diff --git a/src/DefaultHeadersDecorator.php b/src/DefaultHeadersDecorator.php
index 46545066..8b8aa89a 100644
--- a/src/DefaultHeadersDecorator.php
+++ b/src/DefaultHeadersDecorator.php
@@ -12,8 +12,10 @@ class DefaultHeadersDecorator implements MessageDecorator
 {
     private ClassNameInflector $inflector;
     private Clock|ClockInterface $clock;
+    /** @var non-empty-string */
     private string $timeOfRecordingFormat;
 
+    /** @param non-empty-string $timeOfRecordingFormat */
     public function __construct(
         ?ClassNameInflector $inflector = null,
         null|Clock|ClockInterface $clock = null,
diff --git a/src/Message.php b/src/Message.php
index 11e61c60..6ddddc34 100644
--- a/src/Message.php
+++ b/src/Message.php
@@ -9,15 +9,29 @@
 
 /**
  * @template TId of AggregateRootId = AggregateRootId
+ * @template TPayload of object = object
+ *
+ * @phpstan-type HeaderValue int|string|array<mixed>|TId|null|bool|float
+ * @phpstan-type HeadersShape array<non-empty-string, HeaderValue>
  */
 final class Message
 {
     public const TIME_OF_RECORDING_FORMAT = 'Y-m-d H:i:s.uO';
 
+    /**
+     * @param TPayload $payload
+     * @param HeadersShape $headers
+     */
     public function __construct(private object $payload, private array $headers = [])
     {
     }
 
+    /**
+     * @param non-empty-string $key
+     * @param HeaderValue $value
+     *
+     * @return Message<TId, TPayload>
+     */
     public function withHeader(string $key, int|string|array|AggregateRootId|null|bool|float $value): Message
     {
         $clone = clone $this;
@@ -26,6 +40,11 @@ public function withHeader(string $key, int|string|array|AggregateRootId|null|bo
         return $clone;
     }
 
+    /**
+     * @param HeadersShape $headers
+     *
+     * @return Message<TId, TPayload>
+     */
     public function withHeaders(array $headers): Message
     {
         $clone = clone $this;
@@ -34,6 +53,11 @@ public function withHeaders(array $headers): Message
         return $clone;
     }
 
+    /**
+     * @param non-empty-string $format
+     *
+     * @return Message<TId, TPayload>
+     */
     public function withTimeOfRecording(
         DateTimeImmutable $timeOfRecording,
         string $format = self::TIME_OF_RECORDING_FORMAT
@@ -47,6 +71,7 @@ public function withTimeOfRecording(
     public function aggregateVersion(): int
     {
         $version = $this->headers[Header::AGGREGATE_ROOT_VERSION] ?? null;
+        \assert($version === null || \is_numeric($version));
 
         if ($version === null) {
             throw new RuntimeException("Can't get the version if the message has none.");
@@ -60,23 +85,28 @@ public function aggregateVersion(): int
      */
     public function aggregateRootId(): ?AggregateRootId
     {
-        return $this->headers[Header::AGGREGATE_ROOT_ID] ?? null;
+        $aggregateRootId = $this->headers[Header::AGGREGATE_ROOT_ID] ?? null;
+        \assert($aggregateRootId === null || $aggregateRootId instanceof AggregateRootId);
+
+        return $aggregateRootId;
     }
 
     public function aggregateRootType(): ?string
     {
-        return $this->headers[Header::AGGREGATE_ROOT_TYPE] ?? null;
+        $aggregateRootType = $this->headers[Header::AGGREGATE_ROOT_TYPE] ?? null;
+        \assert($aggregateRootType === null || \is_string($aggregateRootType));
+
+        return $aggregateRootType;
     }
 
     public function timeOfRecording(): DateTimeImmutable
     {
         $format = $this->headers[Header::TIME_OF_RECORDING_FORMAT] ?? self::TIME_OF_RECORDING_FORMAT;
+        $header = $this->headers[Header::TIME_OF_RECORDING] ?? '';
+        \assert(\is_string($format));
+        \assert(\is_string($header));
 
-        /* @var DateTimeImmutable */
-        $timeOfRecording = DateTimeImmutable::createFromFormat(
-            '!' . $format,
-            $header = (string) ($this->headers[Header::TIME_OF_RECORDING] ?? '')
-        );
+        $timeOfRecording = \DateTimeImmutable::createFromFormat('!' . $format, $header);
 
         if ( ! $timeOfRecording instanceof DateTimeImmutable) {
             throw UnableToResolveTimeOfRecording::fromFormatAndHeader($format, $header);
@@ -85,16 +115,27 @@ public function timeOfRecording(): DateTimeImmutable
         return $timeOfRecording;
     }
 
-    public function header(string $key): int|string|array|AggregateRootId|null|bool|float
+    /**
+     * @param non-empty-string $key
+     *
+     * @return HeaderValue
+     */
+    public function header(string $key): int|string|array|AggregateRootId|bool|float|null
     {
         return $this->headers[$key] ?? null;
     }
 
+    /**
+     * @return HeadersShape
+     */
     public function headers(): array
     {
         return $this->headers;
     }
 
+    /**
+     * @return TPayload
+     */
     public function payload(): object
     {
         return $this->payload;
@@ -102,6 +143,8 @@ public function payload(): object
 
     /**
      * @deprecated use ->payload instead
+     *
+     * @return TPayload
      */
     public function event(): object
     {