diff --git a/pkg/dotc1z/c1file.go b/pkg/dotc1z/c1file.go
index 548968ce6..0ffd2ed94 100644
--- a/pkg/dotc1z/c1file.go
+++ b/pkg/dotc1z/c1file.go
@@ -276,6 +276,25 @@ func (c *C1File) CloseContext(ctx context.Context) error {
 		if c.readOnly {
 			return cleanupDbDir(c.dbFilePath, ErrReadOnly)
 		}
+
+		// CRITICAL: Ensure database file is synced to disk before reading for compression.
+		// On filesystems with aggressive caching (like ZFS with ARC), data written by
+		// sqlite during Close() may still be in kernel buffers. Without this explicit
+		// fsync, saveC1z() could read stale/incomplete data, producing a truncated
+		// zstd stream that appears valid but is missing the end-of-stream marker.
+		// Note: O_RDWR is required because Sync() on read-only fd fails on Windows.
+		dbFile, err := os.OpenFile(c.dbFilePath, os.O_RDWR, 0)
+		if err != nil {
+			return cleanupDbDir(c.dbFilePath, fmt.Errorf("open db for sync: %w", err))
+		}
+		if err := dbFile.Sync(); err != nil {
+			dbFile.Close()
+			return cleanupDbDir(c.dbFilePath, fmt.Errorf("sync db before compress: %w", err))
+		}
+		if err := dbFile.Close(); err != nil {
+			return cleanupDbDir(c.dbFilePath, fmt.Errorf("close db after sync: %w", err))
+		}
+
 		err = saveC1z(c.dbFilePath, c.outputFilePath, c.encoderConcurrency)
 		if err != nil {
 			return cleanupDbDir(c.dbFilePath, err)
diff --git a/pkg/dotc1z/manager/local/local.go b/pkg/dotc1z/manager/local/local.go
index 3f3190795..3693579e9 100644
--- a/pkg/dotc1z/manager/local/local.go
+++ b/pkg/dotc1z/manager/local/local.go
@@ -168,11 +168,24 @@ func (l *localManager) SaveC1Z(ctx context.Context) error {
 	}
 	defer dstFile.Close()
 
+	// Get source file size before copy for validation
+	srcStat, err := tmpFile.Stat()
+	if err != nil {
+		return fmt.Errorf("failed to stat source file: %w", err)
+	}
+	expectedSize := srcStat.Size()
+
 	size, err := io.Copy(dstFile, tmpFile)
 	if err != nil {
 		return err
 	}
 
+	// CRITICAL: Validate copy was complete. A truncated copy would result in
+	// a corrupt c1z file missing the zstd end-of-stream marker.
+	if size != expectedSize {
+		return fmt.Errorf("copy truncated: copied %d bytes, expected %d bytes", size, expectedSize)
+	}
+
 	// CRITICAL: Sync to ensure data is written before function returns.
 	// This is especially important on ZFS ARC where writes may be cached.
 	if err := dstFile.Sync(); err != nil {