Chat Completions vs OpenAI Responses API: What Actually Changed

OpenAI's API has evolved from Completions to Chat Completions to the newer Responses API, which simplifies request/response structure by replacing message arrays and nested choices with clearer fields like `instructions`, `input`, and `output_text`. The Responses API also streamlines conversation history management and is designed for modern capabilities such as reasoning models, tool usage, and structured outputs, signaling the direction OpenAI is pushing for future AI application development.