@niklaswolf it seems that's not a bug
Usage: $input = new ArrayInput(['command' => 'foo:bar', 'foo' => 'bar', '--bar' => 'foobar']);

but It possible to add a check if it's an array or not (\is_array($param))

If that is not the intended usage, then it is strange why there is handling for the value being an array in every other method of this class, see for example here:

The first argument getFirstArgument() allocated for the command name, so it's normal that accept string only
Maybe if you put a real usage that you need, it'll be more clear to understand

The usecase is this: https://github.com/shopware/shopware/blob/540d526e61e32c7065d1eae6d74a81f5e1728cb9/src/Core/TestBootstrapper.php#L396C42-L396C42

There a command is programmatically triggered, which needs the (only) argument plugins to be of type array, see https://github.com/shopware/shopware/blob/540d526e61e32c7065d1eae6d74a81f5e1728cb9/src/Core/Framework/Plugin/Command/Lifecycle/AbstractPluginLifecycleCommand.php#L45

It's more clear now, Thanks
If i see, maybe problem come from here
https://github.com/shopware/shopware/blob/540d526e61e32c7065d1eae6d74a81f5e1728cb9/src/Core/TestBootstrapper.php#L390C13-L390C13

I think the issue can be resolved in the shopware core

$args = [
                'command' => 'plugin:refresh',
                '--activate' => true,
                '--reinstall' => true,
                'plugins' => [$activePlugin],
            ];

No this is not the solution, I tried it and got the following error:

Symfony\Component\Console\Exception\InvalidArgumentException:
The "command" argument does not exist.

I didn't use shopware before (what i wrote, it was a suggestion that can help you to resolve your problem)
The main idea that you can resolve the issue in shopware core

You can see here https://github.com/shopware/shopware/blob/540d526e61e32c7065d1eae6d74a81f5e1728cb9/src/Core/Framework/Plugin/Command/Lifecycle/PluginInstallCommand.php#L24

To add something like this https://github.com/shopware/shopware/blob/540d526e61e32c7065d1eae6d74a81f5e1728cb9/src/Core/Framework/App/Command/RefreshAppCommand.php#L39

I mean something like that:

protected function configure(): void
    {
        $this->configureCommand(self::LIFECYCLE_METHOD);
        $this->addArgument('plugins', InputArgument::OPTIONAL | InputArgument::IS_ARRAY, 'The plugins list')
            ->addOption('activate', 'a', InputOption::VALUE_NONE, 'Activate plugins after installation.')
            ->addOption('reinstall', null, InputOption::VALUE_NONE, 'Reinstall the plugins');
    }

The docs for that were optimized in https://github.com/symfony/symfony-docs/pull/18741/files, hope this helps.

Okay I created an issue at Shopware for that.

Nevertheless I stand by my point that it is strange that there is handling for the value of the first argument beeing an array in other methods of this class, but not in getFirstArgument.

But if there is no intention to adjust this, this issue can be closed I think.