Shopify app development (Technical). Full-stack app work (Node/React), OAuth, webhooks, billing integration. Requirements: Coding agent + multi-file work. Best pick: GPT-5.3 Codex - Best multi-file coding agent for complex app work. Alternative: Grok Code Fast 1 - Purpose-built coding model; fast for daily dev work. Budget pick: GPT-5.3 Codex-Spark - Fast prototyping; treat outputs as drafts and run tests. Recommended models: GPT-5.3 Codex, GPT-5.3 Codex-Spark, Grok Code Fast 1. Use with caution: GPT-5.2 Thinking, GPT-5.2 Instant, Gemini 3 Pro, Gemini 3 Flash, Claude Opus 4.6, Claude Sonnet 4.5, Claude Haiku 4.5, Grok 4.1 Fast (Non-reasoning), Llama 4 Maverick (Instruct), Llama 4 Scout (Instruct). Roles: Engineering. Stacks: Shopify.