Migrating to GPT-5.x Without Breaking GPT-4: A Practical, Backward-Compatible Playbook (opens in new tab)

The first request your service sends after swapping gpt-4o for gpt-5.1 in production will return HTTP 400. Not in two weeks. On the first call. And the parameter the error points to isn't one you set anywhere in your code - it's bound onto the request by a LangChain helper you've used for two years. This post walks through every breaking change between the GPT-4 and GPT-5 families on Azure OpenAI in Microsoft Foundry, the integration cliffs nobody warns you about, and the small set of files y...