I asked ChatGPT and mostly agree! 
- Assistants API (v2)
- Status: Beta, supported through ~H2 2026 (deprecation announced H1 2026) OpenAI Platform
- When to use: Short-term “hosted” assistants with built-in state (+ threads) and tools (file_search, code interpreter).
- Responses API
- Status: GA, new primary endpoint for chat + tools + state persistence OpenAI Community
- When to use: New projects needing fastest, most flexible chat + function-/tool-calling without rolling your own history.
- OpenAI Agents SDK
- Status: Open-source Python/Node framework on top of Responses/Chat Completions
- When to use: Building complex, multi-agent workflows with orchestration, guardrails, and observability.
Have you started with the Quickstart guide?
The Cookbook (and this forum) are super helpful too.
What are you trying to build?