diff --git a/.env.example b/.env.example
index 3e8ecc3..b1ff81d 100644
--- a/.env.example
+++ b/.env.example
@@ -5,6 +5,10 @@ LOG_LEVEL=debug
 # 日志级别；建议值为 debug、info、warning、error，当前 MVP 预留该配置用于后续日志工厂。
 AUDIT_LOG=runtime/audit.log
 # 审计日志文件路径；建议使用 runtime/*.log，MVP 会按 JSON Lines 格式追加写入。
+LAYLINK_FRAME_ENCRYPTION=none
+# Agent 与 POP Server 之间 Frame 加密方式；可选 none、chacha20，两端必须一致。
+LAYLINK_FRAME_ENCRYPTION_KEY=
+# Frame 加密密钥；LAYLINK_FRAME_ENCRYPTION=chacha20 时必填。可填普通口令，或 hex:32字节十六进制，或 base64:32字节base64。
 
 [client-agent]
 NODE_ID=client-01
diff --git a/.gitignore b/.gitignore
new file mode 100644
index 0000000..e0b9c9d
--- /dev/null
+++ b/.gitignore
@@ -0,0 +1,2 @@
+.env
+runtime/*
\ No newline at end of file
diff --git a/bin/client-agent.php b/bin/client-agent.php
index 71121b3..04c2a24 100755
--- a/bin/client-agent.php
+++ b/bin/client-agent.php
@@ -4,12 +4,17 @@
 declare(strict_types=1);
 
 use LayLink\Agent\AgentClient;
+use LayLink\Protocol\FrameCodec;
 use LayLink\Util\Env;
 use Workerman\Worker;
 
 require dirname(__DIR__) . '/vendor/autoload.php';
 
 Env::load(dirname(__DIR__) . '/.env');
+FrameCodec::configureEncryption(
+    Env::get('LAYLINK_FRAME_ENCRYPTION', 'none'),
+    Env::get('LAYLINK_FRAME_ENCRYPTION_KEY', ''),
+);
 Worker::$logFile = dirname(__DIR__) . '/runtime/workerman.log';
 Worker::$pidFile = dirname(__DIR__) . '/runtime/client-agent.pid';
 
diff --git a/bin/pop-server.php b/bin/pop-server.php
index 5a61f67..c1531d6 100755
--- a/bin/pop-server.php
+++ b/bin/pop-server.php
@@ -4,12 +4,17 @@
 declare(strict_types=1);
 
 use LayLink\Server\PopServer;
+use LayLink\Protocol\FrameCodec;
 use LayLink\Util\Env;
 use Workerman\Worker;
 
 require dirname(__DIR__) . '/vendor/autoload.php';
 
 Env::load(dirname(__DIR__) . '/.env');
+FrameCodec::configureEncryption(
+    Env::get('LAYLINK_FRAME_ENCRYPTION', 'none'),
+    Env::get('LAYLINK_FRAME_ENCRYPTION_KEY', ''),
+);
 Worker::$logFile = dirname(__DIR__) . '/runtime/workerman.log';
 Worker::$pidFile = dirname(__DIR__) . '/runtime/pop-server.pid';
 
diff --git a/contract.md b/contract.md
index e308f99..a85c8a7 100644
--- a/contract.md
+++ b/contract.md
@@ -141,6 +141,16 @@ Completed in this checkpoint:
   * `bin/border-agent.php`
   * sample border node and border policy docs
 * Verified POP now starts with only `laylink-pop-agent-listener`.
+* Fixed SOCKS5 error behavior when POP is not connected:
+  * SOCKS5 method negotiation no longer returns text errors.
+  * POP connection failures during CONNECT now return standard SOCKS5 failure replies.
+* Added Agent-to-POP Frame encryption:
+  * `LAYLINK_FRAME_ENCRYPTION=none|chacha20`
+  * `LAYLINK_FRAME_ENCRYPTION_KEY`
+  * POP Server and Client Agent must use identical encryption settings.
+  * `chacha20` currently uses libsodium XChaCha20 stream encryption with a random nonce per frame.
+* Verified `none` and `chacha20` FrameCodec encode/decode round trips.
+* Verified POP Server starts with `LAYLINK_FRAME_ENCRYPTION=chacha20`.
 
