I've watched two teams ship the exact same product in 2026. One spent a week migrating from GPT-5.3 to Claude Opus 4.7 when Anthropic shipped it. The other spent a quarter — and still hadn't finished when GPT-5.5 launched and made the migration moot. The difference wasn't the model. It was how tightly they'd coupled their agent to one provider. Here's how to build agents that stay portable as the frontier moves. What actually couples you to a vendor Most teams think model lock-in is about the API. It's not — those are interchangeable in an afternoon. The real lock-in lives in five places: 1. Prompts. Every model has its own quirks. Claude responds well to long structured prompts with XML tags. GPT-5 wants concise instructions and clear JSON schemas. Gemini handles multi-document context differently. Prompts tuned for one model regress on another by 5–20% on quality unless you re-tune. 2. Tool / function-call schemas.…