OpenAI’s Assistants API Deprecation: What .NET & Azure Developers Need to Know

TL;DR: OpenAI has announced that its Assistants API will be deprecated on August 26, 2026. If you’re building agents on OpenAI’s platform directly, you’ll need to migrate to the Responses API, which combines features of OpenAI’s Chat Completions API with the tool-use functionality of the Assistants API. But here’s the good news: Azure OpenAI Service is not impacted by this deprecation. Azure OpenAI does not use the Assistants API from OpenAI’s platform. Instead, it offers its own endpoints for chat completions, embeddings, and other model capabilities.

The Deprecation: What’s Changing

OpenAI has sent out emails notifying API customers that its chatgpt-4o-latest model will be retired from the developer platform in mid-February 2026, with access scheduled to end on February 16, 2026, creating a roughly three-month transition period for remaining applications still built on GPT-4o. More significantly, the Assistants API will be deprecated on August 26, 2026. This API allowed developers to build AI agents with memory, tools, and file handling. OpenAI is transitioning developers to a new architecture called the Responses API, which offers more flexibility and better performance for multi-step workflows and tool integrations.

What the Responses API Brings to the Table

The Responses API combines features of OpenAI’s Chat Completions API with the tool-use functionality of the Assistants API. While the Chat Completions API will continue to receive updates, the Responses API is considered its superset. Developers who need built-in tools or multi-step model interactions should use Responses API for new integrations. OpenAI is also making its web search, file search and computer use tools available directly through the Responses API. These tools enable AI agents to access real-world information, retrieve context from documents and interact with digital environments more effectively.

Azure Developers: You’re Covered

The critical distinction for .NET engineers shipping on Azure: Microsoft has not announced any changes related to this deprecation, and users of Azure OpenAI can continue building assistants and agents as usual. If you’re building with OpenAI’s platform directly, you’ll need to migrate to the Responses API before August 2026. If you’re using Azure OpenAI, no action is required at this time. Microsoft will provide updates if any changes are planned, but currently, Azure OpenAI remains stable and unaffected.

Migration Path for Direct OpenAI Users

If you’re using OpenAI’s Assistants API in a .NET project, OpenAI is transitioning developers to a new architecture called the Responses API, which offers more flexibility and better performance for multi-step workflows and tool integrations. The new system supports persistent threads, tool calling, and file handling, but with a different structure. Here’s a quick example of how the Responses API differs in structure. Instead of creating an Assistant and managing threads separately, you now work with a unified interface:

// Old Assistants API pattern (deprecated)
var assistant = await client.BetaAssistants.CreateAssistantAsync(new CreateAssistantRequest { Model = "gpt-4" });
var thread = await client.BetaBeta.Threads.CreateThreadAsync();

// New Responses API pattern
var response = await client.Responses.CreateAsync(new CreateResponseRequest
{
    Model = "gpt-4",
    Input = "What's the weather today?",
    Tools = new[] { weatherTool }
});

The Bigger Picture: Cost & Performance

OpenAI’s latest models achieve an even better score with almost 400 times less cost and less compute associated with it compared to models from a year ago. For teams evaluating whether to migrate now or wait, this efficiency gain is worth factoring into your roadmap.

Practical Takeaway

For .NET teams on Azure: No action needed. Continue using Azure OpenAI Service with confidence.

For .NET teams using OpenAI’s API directly: Start planning your Responses API migration now. You have until August 2026, but the sooner you move, the sooner you’ll benefit from improved tool integration and performance.

The deprecation reflects OpenAI’s push toward simpler, more capable APIs—and it’s worth treating as a gentle nudge to audit your current agent architecture. Treat claims as marketing until you’ve seen working results in your own codebase, with your constraints, and your risk profile. Only believe what you can validate. —

OpenAI's Assistants API Deprecation: What .NET & Azure Developers Need to Know