OpenAI Responses API vs Chat Completions

OpenAI’s Responses API is its recommended interface for new projects as of September 26, 2026; Chat Completions remains supported. Responses accepts input and returns a typed output list that can contain messages and tool-related items. Chat Completions accepts a role-tagged messages array and returns choices containing messages. Both APIs support text generation, and OpenAI documents function calling and structured outputs for each, but request and response schemas differ. Responses offers platform tools such as web search and file search, and its multi-turn options include previous_response_id and the Conversations API. Chat Completions does not automatically provide the same conversation history; the application generally resends the messages it wants the model to consider. State and storage are configurable, so stateful does not mean the service automatically remembers every user across calls. Check current data controls and model compatibility before relying on a feature. For a simple existing integration, Chat Completions may remain a reasonable fit when its capabilities meet the need. A new agentic workflow that needs built-in platform tools may benefit from Responses. Migration involves more than changing an endpoint: update input fields, parse output items, adapt function/tool flow, and test streaming, structured outputs, errors, and storage settings. OpenAI’s documentation should be treated as the authority for the current feature matrix. Endpoint and tool availability can vary by model, so confirm support for the exact model and project before relying on a capability.