 Known MVP limitations:
 
diff --git a/readme.md b/readme.md
index ab15ee7..ec01e4b 100644
--- a/readme.md
+++ b/readme.md
@@ -27,6 +27,29 @@ cp .env.example .env
 
 `.env.example` 中的 `[config]`、`[client-agent]`、`[pop-server]` 是阅读分组标题，当前加载器会忽略这些标题，只读取 `KEY=value` 配置行。
 
+Agent 与 POP Server 之间的 LayLink Frame 支持加密：
+
+```env
+LAYLINK_FRAME_ENCRYPTION=none
+LAYLINK_FRAME_ENCRYPTION_KEY=
+```
+
+可选值：
+
+| 值 | 作用 |
+| --- | --- |
+| `none` | 不加密，开发调试默认值。 |
+| `chacha20` | 使用 libsodium XChaCha20 stream 对 Frame body 加密。 |
+
+启用 `chacha20` 时，POP Server 和 Client Agent 必须配置完全相同的加密方式和密钥：
+
+```env
+LAYLINK_FRAME_ENCRYPTION=chacha20
+LAYLINK_FRAME_ENCRYPTION_KEY=change-this-long-random-secret
+```
+
+密钥支持普通口令，也支持 `hex:` 或 `base64:` 前缀的 32 字节原始密钥。
+
 ## POP Server
 
 POP Server 是控制面和转发入口。
@@ -61,6 +84,8 @@ LOG_LEVEL=debug
 | 变量 | 作用 | 常见值 |
 | --- | --- | --- |
 | `APP_ENV` | 当前运行环境。开发时使用 `dev`，生产可使用 `prod`。 | `dev`、`test`、`prod` |
+| `LAYLINK_FRAME_ENCRYPTION` | Agent 与 POP Server 之间 Frame 加密方式，两端必须一致。 | `none`、`chacha20` |
+| `LAYLINK_FRAME_ENCRYPTION_KEY` | Frame 加密密钥，启用 `chacha20` 时必填。 | 普通口令、`hex:...`、`base64:...` |
 | `POP_AGENT_LISTEN` | POP Server 给 Client Agent 连接的监听地址。Agent 的 `POP_SERVER_ADDRESS` 应指向这里。 | `0.0.0.0:9001`、`127.0.0.1:9001` |
 | `POP_ALLOWED_AGENT_TRANSPORTS` | POP Server 允许 Agent 使用的底层传输协议，逗号分隔。Agent 认证时会上报自己的选择，不在列表内会被拒绝。 | `tcp`、`tcp,kcp`、`tcp,udp,kcp` |
 | `AUDIT_LOG` | 审计日志路径。MVP 使用 JSON Lines 追加写入。 | `runtime/audit.log` |
diff --git a/runtime/workerman.log b/runtime/workerman.log
index 8b07f27..39657db 100644
--- a/runtime/workerman.log
+++ b/runtime/workerman.log
@@ -34,3 +34,30 @@
 2026-05-28 12:08:11 pid:489103 Workerman[pop-server.php] received signal SIGTERM
 2026-05-28 12:08:11 pid:489103 Workerman[pop-server.php] stopping
 2026-05-28 12:08:11 pid:489103 Workerman[pop-server.php] has been stopped
+2026-05-28 12:28:14 pid:489644 Workerman[client-agent.php] start in DEBUG mode
+2026-05-28 12:34:47 pid:490774 Workerman[client-agent.php] restart
+2026-05-28 12:34:47 pid:490774 Workerman[client-agent.php] is stopping ...
+2026-05-28 12:34:47 pid:489644 Workerman[client-agent.php] received signal SIGINT
+2026-05-28 12:34:47 pid:489644 Workerman[client-agent.php] stopping
+2026-05-28 12:34:48 pid:489644 Workerman[client-agent.php] has been stopped
+2026-05-28 12:34:48 pid:490774 Workerman[client-agent.php] stop success
+2026-05-28 12:35:17 pid:490876 Workerman[client-agent.php] stop
+2026-05-28 12:35:17 pid:490876 Workerman[client-agent.php] is stopping ...
+2026-05-28 12:35:17 pid:490774 Workerman[client-agent.php] received signal SIGINT
+2026-05-28 12:35:17 pid:490774 Workerman[client-agent.php] stopping
+2026-05-28 12:35:19 pid:490774 Workerman[client-agent.php] has been stopped
+2026-05-28 12:35:19 pid:490876 Workerman[client-agent.php] stop success
+2026-05-28 12:35:19 pid:490845 Workerman[client-agent.php] start in DAEMON mode
+2026-05-28 12:40:44 pid:491096 Workerman[client-agent.php] restart
+2026-05-28 12:40:44 pid:491096 Workerman[client-agent.php] is stopping ...
+2026-05-28 12:40:44 pid:490878 Workerman[client-agent.php] received signal SIGINT
+2026-05-28 12:40:44 pid:490878 Workerman[client-agent.php] stopping
+2026-05-28 12:40:44 pid:490878 Workerman[client-agent.php] has been stopped
+2026-05-28 12:40:44 pid:491096 Workerman[client-agent.php] stop success
+2026-05-28 13:04:58 pid:492360 Workerman[pop-server.php] start in DEBUG mode
+2026-05-28 13:05:00 pid:492360 Workerman[pop-server.php] received signal SIGTERM
+2026-05-28 13:05:00 pid:492360 Workerman[pop-server.php] stopping
+2026-05-28 13:05:00 pid:492360 Workerman[pop-server.php] has been stopped
+2026-05-28 13:07:21 pid:491096 Workerman[client-agent.php] received signal SIGINT
+2026-05-28 13:07:21 pid:491096 Workerman[client-agent.php] stopping
+2026-05-28 13:07:21 pid:491096 Workerman[client-agent.php] has been stopped
diff --git a/src/Agent/AgentClient.php b/src/Agent/AgentClient.php
index c9121d1..0022121 100644
--- a/src/Agent/AgentClient.php
+++ b/src/Agent/AgentClient.php
@@ -167,12 +167,6 @@ final class AgentClient
 
