diff --git a/src/Client.php b/src/Client.php
index 5424259..b133968 100644
--- a/src/Client.php
+++ b/src/Client.php
@@ -253,11 +253,12 @@ public function createSession(SessionConfig|array $config): CopilotSession
         }
 
         $tools = $config['tools'] ?? [];
-        $toolsForRequest = array_map(fn ($tool) => [
+        $toolsForRequest = array_map(fn ($tool) => array_filter([
             'name' => $tool['name'],
             'description' => $tool['description'] ?? null,
             'parameters' => $tool['parameters'] ?? null,
-        ], $tools);
+            'overridesBuiltInTool' => $tool['overridesBuiltInTool'] ?? null,
+        ], fn ($v) => $v !== null), $tools);
 
         $hooks = $config['hooks'] ?? null;
         $hasHooks = $hooks !== null && ! empty(array_filter(
@@ -334,11 +335,12 @@ public function resumeSession(string $sessionId, ResumeSessionConfig|array $conf
         }
 
         $tools = $config['tools'] ?? [];
-        $toolsForRequest = array_map(fn ($tool) => [
+        $toolsForRequest = array_map(fn ($tool) => array_filter([
             'name' => $tool['name'],
             'description' => $tool['description'] ?? null,
             'parameters' => $tool['parameters'] ?? null,
-        ], $tools);
+            'overridesBuiltInTool' => $tool['overridesBuiltInTool'] ?? null,
+        ], fn ($v) => $v !== null), $tools);
 
         $hooks = $config['hooks'] ?? null;
         $hasHooks = $hooks !== null && ! empty(array_filter(
diff --git a/src/Types/Tool.php b/src/Types/Tool.php
index 4b45b87..705360a 100644
--- a/src/Types/Tool.php
+++ b/src/Types/Tool.php
@@ -17,6 +17,7 @@ public function __construct(
         public ?string $description,
         public ?array $parameters,
         public Closure $handler,
+        public bool $overridesBuiltInTool = false,
     ) {}
 
     /**
@@ -27,14 +28,15 @@ public static function define(
         ?string $description,
         ?array $parameters,
         Closure $handler,
+        bool $overridesBuiltInTool = false,
     ): array {
-        return new self($name, $description, $parameters, $handler)->toArray();
+        return new self($name, $description, $parameters, $handler, $overridesBuiltInTool)->toArray();
     }
 
     /**
      * Create from array.
      *
-     * @param  array{name: string, description?: string, parameters?: array, handler: callable}  $data
+     * @param  array{name: string, description?: string, parameters?: array, handler: callable, overridesBuiltInTool?: bool}  $data
      */
     public static function fromArray(array $data): self
     {
@@ -43,21 +45,28 @@ public static function fromArray(array $data): self
             description: $data['description'] ?? null,
             parameters: $data['parameters'] ?? null,
             handler: $data['handler'],
+            overridesBuiltInTool: $data['overridesBuiltInTool'] ?? false,
         );
     }
 
     /**
      * Convert to array.
      *
-     * @return array{state: string, terms: string}
+     * @return array{name: string, description: string|null, parameters: array|null, handler: Closure, overridesBuiltInTool?: bool}
      */
     public function toArray(): array
     {
-        return [
+        $array = [
             'name' => $this->name,
             'description' => $this->description,
             'parameters' => $this->parameters,
             'handler' => $this->handler,
         ];
+
+        if ($this->overridesBuiltInTool) {
+            $array['overridesBuiltInTool'] = true;
+        }
+
+        return $array;
     }
 }
diff --git a/tests/Unit/Types/ToolTest.php b/tests/Unit/Types/ToolTest.php
index ed6fd5a..e1c2bcb 100644
--- a/tests/Unit/Types/ToolTest.php
+++ b/tests/Unit/Types/ToolTest.php
@@ -32,7 +32,8 @@
         expect($tool->name)->toBe('simple_tool')
             ->and($tool->description)->toBeNull()
             ->and($tool->parameters)->toBeNull()
-            ->and($tool->handler)->toBe($handler);
+            ->and($tool->handler)->toBe($handler)
+            ->and($tool->overridesBuiltInTool)->toBeFalse();
     });
 
     it('can define a tool statically', function () {
@@ -97,7 +98,66 @@
         expect($array)->toHaveKey('description')
             ->and($array['description'])->toBeNull()
             ->and($array)->toHaveKey('parameters')
-            ->and($array['parameters'])->toBeNull();
+            ->and($array['parameters'])->toBeNull()
+            ->and($array)->not->toHaveKey('overridesBuiltInTool');
+    });
+
+    it('includes overridesBuiltInTool in toArray when true', function () {
+        $handler = fn () => null;
+
+        $tool = new Tool(
+            name: 'override_tool',
+            description: null,
+            parameters: null,
+            handler: $handler,
+            overridesBuiltInTool: true,
+        );
+
+        $array = $tool->toArray();
+
+        expect($array)->toHaveKey('overridesBuiltInTool')
+            ->and($array['overridesBuiltInTool'])->toBeTrue();
+    });
+
+    it('omits overridesBuiltInTool from toArray when false', function () {
+        $handler = fn () => null;
+
+        $tool = new Tool(
+            name: 'normal_tool',
+            description: null,
+            parameters: null,
+            handler: $handler,
+            overridesBuiltInTool: false,
+        );
+
+        expect($tool->toArray())->not->toHaveKey('overridesBuiltInTool');
+    });
+
+    it('can define a tool with overridesBuiltInTool', function () {
+        $handler = fn () => null;
+
+        $toolArray = Tool::define(
+            name: 'edit_file',
+            description: 'Custom file editor',
+            parameters: null,
+            handler: $handler,
+            overridesBuiltInTool: true,
+        );
+
+        expect($toolArray)->toHaveKey('overridesBuiltInTool')
+            ->and($toolArray['overridesBuiltInTool'])->toBeTrue();
+    });
+
+    it('can create from array with overridesBuiltInTool', function () {
+        $handler = fn () => null;
+
+        $tool = Tool::fromArray([
+            'name' => 'edit_file',
+            'handler' => $handler,
+            'overridesBuiltInTool' => true,
+        ]);
+
+        expect($tool->overridesBuiltInTool)->toBeTrue();
     });
 
     it('handler can be executed', function () {