diff --git a/examples/chat-system-prompt.php b/examples/chat-system-prompt.php
new file mode 100755
index 0000000..81b71ec
--- /dev/null
+++ b/examples/chat-system-prompt.php
@@ -0,0 +1,28 @@
+<?php
+
+use PhpLlm\LlmChain\Bridge\OpenAI\GPT;
+use PhpLlm\LlmChain\Bridge\OpenAI\PlatformFactory;
+use PhpLlm\LlmChain\Chain;
+use PhpLlm\LlmChain\Chain\InputProcessor\SystemPromptInputProcessor;
+use PhpLlm\LlmChain\Model\Message\Message;
+use PhpLlm\LlmChain\Model\Message\MessageBag;
+use Symfony\Component\Dotenv\Dotenv;
+
+require_once dirname(__DIR__).'/vendor/autoload.php';
+(new Dotenv())->loadEnv(dirname(__DIR__).'/.env');
+
+if (empty($_ENV['OPENAI_API_KEY'])) {
+    echo 'Please set the OPENAI_API_KEY environment variable.'.PHP_EOL;
+    exit(1);
+}
+
+$platform = PlatformFactory::create($_ENV['OPENAI_API_KEY']);
+$llm = new GPT(GPT::GPT_4O_MINI);
+
+$processor = new SystemPromptInputProcessor('You are Yoda and write like he speaks. But short.');
+
+$chain = new Chain($platform, $llm, [$processor]);
+$messages = new MessageBag(Message::ofUser('What is the meaning of life?'));
+$response = $chain->call($messages);
+
+echo $response->getContent().PHP_EOL;
diff --git a/src/Chain/LlmOverrideInputProcessor.php b/src/Chain/InputProcessor/LlmOverrideInputProcessor.php
similarity index 83%
rename from src/Chain/LlmOverrideInputProcessor.php
rename to src/Chain/InputProcessor/LlmOverrideInputProcessor.php
index 83241e3..0d50957 100644
--- a/src/Chain/LlmOverrideInputProcessor.php
+++ b/src/Chain/InputProcessor/LlmOverrideInputProcessor.php
@@ -2,8 +2,10 @@
 
 declare(strict_types=1);
 
-namespace PhpLlm\LlmChain\Chain;
+namespace PhpLlm\LlmChain\Chain\InputProcessor;
 
+use PhpLlm\LlmChain\Chain\Input;
+use PhpLlm\LlmChain\Chain\InputProcessor;
 use PhpLlm\LlmChain\Exception\InvalidArgumentException;
 use PhpLlm\LlmChain\Model\LanguageModel;
 
diff --git a/src/Chain/InputProcessor/SystemPromptInputProcessor.php b/src/Chain/InputProcessor/SystemPromptInputProcessor.php
new file mode 100644
index 0000000..1d0058d
--- /dev/null
+++ b/src/Chain/InputProcessor/SystemPromptInputProcessor.php
@@ -0,0 +1,33 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpLlm\LlmChain\Chain\InputProcessor;
+
+use PhpLlm\LlmChain\Chain\Input;
+use PhpLlm\LlmChain\Chain\InputProcessor;
+use PhpLlm\LlmChain\Model\Message\Message;
+use Psr\Log\LoggerInterface;
+use Psr\Log\NullLogger;
+
+final readonly class SystemPromptInputProcessor implements InputProcessor
+{
+    public function __construct(
+        private string $systemPrompt,
+        private LoggerInterface $logger = new NullLogger(),
+    ) {
+    }
+
+    public function processInput(Input $input): void
+    {
+        $messages = $input->messages;
+
+        if (null !== $messages->getSystemMessage()) {
+            $this->logger->debug('Skipping system prompt injection since MessageBag already contains a system message.');
+
+            return;
+        }
+
+        $input->messages = $messages->prepend(Message::forSystem($this->systemPrompt));
+    }
+}
diff --git a/src/Exception/ToolBoxException.php b/src/Exception/ToolBoxException.php
index 5bcbba7..0c87561 100644
--- a/src/Exception/ToolBoxException.php
+++ b/src/Exception/ToolBoxException.php
@@ -8,7 +8,7 @@
 
 final class ToolBoxException extends RuntimeException
 {
-    public ?ToolCall $toolCall;
+    public ?ToolCall $toolCall = null;
 
     public static function notFoundForToolCall(ToolCall $toolCall): self
     {
diff --git a/tests/Chain/LlmOverrideInputProcessorTest.php b/tests/Chain/InputProcessor/LlmOverrideInputProcessorTest.php
similarity index 93%
rename from tests/Chain/LlmOverrideInputProcessorTest.php
rename to tests/Chain/InputProcessor/LlmOverrideInputProcessorTest.php
index 84476cb..6ea0991 100644
--- a/tests/Chain/LlmOverrideInputProcessorTest.php
+++ b/tests/Chain/InputProcessor/LlmOverrideInputProcessorTest.php
@@ -2,13 +2,13 @@
 
 declare(strict_types=1);
 
