Eloquent: how to create() with json among other values ?

If you check documentation https://www.php.net/manual/en/function.json-encode.php json_encode returns a string|false

you code works as expected

JSON is a string format, so yeah that what it should do.

But it you want to use that field as a normal object in your application, I recommend to use Accessors & Mutators: https://laravel.com/docs/8.x/eloquent-mutators#accessors-and-mutators

In your model

// To set the value
public function setDataAttribute($value){
	$this->attributes['data'] = json_encode($value);
}
// To get the value
public function setDataAttribute($value){
	return json_decode($this->attributes['data']);
}

In your controller.

$i = MyModel::create([
    'foo_id' => $r->foo,
    'bar_id' => $r->bar,
    'data' => [
        'abc' => $r->my_value,
    ],
    'created_at' => $r->created_at,
]);

Why are you wanting to store something as json? There are almost no good reason for it at all.