     private function handleInitialRequest(TcpConnection $connection, string $data): void
     {
-        if (!$this->authenticated || $this->pop === null) {
-            $connection->send("ERR pop_not_connected\n");
-            $connection->close();
-            return;
-        }
-
         $buffer = ($this->initialBuffers[$connection->id] ?? '') . $data;
         $this->initialBuffers[$connection->id] = $buffer;
 
@@ -428,6 +422,11 @@ final class AgentClient
 
     private function startPopSession(TcpConnection $connection, array $request, string $payloadBytes, string $ingressProtocol): void
     {
+        if (!$this->authenticated || $this->pop === null) {
+            $this->failOpeningLocalClient($connection, $ingressProtocol, 'pop_not_connected');
+            return;
+        }
+
         $sessionId = Uuid::v4();
         $this->connectionSessionIds[$connection->id] = $sessionId;
         $this->clients[$sessionId] = $connection;
@@ -448,6 +447,18 @@ final class AgentClient
         ]));
     }
 
+    private function failOpeningLocalClient(TcpConnection $connection, string $ingressProtocol, string $reason): void
+    {
+        if ($ingressProtocol === 'socks5') {
+            $connection->send($this->socks5Reply(1));
+        } elseif (str_starts_with($ingressProtocol, 'http')) {
+            $connection->send("HTTP/1.1 502 Bad Gateway\r\nConnection: close\r\nX-LayLink-Error: {$reason}\r\n\r\n");
+        } else {
+            $connection->send("ERR {$reason}\n");
+        }
+        $connection->close();
+    }
+
     private function onSocks5UdpMessage(UdpConnection $connection, string $data): void
     {
         if (!$this->authenticated || $this->pop === null || strlen($data) < 10) {
diff --git a/src/Protocol/FrameCodec.php b/src/Protocol/FrameCodec.php
index 6109e9a..a84d26d 100644
--- a/src/Protocol/FrameCodec.php
+++ b/src/Protocol/FrameCodec.php
@@ -9,6 +9,34 @@ use InvalidArgumentException;
 final class FrameCodec
 {
     public const MAX_FRAME_LENGTH = 16 * 1024 * 1024;
+    private const ENCRYPTION_NONE = 'none';
+    private const ENCRYPTION_CHACHA20 = 'chacha20';
+
+    private static string $encryption = self::ENCRYPTION_NONE;
+    private static ?string $key = null;
+
+    public static function configureEncryption(string $method, string $keyMaterial = ''): void
+    {
+        $method = strtolower(trim($method));
+        if ($method === '') {
+            $method = self::ENCRYPTION_NONE;
+        }
+
+        if (!in_array($method, [self::ENCRYPTION_NONE, self::ENCRYPTION_CHACHA20], true)) {
+            throw new InvalidArgumentException('unsupported_frame_encryption');
+        }
+
+        if ($method === self::ENCRYPTION_CHACHA20) {
+            if (!function_exists('sodium_crypto_stream_xchacha20_xor')) {
+                throw new InvalidArgumentException('chacha20_unavailable');
+            }
+            self::$key = self::normalizeKey($keyMaterial);
