GPT-5.4 shipped on March 5. Seven weeks later, on April 23, 2026, OpenAI released GPT-5.5 — and the pace alone tells you something about where this race is headed. This isn't iteration for iteration's sake. GPT-5.5 is a genuinely different model from the ground up, and if you're building on top of OpenAI's stack, the changes matter in ways that go beyond the benchmark table. Here's everything developers need to know. The Problem It's Solving The core complaint with every prior GPT-5.x model was the same: impressive on individual tasks, but brittle on anything that required sustained, multi-step reasoning. You'd hand it a complex task, get a decent first pass, and then spend the next hour managing every subsequent step yourself. GPT-5.5 is designed to handle messy, multi-part tasks where you can trust it to plan, use tools, check its own work, navigate ambiguity, and keep going without babysitting.…