-namespace PhpLlm\LlmChain\Tests\Chain;
+namespace PhpLlm\LlmChain\Tests\Chain\InputProcessor;
 
 use PhpLlm\LlmChain\Bridge\Anthropic\Claude;
 use PhpLlm\LlmChain\Bridge\OpenAI\Embeddings;
 use PhpLlm\LlmChain\Bridge\OpenAI\GPT;
 use PhpLlm\LlmChain\Chain\Input;
-use PhpLlm\LlmChain\Chain\LlmOverrideInputProcessor;
+use PhpLlm\LlmChain\Chain\InputProcessor\LlmOverrideInputProcessor;
 use PhpLlm\LlmChain\Exception\InvalidArgumentException;
 use PhpLlm\LlmChain\Model\Message\MessageBag;
 use PHPUnit\Framework\Attributes\CoversClass;
diff --git a/tests/Chain/InputProcessor/SystemPromptInputProcessorTest.php b/tests/Chain/InputProcessor/SystemPromptInputProcessorTest.php
new file mode 100644
index 0000000..ce6b589
--- /dev/null
+++ b/tests/Chain/InputProcessor/SystemPromptInputProcessorTest.php
@@ -0,0 +1,60 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpLlm\LlmChain\Tests\Chain\InputProcessor;
+
+use PhpLlm\LlmChain\Bridge\OpenAI\GPT;
+use PhpLlm\LlmChain\Chain\Input;
+use PhpLlm\LlmChain\Chain\InputProcessor\SystemPromptInputProcessor;
+use PhpLlm\LlmChain\Model\Message\Message;
+use PhpLlm\LlmChain\Model\Message\MessageBag;
+use PhpLlm\LlmChain\Model\Message\SystemMessage;
+use PhpLlm\LlmChain\Model\Message\UserMessage;
+use PHPUnit\Framework\Attributes\CoversClass;
+use PHPUnit\Framework\Attributes\Small;
+use PHPUnit\Framework\Attributes\Test;
+use PHPUnit\Framework\Attributes\UsesClass;
+use PHPUnit\Framework\TestCase;
+
+#[CoversClass(SystemPromptInputProcessor::class)]
+#[UsesClass(GPT::class)]
+#[UsesClass(Message::class)]
+#[UsesClass(MessageBag::class)]
+#[Small]
+final class SystemPromptInputProcessorTest extends TestCase
+{
+    #[Test]
+    public function processInputAddsSystemMessageWhenNoneExists(): void
+    {
+        $processor = new SystemPromptInputProcessor('This is a system prompt');
+
+        $input = new Input(new GPT(), new MessageBag(Message::ofUser('This is a user message')), []);
+        $processor->processInput($input);
+
+        $messages = $input->messages->getMessages();
+        self::assertCount(2, $messages);
+        self::assertInstanceOf(SystemMessage::class, $messages[0]);
+        self::assertInstanceOf(UserMessage::class, $messages[1]);
+        self::assertSame('This is a system prompt', $messages[0]->content);
+    }
+
+    #[Test]
+    public function processInputDoesNotAddSystemMessageWhenOneExists(): void
+    {
+        $processor = new SystemPromptInputProcessor('This is a system prompt');
+
+        $messages = new MessageBag(
+            Message::forSystem('This is already a system prompt'),
+            Message::ofUser('This is a user message'),
+        );
+        $input = new Input(new GPT(), $messages, []);
+        $processor->processInput($input);
+
+        $messages = $input->messages->getMessages();
+        self::assertCount(2, $messages);
+        self::assertInstanceOf(SystemMessage::class, $messages[0]);
+        self::assertInstanceOf(UserMessage::class, $messages[1]);
+        self::assertSame('This is already a system prompt', $messages[0]->content);
+    }
+}