Is a production React or Next.js application failing during login, API loading, checkout, form submission, responsive rendering, or deployment? I will reproduce the exact failure, trace it through the frontend, server routes, authentication, API integration, and data boundary, then implement the smallest reliable fix. Before work starts, we agree on the failing path, expected result, exclusions, environment, and acceptance criteria. I verify both success and failure paths and provide concise root-cause and handoff notes. Credentials remain in a protected client-controlled path. Risky changes should be tested in staging before production deployment.