context) {
 	if h == nil {
 		c.JSON(http.StatusInternalServerError, gin.H{"error": "handler unavailable"})
@@ -756,6 +762,8 @@ func locateLogCursorFile(files []string, cursor logCursor) (int, bool, error) {
 		if matches && !truncated {
 			if shouldDeferEmptyMainCursorToRotated(files, cursor) {
 				deferEmptyMainMatch = true
+			} else if shouldResetAmbiguousEmptyMainCursor(files, index, cursor) {
+				return 0, false, nil
 			} else {
 				return index, true, nil
 			}
@@ -804,6 +812,32 @@ func shouldDeferEmptyMainCursorToRotated(files []string, cursor logCursor) bool
 	return false
 }
 
+func shouldResetAmbiguousEmptyMainCursor(files []string, mainIndex int, cursor logCursor) bool {
+	if cursor.File != defaultLogFileName || cursor.Offset != 0 || cursor.Size != 0 {
+		return false
+	}
+	info, errStat := os.Stat(files[mainIndex])
+	if errStat != nil || info.IsDir() {
+		return false
+	}
+	if info.Size() == cursor.Size && info.ModTime().UnixNano() == cursorModTimeUnixNano(cursor) {
+		return false
+	}
+	for i := range files {
+		if i == mainIndex || filepath.Base(files[i]) == defaultLogFileName {
+			continue
+		}
+		rotatedInfo, errRotated := os.Stat(files[i])
+		if errRotated != nil || rotatedInfo.IsDir() || rotatedInfo.Size() == 0 {
+			continue
+		}
+		if !logFileChangedAfterCursor(files[i], cursor) {
+			return true
+		}
+	}
+	return false
+}
+
 func logFileChangedAfterCursor(path string, cursor logCursor) bool {
 	info, errStat := os.Stat(path)
 	if errStat != nil || info.IsDir() || info.Size() == 0 {

internal/api/handlers/management/logs_test.go

@@ -474,6 +474,53 @@ func TestGetLogsZeroOffsetCursorWithEmptyFileReadsAcrossRotation(t *testing.T) {
 	}
 }
 
+func TestGetLogsZeroOffsetCursorWithEmptyFileResetsWhenRotationModTimeAmbiguous(t *testing.T) {
+	dir := t.TempDir()
+	mainPath := filepath.Join(dir, defaultLogFileName)
+	fixedModTime := time.Date(2026, 6, 15, 10, 0, 0, 0, time.Local)
+	writeMainLog(t, dir, "")
+	if err := os.Chtimes(mainPath, fixedModTime, fixedModTime); err != nil {
+		t.Fatalf("set initial main mtime: %v", err)
+	}
+	initial := performGetLogs(t, newLogsTestHandler(dir, true), "/v0/management/logs?limit=1")
+	if initial.NextCursor == "" {
+		t.Fatal("initial next-cursor is empty")
+	}
+	cursor, errCursor := decodeLogCursor(initial.NextCursor)
+	if errCursor != nil {
+		t.Fatalf("decode initial cursor: %v", errCursor)
+	}
+	if cursor.Offset != 0 || cursor.Size != 0 {
+		t.Fatalf("cursor offset/size = %d/%d, want empty zero offset", cursor.Offset, cursor.Size)
+	}
+
+	first := "[2026-06-15 10:00:01] first"
+	second := "[2026-06-15 10:00:02] second"
+	appendMainLog(t, dir, first+"\n")
+	if err := os.Chtimes(mainPath, fixedModTime, fixedModTime); err != nil {
+		t.Fatalf("set rotated mtime: %v", err)
+	}
+	if err := os.Rename(mainPath, filepath.Join(dir, defaultLogFileName+".1")); err != nil {
+		t.Fatalf("rotate main log: %v", err)
+	}
+	writeMainLog(t, dir, second+"\n")
+	if err := os.Chtimes(mainPath, fixedModTime, fixedModTime); err != nil {
+		t.Fatalf("set new main mtime: %v", err)
+	}
+
+	resp := performGetLogs(t, newLogsTestHandler(dir, true), "/v0/management/logs?cursor="+url.QueryEscape(initial.NextCursor)+"&limit=2")
+	wantLines := []string{first, second}
+	if !reflect.DeepEqual(resp.Lines, wantLines) {
+		t.Fatalf("lines = %#v, want %#v", resp.Lines, wantLines)
+	}
+	if !resp.CursorReset {
+		t.Fatal("cursor-reset = false, want true for ambiguous empty cursor rotation")
+	}
+	if resp.LineCount != len(wantLines) {
+		t.Fatalf("line-count = %d, want returned line count %d", resp.LineCount, len(wantLines))
+	}
+}
+
 func TestGetLogsInvalidCursorResetsToTail(t *testing.T) {
 	dir := t.TempDir()
 	lines := []string{