diff --git a/composer.json b/composer.json
index 4fa1bda..0c72980 100644
--- a/composer.json
+++ b/composer.json
@@ -27,6 +27,7 @@
         "phpunit/phpunit": "^9.5"
     },
     "require": {
+        "ext-json": "*",
         "rybakit/msgpack": "^0.9.0"
     },
     "suggest": {
diff --git a/src/Buybrain/Nervus/Adapter/Config/AdapterConfig.php b/src/Buybrain/Nervus/Adapter/Config/AdapterConfig.php
index b0403ae..0714ed4 100644
--- a/src/Buybrain/Nervus/Adapter/Config/AdapterConfig.php
+++ b/src/Buybrain/Nervus/Adapter/Config/AdapterConfig.php
@@ -30,10 +30,7 @@ public function __construct($codec, $adapterType, ExtraAdapterConfig $extra = nu
         $this->extra = $extra;
     }
 
-    /**
-     * @return array
-     */
-    function jsonSerialize()
+    function jsonSerialize(): array
     {
         return [
             'codec' => $this->codec,
diff --git a/src/Buybrain/Nervus/Adapter/Config/SignalAdapterConfig.php b/src/Buybrain/Nervus/Adapter/Config/SignalAdapterConfig.php
index d4ba3d8..c257b84 100644
--- a/src/Buybrain/Nervus/Adapter/Config/SignalAdapterConfig.php
+++ b/src/Buybrain/Nervus/Adapter/Config/SignalAdapterConfig.php
@@ -23,10 +23,7 @@ public function __construct($interval, $priority)
         $this->priority = (int)$priority;
     }
 
-    /**
-     * @return array
-     */
-    function jsonSerialize()
+    function jsonSerialize(): array
     {
         return [
             'interval' => $this->interval,
diff --git a/src/Buybrain/Nervus/Adapter/Config/TypedAdapterConfig.php b/src/Buybrain/Nervus/Adapter/Config/TypedAdapterConfig.php
index 7877313..e47936f 100644
--- a/src/Buybrain/Nervus/Adapter/Config/TypedAdapterConfig.php
+++ b/src/Buybrain/Nervus/Adapter/Config/TypedAdapterConfig.php
@@ -19,10 +19,7 @@ public function __construct(array $entityTypes = null)
         $this->entityTypes = $entityTypes;
     }
 
-    /**
-     * @return array
-     */
-    function jsonSerialize()
+    function jsonSerialize(): array
     {
         return [
             'entityTypes' => $this->entityTypes,
diff --git a/src/Buybrain/Nervus/Adapter/Message/AbstractResponse.php b/src/Buybrain/Nervus/Adapter/Message/AbstractResponse.php
index 8e7203f..49fe504 100644
--- a/src/Buybrain/Nervus/Adapter/Message/AbstractResponse.php
+++ b/src/Buybrain/Nervus/Adapter/Message/AbstractResponse.php
@@ -55,10 +55,7 @@ public static function fromArray(array $data, StructMapper $mapper)
         return new static($data['status'], $data['error']);
     }
 
-    /**
-     * @return array
-     */
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return [
             'status' => $this->status,
diff --git a/src/Buybrain/Nervus/Adapter/Message/ReadRequest.php b/src/Buybrain/Nervus/Adapter/Message/ReadRequest.php
index 5f4496f..4fe39e8 100644
--- a/src/Buybrain/Nervus/Adapter/Message/ReadRequest.php
+++ b/src/Buybrain/Nervus/Adapter/Message/ReadRequest.php
@@ -43,10 +43,7 @@ public static function fromArray(array $data, StructMapper $mapper)
         return new self($ids);
     }
 
-    /**
-     * @return array
-     */
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return ['ids' => $this->ids];
     }
diff --git a/src/Buybrain/Nervus/Adapter/Message/ReadResponse.php b/src/Buybrain/Nervus/Adapter/Message/ReadResponse.php
index 51bd9f0..2b4ddfd 100644
--- a/src/Buybrain/Nervus/Adapter/Message/ReadResponse.php
+++ b/src/Buybrain/Nervus/Adapter/Message/ReadResponse.php
@@ -42,10 +42,7 @@ public static function fromArray(array $data, StructMapper $mapper)
         return $res;
     }
 
