Azure OpenAI’s Responses API: Time to Migrate Your Agents Before August 2026

TL;DR OpenAI’s Assistants API will be discontinued in the first half of 2026 , with deprecation on August 26, 2026 . The Responses API combines features of OpenAI’s Chat Completions API with the tool-use functionality of the Assistants API . For .NET developers on Azure, Azure OpenAI Service is not impacted by this deprecation, as it does not use the Assistants API from OpenAI’s platform but offers its own endpoints for chat completions, embeddings, and other model capabilities . However, if you’re calling OpenAI’s platform directly, migration is mandatory. Plus: GPT-5.2 introduces deeper logical chains, richer context handling, and agentic execution that prompts shippable artifacts like design docs, runnable code, unit tests, and deployment scripts with fewer iterations .

What’s Changing and Why It Matters

If you’ve built AI agents using OpenAI’s Assistants API, mark your calendar: OpenAI has notified 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 . More significantly, the Assistants API itself is on borrowed time. The Responses API combines features of OpenAI’s Chat Completions API with the tool-use functionality of the Assistants API, and 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 .

The good news: OpenAI is making its web search, file search and computer use tools available directly through the Responses API, enabling AI agents to access real-world information, retrieve context from documents and interact with digital environments more effectively .

For Azure Developers: You’re Mostly Safe (For Now)

Here’s where it gets interesting for the .NET + Azure crowd. Azure OpenAI Service is not impacted by this deprecation, as it does not use the Assistants API from OpenAI’s platform but offers its own endpoints for chat completions, embeddings, and other model capabilities; Microsoft has not announced any changes related to this deprecation, and users of Azure OpenAI can continue building assistants and agents as usual .

Translation: If you’re using Azure.AI.OpenAI in C#/.NET and deploying through Azure OpenAI Service endpoints, you don’t need to panic. Your existing code will keep working.

But—and this is a big but— Azure OpenAI previously required the extra step of using Azure specific clients which created overhead when migrating code between OpenAI and Azure OpenAI; the v1 API removes this dependency by adding automatic token refresh support to the OpenAI() client .

The Migration Path (If You’re on OpenAI Direct)

If you’re calling api.openai.com directly instead of Azure, here’s what you need to know: 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 .

The Responses API is now available in the OpenAI SDK. A basic example in Python:

from openai import OpenAI

client = OpenAI(api_key="sk-...")
response = client.responses.create(
    model="gpt-5.2",
    input="Fetch the latest weather and summarize it.",
    tools=[
        {
            "type": "function",
            "function": {
                "name": "get_weather",
                "description": "Get weather for a location",
                "parameters": {
                    "type": "object",
                    "properties": {
                        "location": {"type": "string"}
                    }
                }
            }
        }
    ]
)

For .NET, the OpenAI NuGet package supports this as well—check your package version and update if needed.

The Cost Wild Card: GPT-5.2 Pricing

Here’s where budgets get interesting. GPT-5.2 Thinking is priced 40% higher in the API than the standard GPT-5.1, signaling that OpenAI views the new reasoning capabilities as a tangible value-add; the high-end GPT-5.2 Pro follows the same pattern, costing 40% more than the previous GPT-5 Pro .

But there’s a silver lining: OpenAI argues that despite the higher per-token cost, the model’s “greater token efficiency” and ability to solve tasks in fewer turns make it economically viable for high-value enterprise workflows .

For your spreadsheet: If you’re building agents that run complex reasoning tasks (code generation, document analysis, multi-step planning), GPT-5.2 might actually reduce your per-task cost, even at higher per-token rates. Test it in a sandbox first.

Action Items for .NET Teams

If you’re on Azure OpenAI: No immediate action required. Monitor Microsoft Learn for any future announcements, but your code is safe.
If you’re calling OpenAI’s platform directly: Start a migration spike. Review your Assistants API usage, test the Responses API in a dev environment, and plan a rollout before August 2026.
If you’re evaluating new agent frameworks: Consider Microsoft Extensions for AI (MEAI) as unified abstractions for interacting with models, and Microsoft Extensions for Vector Data as standard interfaces for vector databases used in RAG systems .
Cost optimization: Benchmark GPT-5.2 against GPT-5.1 for your specific workloads. The 40% premium might pay for itself in token efficiency.

Azure OpenAI's Responses API: Time to Migrate Your Agents Before August 2026