diff --git a/src/Chain/ToolBox/ToolBox.php b/src/Chain/ToolBox/ToolBox.php
index bc3f852e..bca547a1 100644
--- a/src/Chain/ToolBox/ToolBox.php
+++ b/src/Chain/ToolBox/ToolBox.php
@@ -4,7 +4,7 @@
 
 namespace PhpLlm\LlmChain\Chain\ToolBox;
 
-use PhpLlm\LlmChain\Exception\RuntimeException;
+use PhpLlm\LlmChain\Exception\ToolNotFoundException;
 use PhpLlm\LlmChain\Model\Response\ToolCall;
 
 final class ToolBox implements ToolBoxInterface
@@ -55,6 +55,6 @@ public function execute(ToolCall $toolCall): string
             }
         }
 
-        throw new RuntimeException('Tool not found');
+        throw ToolNotFoundException::forToolCall($toolCall);
     }
 }
diff --git a/src/Exception/ToolNotFoundException.php b/src/Exception/ToolNotFoundException.php
new file mode 100644
index 00000000..d68a93a4
--- /dev/null
+++ b/src/Exception/ToolNotFoundException.php
@@ -0,0 +1,15 @@
+<?php
+
+declare(strict_types=1);
+
+namespace PhpLlm\LlmChain\Exception;
+
+use PhpLlm\LlmChain\Model\Response\ToolCall;
+
+final class ToolNotFoundException extends RuntimeException
+{
+    public static function forToolCall(ToolCall $toolCall): self
+    {
+        return new self(sprintf('Tool not found for call: %s', $toolCall->name));
+    }
+}
diff --git a/tests/Chain/ToolBox/ToolBoxTest.php b/tests/Chain/ToolBox/ToolBoxTest.php
index a02b3444..c494d60f 100644
--- a/tests/Chain/ToolBox/ToolBoxTest.php
+++ b/tests/Chain/ToolBox/ToolBoxTest.php
@@ -9,6 +9,7 @@
 use PhpLlm\LlmChain\Chain\ToolBox\ParameterAnalyzer;
 use PhpLlm\LlmChain\Chain\ToolBox\ToolAnalyzer;
 use PhpLlm\LlmChain\Chain\ToolBox\ToolBox;
+use PhpLlm\LlmChain\Exception\ToolNotFoundException;
 use PhpLlm\LlmChain\Model\Response\ToolCall;
 use PhpLlm\LlmChain\Tests\Fixture\Tool\ToolNoParams;
 use PhpLlm\LlmChain\Tests\Fixture\Tool\ToolOptionalParam;
@@ -101,6 +102,15 @@ public function toolsMap(): void
         self::assertSame(json_encode($expected), json_encode($actual));
     }
 
+    #[Test]
+    public function executeWithUnknownTool(): void
+    {
+        self::expectException(ToolNotFoundException::class);
+        self::expectExceptionMessage('Tool not found for call: foo_bar_baz');
+
+        $this->toolBox->execute(new ToolCall('call_1234', 'foo_bar_baz'));
+    }
+
     #[Test]
     public function execute(): void
     {