diff --git a/app/app.cli.php b/app/app.cli.php
index 06edf72..6ae3227 100644
--- a/app/app.cli.php
+++ b/app/app.cli.php
@@ -1,4 +1,5 @@
 <?php
+if(!Chain::isValid()) die('Chain is not valid.'.eol);
 function stdin() {
     return trim(fgets(STDIN));
 }
diff --git a/app/web/index.html b/app/web/index.html
index 465dccd..34dada7 100644
--- a/app/web/index.html
+++ b/app/web/index.html
@@ -95,11 +95,13 @@ <h5 class="modal-title" id="statusModalCenterTitle">...</h5>
                     data: $('#transaction_form').serialize(),
                     success: function(response){
                         if(response[0] == 200) updateModal('Transaction', '<span class="badge badge-success">Done</span><br>Account: '+response[1]+'<br>Amount: '+response[2]+'<br>Balance: '+response[3]);
+                        else if(response[0] == 100) updateModal('Transaction', '<span class="badge badge-danger">Error</span><br>'+response[1]);
                         $('#add_btn').html('Add');
                         $('#transaction_form')[0].reset();
                     },
                     error: function(response){
                         updateModal('Transaction', '<span class="badge badge-danger">Error</span><br>'+response);
+                        console.log(response);
                         $('#add_btn').html('Add');
                     }
                 });
@@ -114,16 +116,23 @@ <h5 class="modal-title" id="statusModalCenterTitle">...</h5>
                 data: {get_transactionchain: "1"},
                 success: function(response){
                     var s = "";
-                    var keys = ['prev', 'date', 'amount', 'account', 'hash'];
-                    for(var r in response) {
-                        s += '<div class="tx_outer alert alert-secondary" id="tx_outer_index_'+r+'">';
-                        for(var k in keys) s += '<div class="tx_inner">'+keys[k]+': '+response[r][keys[k]]+'</div>';
-                        s += '</div>';
+                    if(response[0] === 100) {
+                        updateModal('TransactionChain', '<span class="badge badge-danger">Error</span><br>'+response[1]);
+                        s = response[1];
+                    }
+                    else {
+                        var keys = ['prev', 'date', 'amount', 'account', 'hash'];
+                        for(var r in response) {
+                            s += '<div class="tx_outer alert alert-secondary" id="tx_outer_index_'+r+'">';
+                            for(var k in keys) s += '<div class="tx_inner">'+keys[k]+': '+response[r][keys[k]]+'</div>';
+                            s += '</div>';
+                        }
                     }
                     $('#transactionchain_section').html(s);
                 },
                 error: function(response){
                     updateModal('TransactionChain', '<span class="badge badge-danger">Error</span><br>'+response);
+                    console.log(response);
                     $('#transactionchain_section').html('<span class="badge badge-danger">Error</span>');
                 }
             });
diff --git a/app/web/source.php b/app/web/source.php
index 4f7a797..554fff0 100644
--- a/app/web/source.php
+++ b/app/web/source.php
@@ -16,6 +16,8 @@ function getChain() {
     Chain::show(true);
 }
 // ---------------------
+if(!Chain::isValid()) die(json_encode([100, 'Chain is not valid.']));
+// ---------------------
 if($_SERVER['REQUEST_METHOD'] == 'POST' && isset($_POST['action']) && isset($_POST['account']) && isset($_POST['amount'])) {
     $account = $_POST['account'];
     $amount = round($_POST['amount'], ROUND_PREC);
diff --git a/db/chain.db.json b/db/chain.db.json
index eea332a..fa62afb 100644
--- a/db/chain.db.json
+++ b/db/chain.db.json
@@ -20,4 +20,4 @@
         "account": "webAcc",
         "hash": "22af9a6b2f8f6a7417c35405a807763dca4d00d838c6c045194578d82ef21122"
     }
-]
+]
\ No newline at end of file
diff --git a/main.php b/main.php
index 4f9b0a0..60ed6ba 100644
--- a/main.php
+++ b/main.php
@@ -3,4 +3,4 @@
 require_once 'inc.php';
 // ---------------------
 require_once DIR_APP.'app.cli.php';
-?>
+?>
\ No newline at end of file
diff --git a/src/Chain.class.php b/src/Chain.class.php
index a7296aa..e223471 100644
--- a/src/Chain.class.php
+++ b/src/Chain.class.php
@@ -8,11 +8,13 @@ public static function balanceOf(string $account):float {
         }
         return round($balance, ROUND_PREC);
     }
-    public static function save():void {
+    public static function save():bool {
         if(self::isValid()) {
             global $transactions;
             file_put_contents(TRANSACTIONS_DB_FILE, json_encode($transactions, JSON_PRETTY_PRINT));
+            return true;
         }
+        return false;
     }
     public static function show(?bool $current = false):void {
         global $transactions;
@@ -21,15 +23,10 @@ public static function show(?bool $current = false):void {
     }
     public static function isValid():bool {
         global $transactions;
-        $txo = [];
-        $i = 0;
         foreach($transactions as $tx) {
-            $txo[] = new Transaction($txo[count($txo)-1], $tx['amount'], $tx['account'], $tx['date'], true);
-            if(!$txo[$i]->isValid()) return false;
-            if(!PoW::verifyHash($txo[$i]->prev.':'.$txo[$i]->amount.':'.$txo[$i]->date.':'.$txo[$i]->account, $txo[$i]->hash)) return false;
-            $i++;
+            if(!PoW::verifyHash($tx['prev'].':'.$tx['amount'].':'.$tx['date'].':'.$tx['account'], $tx['hash'])) return false;
         }
         return true;
     }
 }
-?>
+?>
\ No newline at end of file
diff --git a/src/Transaction.class.php b/src/Transaction.class.php
index a171de0..d1986a5 100644
--- a/src/Transaction.class.php
+++ b/src/Transaction.class.php
@@ -14,10 +14,7 @@ public function __construct($prev, ?float $amount, ?string $account = null, $dat
         $this->account = $account ? $account : null;
         $this->toHash = $this->prev.':'.$this->amount.':'.$this->date.':'.$this->account;
         $this->hash = PoW::hash($this->toHash);
-        if(!$isFake) if($this->isValid()) $this->save();
-    }
-    public function isValid():bool {
-        return PoW::verifyHash($this->toHash, $this->hash);
+        if(!$isFake) $this->save();
     }
     public function save():void {
         global $transactions;
diff --git a/test/1.test.php b/test/1.test.php
new file mode 100644
index 0000000..b805e2f
--- /dev/null
+++ b/test/1.test.php
@@ -0,0 +1,10 @@
+<?php
+define('fds', '../');
+require_once fds.'inc.php';
+// ---------------------
+var_dump(Chain::isValid()); // current chain validate
+var_dump(Chain::save()); // try to save the chain, current
+$transactions[1]['amount'] = 500; // change a transaction
+var_dump(Chain::save()); // try to save the chain, after changing a transaction
+var_dump(Chain::isValid()); // chain validate after changing a transaction
+?>
\ No newline at end of file