diff --git a/src/Resource/BlobResourceContents.php b/src/Resource/BlobResourceContents.php
index 441add5..b5eebb4 100644
--- a/src/Resource/BlobResourceContents.php
+++ b/src/Resource/BlobResourceContents.php
@@ -28,15 +28,11 @@ public function __construct(
      * Converts the blob resource contents to an array format.
      * Includes 'uri', 'blob', and 'mimeType'.
      *
-     * @return array{uri: string, blob: string, mimeType?: string} The array representation.
+     * @return array{uri: string, blob: string, mimeType: string} The array representation.
      */
     public function toArray(): array
     {
-        $data = ['uri' => $this->uri, 'blob' => $this->blob];
-        // $this->mimeType is guaranteed by constructor, but check doesn't hurt for consistency
-        if ($this->mimeType !== null) {
-            $data['mimeType'] = $this->mimeType;
-        }
+        $data = ['uri' => $this->uri, 'blob' => $this->blob, 'mimeType' => $this->mimeType];
         return $data;
     }
 }
diff --git a/src/Resource/Resource.php b/src/Resource/Resource.php
index a1404c3..1116177 100644
--- a/src/Resource/Resource.php
+++ b/src/Resource/Resource.php
@@ -145,7 +145,7 @@ protected function text(string $text, ?string $mimeType = null, array $parameter
         return new TextResourceContents(
             $this->resolveUri($this->getUri(), $parameters),
             $text,
-            $mimeType
+            $mimeType ?? 'text/plain'
         );
     }
 
diff --git a/src/Resource/ResourceContents.php b/src/Resource/ResourceContents.php
index 887d02e..8afcc97 100644
--- a/src/Resource/ResourceContents.php
+++ b/src/Resource/ResourceContents.php
@@ -11,17 +11,33 @@
  */
 abstract class ResourceContents
 {
+    /**
+     * The MIME type of the resource content.
+     * @var string
+     */
+    public readonly string $mimeType;
+
     /**
      * Constructs a ResourceContents instance.
      *
      * @param string $uri The URI of the resource content. This might be a resolved URI if the resource URI was a template.
-     * @param string|null $mimeType The MIME type of the resource content, if applicable.
+     * @param string $mimeType The MIME type of the resource content.
      */
     public function __construct(
         /** The URI of the resource content. */
         public readonly string $uri,
-        /** The MIME type of the resource content, if applicable. */
-        public readonly ?string $mimeType = null
+        string $mimeType
     ) {
+        $this->mimeType = $mimeType;
+    }
+
+    /**
+     * Converts the resource contents to an array format.
+     *
+     * @return array{uri: string, mimeType: string} The array representation.
+     */
+    public function toArray(): array
+    {
+        return ['uri' => $this->uri, 'mimeType' => $this->mimeType];
     }
 }
diff --git a/src/Resource/TextResourceContents.php b/src/Resource/TextResourceContents.php
index d92787d..7cd9f85 100644
--- a/src/Resource/TextResourceContents.php
+++ b/src/Resource/TextResourceContents.php
@@ -15,28 +15,26 @@ class TextResourceContents extends ResourceContents
      *
      * @param string $uri The URI of the resource.
      * @param string $text The text content.
-     * @param string|null $mimeType The MIME type of the content. Defaults to "text/plain" if null.
+     * @param string $mimeType The MIME type of the resource. Defaults to 'text/plain'.
      */
     public function __construct(
         string $uri,
         public readonly string $text,
-        ?string $mimeType = null
+        string $mimeType = 'text/plain'
     ) {
-        parent::__construct($uri, $mimeType ?? 'text/plain');
+        parent::__construct($uri, $mimeType);
     }
 
     /**
      * Converts the text resource contents to an array format.
-     * Includes 'uri', 'text', and 'mimeType'.
+     * Includes 'uri', 'mimeType', and 'text'.
      *
-     * @return array{uri: string, text: string, mimeType?: string} The array representation.
+     * @return array{uri: string, mimeType: string, text: string} The array representation.
      */
     public function toArray(): array
     {
-        $data = ['uri' => $this->uri, 'text' => $this->text];
-        if ($this->mimeType !== null) {
-            $data['mimeType'] = $this->mimeType;
-        }
+        $data = parent::toArray(); // Gets 'uri' and 'mimeType' from ResourceContents
+        $data['text'] = $this->text;
         return $data;
     }
 }
diff --git a/tests/Resource/BlobResourceContentsTest.php b/tests/Resource/BlobResourceContentsTest.php
new file mode 100644
index 0000000..395c19d
--- /dev/null
+++ b/tests/Resource/BlobResourceContentsTest.php
@@ -0,0 +1,28 @@
+<?php
+
+declare(strict_types=1);
+
+namespace MCP\Tests\Server\Resource;
+
+use MCP\Server\Resource\BlobResourceContents;
+use PHPUnit\Framework\TestCase;
+
+class BlobResourceContentsTest extends TestCase
+{
+    public function testToArrayReturnsCorrectStructureAndValues(): void
+    {
+        $uri = 'test://blob/123';
+        $blobData = base64_encode('Hello World');
+        $mimeType = 'application/octet-stream';
+
+        $blobContents = new BlobResourceContents($uri, $blobData, $mimeType);
+
+        $expectedArray = [
+            'uri' => $uri,
+            'blob' => $blobData,
+            'mimeType' => $mimeType,
+        ];
+
+        $this->assertSame($expectedArray, $blobContents->toArray());
+    }
+}
diff --git a/tests/Resource/TextResourceContentsTest.php b/tests/Resource/TextResourceContentsTest.php
new file mode 100644
index 0000000..25c4a5d
--- /dev/null
+++ b/tests/Resource/TextResourceContentsTest.php
@@ -0,0 +1,52 @@
+<?php
+
+declare(strict_types=1);
+
+namespace MCP\Tests\Server\Resource;
+
+use MCP\Server\Resource\TextResourceContents;
+use PHPUnit\Framework\TestCase;
+
+class TextResourceContentsTest extends TestCase
+{
+    public function testConstructorAndToArrayWithExplicitMimeType(): void
+    {
+        $uri = 'test://text/1';
+        $text = 'Hello world';
+        $mimeType = 'application/xml'; // Explicitly different from default
+
+        $contents = new TextResourceContents($uri, $text, $mimeType);
+
+        $this->assertSame($uri, $contents->uri);
+        $this->assertSame($text, $contents->text);
+        $this->assertSame($mimeType, $contents->mimeType);
+
+        $expectedArray = [
+            'uri' => $uri,
+            'mimeType' => $mimeType,
+            'text' => $text,
+        ];
+        $this->assertSame($expectedArray, $contents->toArray());
+    }
+
+    public function testConstructorAndToArrayWithDefaultMimeType(): void
+    {
+        $uri = 'test://text/2';
+        $text = 'Another message';
+        $defaultMimeType = 'text/plain'; // Expected default
+
+        // Instantiate without providing mimeType to use the default
+        $contents = new TextResourceContents($uri, $text);
+
+        $this->assertSame($uri, $contents->uri);
+        $this->assertSame($text, $contents->text);
+        $this->assertSame($defaultMimeType, $contents->mimeType);
+
+        $expectedArray = [
+            'uri' => $uri,
+            'mimeType' => $defaultMimeType,
+            'text' => $text,
+        ];
+        $this->assertSame($expectedArray, $contents->toArray());
+    }
+}