Sandbox Gate - check your API test mode before integrators go live

Check an API platform's test mode so integrators validate every outcome before they touch real money, messages or data, then write the test-mode design and the cutover to live. Free in your browser: failure-path coverage against the 100% gate, every magic value held to your real error codes, one value meaning two things caught, the Luhn digit of every test card verified, event and time coverage, mode-prefixed keys and the wrong-environment error, seed, reset, quotas and card-testing controls, the cutover checklist and the five pitfalls, and the isolation and simulation choices ranked from a short interview, plus a Luhn tool and your test-value catalog as Markdown. A metered run writes the test-mode design, or the cutover to live. Built on the SkillSafe skill @samber/api-test-mode-design, from samber/developer-platform-skills (MIT), by Samuel Berthe.