diff --git a/src/Providers/AWS/MessageMapper.php b/src/Providers/AWS/MessageMapper.php
index 15ac372e..33c49bf4 100644
--- a/src/Providers/AWS/MessageMapper.php
+++ b/src/Providers/AWS/MessageMapper.php
@@ -15,6 +15,7 @@
 use function array_map;
 use function array_merge;
 use function array_filter;
+use function array_values;
 
 class MessageMapper implements MessageMapperInterface
 {
@@ -90,7 +91,7 @@ protected function mapMessage(Message $message): array
      */
     protected function mapBlocks(array $blocks): array
     {
-        return array_filter(array_map($this->mapContentBlock(...), $blocks));
+        return array_values(array_filter(array_map($this->mapContentBlock(...), $blocks)));
     }
 
     protected function mapContentBlock(ContentBlockInterface $block): ?array
diff --git a/src/Providers/Anthropic/MessageMapper.php b/src/Providers/Anthropic/MessageMapper.php
index 7751ae70..34c55ec7 100644
--- a/src/Providers/Anthropic/MessageMapper.php
+++ b/src/Providers/Anthropic/MessageMapper.php
@@ -55,7 +55,7 @@ protected function mapMessage(Message $message): array
 
     protected function mapBlocks(array $blocks): array
     {
-        return array_filter(array_map($this->mapSingleBlock(...), $blocks));
+        return array_values(array_filter(array_map($this->mapSingleBlock(...), $blocks)));
     }
 
     protected function mapSingleBlock(ContentBlockInterface $block): ?array
diff --git a/src/Providers/Gemini/MessageMapper.php b/src/Providers/Gemini/MessageMapper.php
index 7d2e1ea1..fe6559e4 100644
--- a/src/Providers/Gemini/MessageMapper.php
+++ b/src/Providers/Gemini/MessageMapper.php
@@ -25,6 +25,7 @@
 
 use function array_filter;
 use function array_map;
+use function array_values;
 
 class MessageMapper implements MessageMapperInterface
 {
@@ -59,7 +60,7 @@ protected function mapMessage(Message $message): array
 
     protected function mapBlocks(array $blocks): array
     {
-        return array_filter(array_map($this->mapContentBlock(...), $blocks));
+        return array_values(array_filter(array_map($this->mapContentBlock(...), $blocks)));
     }
 
     protected function mapContentBlock(ContentBlock $block): ?array
diff --git a/src/Providers/Mistral/MessageMapper.php b/src/Providers/Mistral/MessageMapper.php
index eef3a111..5e882cac 100644
--- a/src/Providers/Mistral/MessageMapper.php
+++ b/src/Providers/Mistral/MessageMapper.php
@@ -24,6 +24,7 @@
 
 use function array_filter;
 use function array_map;
+use function array_values;
 use function json_encode;
 
 class MessageMapper implements MessageMapperInterface
@@ -58,7 +59,7 @@ protected function mapMessage(Message $message): void
 
     protected function mapBlocks(array $blocks): array
     {
-        return array_filter(array_map($this->mapContentBlock(...), $blocks));
+        return array_values(array_filter(array_map($this->mapContentBlock(...), $blocks)));
     }
 
     protected function mapContentBlock(ContentBlockInterface $block): ?array
diff --git a/src/Providers/OpenAI/MessageMapper.php b/src/Providers/OpenAI/MessageMapper.php
index 956cc9c1..1f6a2038 100644
--- a/src/Providers/OpenAI/MessageMapper.php
+++ b/src/Providers/OpenAI/MessageMapper.php
@@ -24,6 +24,7 @@
 use function array_is_list;
 use function array_map;
 use function array_merge;
+use function array_values;
 use function json_encode;
 
 class MessageMapper implements MessageMapperInterface
@@ -64,7 +65,7 @@ protected function mapMessage(Message $message): array
 
     protected function mapBlocks(array $blocks): array
     {
-        return array_filter(array_map($this->mapContentBlock(...), $blocks));
+        return array_values(array_filter(array_map($this->mapContentBlock(...), $blocks)));
     }
 
     protected function mapContentBlock(ContentBlockInterface $block): ?array
diff --git a/src/Providers/OpenAI/Responses/MessageMapper.php b/src/Providers/OpenAI/Responses/MessageMapper.php
index 500213ab..62861ae6 100644
--- a/src/Providers/OpenAI/Responses/MessageMapper.php
+++ b/src/Providers/OpenAI/Responses/MessageMapper.php
@@ -22,6 +22,7 @@
 use function array_filter;
 use function array_map;
 use function array_merge;
+use function array_values;
 use function json_encode;
 
 class MessageMapper implements MessageMapperInterface
@@ -62,10 +63,10 @@ protected function mapMessage(Message $message): void
      */
     protected function mapBlocks(array $blocks, bool $isUser): array
     {
-        return array_filter(array_map(
+        return array_values(array_filter(array_map(
             fn (ContentBlockInterface $item): ?array => $this->mapContentBlock($item, $isUser),
             $blocks
-        ));
+        )));
     }
 
