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AI / Core OpenAI Codex Application Fundamentals Interview Questions

What is the OpenAI Assistants API and what is its deprecation timeline?

The Assistants API was OpenAI's original high-level framework for building stateful, multi-turn AI assistants with persistent threads, file handling, and built-in tools. It introduced key concepts like Threads (conversation state), Runs (execution instances), and Vector Stores (document retrieval).

Assistants API key concepts
ConceptDescription
AssistantA configured AI model with instructions, tools, and optional files
ThreadA persistent conversation history (persisted server-side)
RunAn execution of an assistant on a thread - produces a response
MessageA single turn added to a Thread
Vector StoreA server-side store of embedded documents for file search

Deprecation timeline: OpenAI formally announced the Assistants API deprecation in 2025. The planned sunset date is mid-2026, once full feature parity is achieved in the Responses API. Key migration points:

  • Threads - replaced by previous_response_id chaining or the Conversations API in Responses
  • Vector Stores - now a standalone resource, still used with file_search tool in Responses API
  • Code Interpreter - available as a built-in tool in Responses API
  • Assistant objects - replaced by the system instructions field in Responses calls

Recommendation: all new projects should use the Responses API with the Agents SDK. Existing Assistants API integrations should plan migration before mid-2026 to avoid disruption.

What is the planned sunset date for the OpenAI Assistants API?
In the migration from Assistants API to Responses API, what replaces persistent Thread objects?

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