Claude wrote 200 lines of code on the first shot — should I trust it?

No, and the warning sign is exactly that — large, clean-looking output with no error feedback. Without an eval loop (php -l, phpcs, phpstan, MFTF) running automatically, the model is operating blind. Common silent failures in Magento: (a) typed against an interface that doesn't exist in your version (e.g. used Magento\\Customer\\Api\\Data\\AddressInterface when the calling code wanted QuoteAddressInterface); (b) wired plugin in etc/di.xml instead of etc/frontend/di.xml; (c) used GraphQL @resolver syntax that compiled, but the schema cache still serves the old AST. Always run setup:upgrade and reload the affected page.