OpenAI GPT5 Chat is the dialog-tuned member of the GPT-5 family, built to anchor long, natural conversations in both ChatGPT and the Chat Completions API. It inherits the core GPT-5 architecture with its 400 k token window, multimodal reasoning, and JSON-based tool calling, yet layers on an extra “conversation controller” that tracks turn-level intent, speaker roles, and safety context so replies stay consistent across hundreds of messages. OpenAI’s router keeps latency low by starting each turn in the compact inference path and only activating the heavier “thinking” experts when the user signals deeper analysis. In internal evals this cut factual errors in chat by roughly 35 percent compared with GPT-4o, while still matching GPT-5 on SWE-bench Verified at the high reasoning setting.