As of API version 2026-04, the Storefront API will return a MERCHANDISE_LINE_TRANSFORMERS_RUN_ERROR cart error code when a Cart Transform Function encounters a runtime error during cart operations such as cartCreate or cartLinesAdd.
This is a companion discussion topic for the original entry at https://shopify.dev/changelog/storefront-api-now-returns-a-specific-error-code-when-cart-transform-functions-fail