-    /**
-     * @return array
-     */
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return array_merge(parent::jsonSerialize(), ['entities' => $this->entities]);
     }
diff --git a/src/Buybrain/Nervus/Adapter/Message/Signal.php b/src/Buybrain/Nervus/Adapter/Message/Signal.php
index ab54f8a..01561f7 100644
--- a/src/Buybrain/Nervus/Adapter/Message/Signal.php
+++ b/src/Buybrain/Nervus/Adapter/Message/Signal.php
@@ -41,10 +41,7 @@ public static function fromArray(array $data, StructMapper $mapper)
         }, $data['ids']));
     }
 
-    /**
-     * @return array
-     */
-    function jsonSerialize()
+    function jsonSerialize(): array
     {
         return [
             'ids' => $this->ids,
diff --git a/src/Buybrain/Nervus/Adapter/Message/SignalAckRequest.php b/src/Buybrain/Nervus/Adapter/Message/SignalAckRequest.php
index dcda696..1378a19 100644
--- a/src/Buybrain/Nervus/Adapter/Message/SignalAckRequest.php
+++ b/src/Buybrain/Nervus/Adapter/Message/SignalAckRequest.php
@@ -36,10 +36,7 @@ public static function fromArray(array $data)
         return new self($data['ack']);
     }
 
-    /**
-     * @return array
-     */
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return ['ack' => $this->ack];
     }
diff --git a/src/Buybrain/Nervus/Adapter/Message/SignalRequest.php b/src/Buybrain/Nervus/Adapter/Message/SignalRequest.php
index bddd59b..47452fc 100644
--- a/src/Buybrain/Nervus/Adapter/Message/SignalRequest.php
+++ b/src/Buybrain/Nervus/Adapter/Message/SignalRequest.php
@@ -11,7 +11,7 @@ class SignalRequest implements JsonSerializable
     /**
      * @return array
      */
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return [];
     }
diff --git a/src/Buybrain/Nervus/Adapter/Message/SignalResponse.php b/src/Buybrain/Nervus/Adapter/Message/SignalResponse.php
index ed30769..2c96dd7 100644
--- a/src/Buybrain/Nervus/Adapter/Message/SignalResponse.php
+++ b/src/Buybrain/Nervus/Adapter/Message/SignalResponse.php
@@ -37,10 +37,7 @@ public static function fromArray(array $data, StructMapper $mapper)
         return $res;
     }
 
-    /**
-     * @return array
-     */
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return array_merge(parent::jsonSerialize(), ['signal' => $this->signal]);
     }
diff --git a/src/Buybrain/Nervus/Adapter/Message/WriteRequest.php b/src/Buybrain/Nervus/Adapter/Message/WriteRequest.php
index f13ffe5..bf37e62 100644
--- a/src/Buybrain/Nervus/Adapter/Message/WriteRequest.php
+++ b/src/Buybrain/Nervus/Adapter/Message/WriteRequest.php
@@ -42,10 +42,7 @@ public static function fromArray(array $data, StructMapper $mapper)
         return new self($entities);
     }
 
-    /**
-     * @return array
-     */
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return [
             'entities' => $this->entities,
diff --git a/src/Buybrain/Nervus/Entity.php b/src/Buybrain/Nervus/Entity.php
index 04c1f06..d633306 100644
--- a/src/Buybrain/Nervus/Entity.php
+++ b/src/Buybrain/Nervus/Entity.php
@@ -49,10 +49,7 @@ public function getData()
         return $this->data;
     }
 
-    /**
-     * return array
-     */
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return [
             'id' => $this->id,
diff --git a/src/Buybrain/Nervus/EntityId.php b/src/Buybrain/Nervus/EntityId.php
index b3e4870..0e4683f 100644
--- a/src/Buybrain/Nervus/EntityId.php
+++ b/src/Buybrain/Nervus/EntityId.php
@@ -42,10 +42,7 @@ public function getId()
         return $this->id;
     }
 
-    /**
-     * @return array
-     */
-    public function jsonSerialize()
+    public function jsonSerialize(): array
     {
         return [
             'type' => $this->type,