     protected function mapContentBlock(ContentBlockInterface $block, bool $isUser): ?array
diff --git a/tests/Providers/MessageMapperReindexingTest.php b/tests/Providers/MessageMapperReindexingTest.php
new file mode 100644
index 00000000..382e98ea
--- /dev/null
+++ b/tests/Providers/MessageMapperReindexingTest.php
@@ -0,0 +1,123 @@
+<?php
+
+declare(strict_types=1);
+
+namespace NeuronAI\Tests\Providers;
+
+use NeuronAI\Chat\Enums\SourceType;
+use NeuronAI\Chat\Messages\AssistantMessage;
+use NeuronAI\Chat\Messages\ContentBlocks\FileContent;
+use NeuronAI\Chat\Messages\ContentBlocks\ImageContent;
+use NeuronAI\Chat\Messages\ContentBlocks\ReasoningContent;
+use NeuronAI\Chat\Messages\ContentBlocks\TextContent;
+use NeuronAI\Providers\AWS\MessageMapper as AWSMessageMapper;
+use NeuronAI\Providers\Anthropic\MessageMapper as AnthropicMessageMapper;
+use NeuronAI\Providers\Gemini\MessageMapper as GeminiMessageMapper;
+use NeuronAI\Providers\MessageMapperInterface;
+use NeuronAI\Providers\Mistral\MessageMapper as MistralMessageMapper;
+use NeuronAI\Providers\OpenAI\MessageMapper as OpenAIMessageMapper;
+use NeuronAI\Providers\OpenAI\Responses\MessageMapper as OpenAIResponsesMessageMapper;
+use PHPUnit\Framework\Attributes\DataProvider;
+use PHPUnit\Framework\TestCase;
+
+use function array_is_list;
+use function json_decode;
+use function json_encode;
+
+class MessageMapperReindexingTest extends TestCase
+{
+    /**
+     * @param array<int, array<string, mixed>> $expectedBlocks
+     */
+    #[DataProvider('mapperProvider')]
+    public function test_filtered_blocks_serialize_as_lists(
+        MessageMapperInterface $mapper,
+        AssistantMessage $message,
+        string $blocksKey,
+        array $expectedBlocks,
+    ): void {
+        $payload = json_decode(json_encode($mapper->map([$message]), JSON_THROW_ON_ERROR), true, 512, JSON_THROW_ON_ERROR);
+
+        $this->assertIsArray($payload[0][$blocksKey]);
+        $this->assertTrue(array_is_list($payload[0][$blocksKey]));
+        $this->assertSame($expectedBlocks, $payload[0][$blocksKey]);
+    }
+
+    /**
+     * @return iterable<string, array{0: MessageMapperInterface, 1: AssistantMessage, 2: string, 3: array<int, array<string, mixed>>}>
+     */
+    public static function mapperProvider(): iterable
+    {
+        yield 'openai responses reasoning then text' => [
+            new OpenAIResponsesMessageMapper(),
+            new AssistantMessage([
+                new ReasoningContent('Thinking'),
+                new TextContent('Hello'),
+            ]),
+            'content',
+            [
+                ['type' => 'output_text', 'text' => 'Hello'],
+            ],
+        ];
+
+        yield 'openai reasoning then text' => [
+            new OpenAIMessageMapper(),
+            new AssistantMessage([
+                new ReasoningContent('Thinking'),
+                new TextContent('Hello'),
+            ]),
+            'content',
+            [
+                ['type' => 'text', 'text' => 'Hello'],
+            ],
+        ];
+
+        yield 'anthropic unsupported image id then text' => [
+            new AnthropicMessageMapper(),
+            new AssistantMessage([
+                new ImageContent('file_123', SourceType::ID),
+                new TextContent('Hello'),
+            ]),
+            'content',
+            [
+                ['type' => 'text', 'text' => 'Hello'],
+            ],
+        ];
+
+        yield 'mistral unsupported base64 file then text' => [
+            new MistralMessageMapper(),
+            new AssistantMessage([
+                new FileContent('ZmFrZQ==', SourceType::BASE64, 'application/pdf', 'test.pdf'),
+                new TextContent('Hello'),
+            ]),
+            'content',
+            [
+                ['type' => 'text', 'text' => 'Hello'],
+            ],
+        ];
+
+        yield 'gemini unsupported image id then text' => [
+            new GeminiMessageMapper(),
+            new AssistantMessage([
+                new ImageContent('file_123', SourceType::ID, 'image/png'),
+                new TextContent('Hello'),
+            ]),
+            'parts',
+            [
+                ['text' => 'Hello'],
+            ],
+        ];
+
+        yield 'aws unsupported image then text' => [
+            new AWSMessageMapper(),
+            new AssistantMessage([
+                new ImageContent('https://example.com/image.png', SourceType::URL),
+                new TextContent('Hello'),
+            ]),
+            'content',
+            [
+                ['text' => 'Hello'],
+            ],
+        ];
+    }
+}