+        } else {
+            self::$key = null;
+        }
+
+        self::$encryption = $method;
+    }
 
     public static function encode(Frame $frame): string
     {
@@ -17,11 +45,14 @@ final class FrameCodec
             throw new InvalidArgumentException('invalid_frame_payload');
         }
 
-        return pack('N', strlen($json)) . $json;
+        $body = self::encrypt($json);
+
+        return pack('N', strlen($body)) . $body;
     }
 
-    public static function decode(string $json): Frame
+    public static function decode(string $body): Frame
     {
+        $json = self::decrypt($body);
         $data = json_decode($json, true);
         if (!is_array($data) || !isset($data['type']) || !is_string($data['type'])) {
             throw new InvalidArgumentException('invalid_frame');
@@ -44,4 +75,56 @@ final class FrameCodec
             (int)($data['version'] ?? 1),
         );
     }
+
+    private static function encrypt(string $plaintext): string
+    {
+        if (self::$encryption === self::ENCRYPTION_NONE) {
+            return $plaintext;
+        }
+
+        $nonce = random_bytes(SODIUM_CRYPTO_STREAM_XCHACHA20_NONCEBYTES);
+        return $nonce . sodium_crypto_stream_xchacha20_xor($plaintext, $nonce, self::$key ?? '');
+    }
+
+    private static function decrypt(string $body): string
+    {
+        if (self::$encryption === self::ENCRYPTION_NONE) {
+            return $body;
+        }
+
+        $nonceLength = SODIUM_CRYPTO_STREAM_XCHACHA20_NONCEBYTES;
+        if (strlen($body) < $nonceLength) {
+            throw new InvalidArgumentException('invalid_encrypted_frame');
+        }
+
+        $nonce = substr($body, 0, $nonceLength);
+        $ciphertext = substr($body, $nonceLength);
+        return sodium_crypto_stream_xchacha20_xor($ciphertext, $nonce, self::$key ?? '');
+    }
+
+    private static function normalizeKey(string $keyMaterial): string
+    {
+        $keyMaterial = trim($keyMaterial);
+        if ($keyMaterial === '') {
+            throw new InvalidArgumentException('missing_frame_encryption_key');
+        }
+
+        if (str_starts_with($keyMaterial, 'hex:')) {
+            $decoded = hex2bin(substr($keyMaterial, 4));
+            if ($decoded !== false && strlen($decoded) === SODIUM_CRYPTO_STREAM_XCHACHA20_KEYBYTES) {
+                return $decoded;
+            }
+            throw new InvalidArgumentException('invalid_hex_frame_encryption_key');
+        }
+
+        if (str_starts_with($keyMaterial, 'base64:')) {
+            $decoded = base64_decode(substr($keyMaterial, 7), true);
+            if ($decoded !== false && strlen($decoded) === SODIUM_CRYPTO_STREAM_XCHACHA20_KEYBYTES) {
+                return $decoded;
+            }
+            throw new InvalidArgumentException('invalid_base64_frame_encryption_key');
+        }
+
+        return hash('sha256', $keyMaterial, true);
+    }
 }