GPT-Realtime-1.5 and GPT-5.3-Codex Land in Azure AI Foundry: What .NET Engineers Need to Know Right Now

Published: February 26, 2026

Ecosystem: Azure · Azure OpenAI · .NET

TL;DR

Starting this week, GPT-Realtime-1.5, GPT-Audio-1.5, and GPT-5.3-Codex are rolling out into Microsoft Foundry. If you’re building voice assistants, live-transcription pipelines, or agentic coding tools on Azure, these three models directly affect your architecture choices — and at least one of them gives you an excuse to finally touch the Realtime API. Read on before you hardcode another model string.

The Drop: Three Models, One Coherent Story

Microsoft didn’t just quietly push a version bump this week. OpenAI’s latest models address the shared challenge of continuity and reliability, pushing the needle from short, stateless interactions toward AI systems that can reason, act, and collaborate over time. That’s a polite way of saying: your old “fire a prompt, get a string back” pattern is increasingly table stakes.

GPT-Realtime-1.5 & GPT-Audio-1.5: Measurably Better, Not Just “Improved™”

The gpt-realtime-1.5-2026-02-23 and gpt-audio-1.5-2026-02-23 models are now available on Azure AI Foundry. These aren’t marketing-only upgrades. The models deliver measurable gains in reasoning and speech understanding for real-time voice interactions on Microsoft Foundry. In OpenAI’s evaluations, they show a +5% lift on Big Bench Audio (reasoning), a +10.23% improvement in alphanumeric transcription, and a +7% gain in instruction following, while maintaining low-latency performance. That alphanumeric transcription bump is significant for anyone building voice-driven DevOps bots or phone-based order systems — “ticket-ID XR-4892” is no longer a coin flip.

Key quality improvements include:

More natural-sounding speech: Audio output is smoother and more conversational, with improved pacing and prosody.
Higher audio quality: Clearer, more consistent audio output across supported voices.
Improved instruction following: Better alignment with developer-provided system and user instructions during live interactions. The Realtime API itself supports WebRTC, SIP, and WebSocket for sending audio input to the model and receiving audio responses in real time. That’s three integration paths for three different architectural needs — browser-native, telephony, and server-side streaming.

GPT-5.3-Codex: The “One Model to Rule the Repo” Release

GPT-5.3-Codex brings together advanced coding capability with broader reasoning and professional problem-solving in a single model built for real engineering work. It unifies the frontier coding performance of GPT-5.2-Codex with the reasoning and professional knowledge capabilities of GPT-5.2 in one system — shifting the experience from optimizing isolated outputs to supporting longer-running development efforts, where repositories are large, changes span multiple steps, and requirements aren’t always fully specified at the start. In other words: it’s been trained for the messy reality of enterprise codebases, not competitive benchmark puzzles. Developers and teams can apply GPT-5.3-Codex across a wide range of scenarios, including refactoring and modernizing large or legacy applications. Yes, that legacy ASP.NET Web Forms project counts.

What This Means for .NET Developers on Azure

1. Upgrade Your Realtime Model Deployment

If you deployed gpt-realtime or gpt-realtime-mini-2025-12-15 before this week, you’re running an older model. Microsoft highly recommends that all customers transition to the newly launched GA models to take full advantage of the latest features — visit the Azure OpenAI documentation and Azure AI Foundry Playground to explore capabilities. Deploying the new model via the Azure CLI is straightforward:

# List available realtime models
az cognitiveservices account deployment list \
  --name <your-resource> \
  --resource-group <your-rg> \
  --query "[?contains(name,'realtime')]"

# Deploy gpt-realtime-1.5
az cognitiveservices account deployment create \
  --name <your-resource> \
  --resource-group <your-rg> \
  --deployment-name gpt-realtime-1-5 \
  --model-name gpt-realtime-1.5-2026-02-23 \
  --model-version 2026-02-23 \
  --model-format OpenAI \
  --sku-capacity 1 \
  --sku-name Standard

2. Calling the Realtime API from .NET — The v1 API Path

The v1 Azure OpenAI API simplifies authentication, removes the need for dated api-version parameters, and supports cross-provider model calls. That’s a meaningful quality-of-life improvement — no more api-version=2024-02-01 littered through your appsettings.json.

Here’s a minimal C# example using the v1 endpoint and DefaultAzureCredential (the recommended approach):

using Azure.Identity;
using OpenAI;
using OpenAI.Chat;
using System.ClientModel.Primitives;

// Use DefaultAzureCredential — works locally (az login) and in managed identity on Azure
BearerTokenPolicy tokenPolicy = new(
    new DefaultAzureCredential(),
    "https://cognitiveservices.azure.com/.default");

ChatClient client = new(
    model: "gpt-realtime-1-5",   // your deployment name
    authenticationPolicy: tokenPolicy,
    options: new OpenAIClientOptions
    {
        Endpoint = new Uri("https://<your-resource>.openai.azure.com/openai/v1")
    });

⚠️ Note: api-version is no longer required on the v1 path, but make sure your Azure.AI.OpenAI NuGet package is up to date. The api-version is no longer a required parameter with the v1 GA API.

3. The Responses API: Your New Stateful Best Friend

While you’re in there, take a look at the Responses API — it landed alongside these models and deserves attention. The Responses API is a new stateful API from Azure OpenAI. It brings together the best capabilities of the Chat Completions and Assistants APIs in one unified experience, and also adds support for the new computer-use-preview model. The magic for multi-turn conversations is previous_response_id — no more manually stitching chat history arrays:

// First turn
var first = await client.Responses.CreateAsync(new ResponseCreateParams
{
    Model = "gpt-5",
    Input = "Explain the Outbox pattern in distributed systems."
});

// Second turn — passes full context automatically
var second = await client.Responses.CreateAsync(new ResponseCreateParams
{
    Model = "gpt-5",
    PreviousResponseId = first.Id,
    Input = new[] { new UserMessage("Now show me a C# implementation.") }
});

4. Don’t Forget the Assistants API Sunset

While you’re modernizing: the Assistants API is deprecated and will be retired on August 26, 2026. Use the generally available Microsoft Foundry Agents service and follow the migration guide to update your workloads. If your team built on the Assistants API last year, that clock is ticking.

Quick-Reference: What’s Changed and What to Do

Area	Change	Action
Realtime model	`gpt-realtime-1.5-2026-02-23` available	Redeploy in Foundry portal
Audio quality	+10.23% alphanumeric transcription	Test existing voice flows
Coding agent	GPT-5.3-Codex in Foundry	Evaluate for agentic PR review pipelines
API surface	Responses API GA, stateful by `previous_response_id`	Replace manual history arrays
Legacy deadline	Assistants API retires Aug 26, 2026	Plan migration to Foundry Agents
SDK hygiene	v1 API path, no `api-version` needed	Update `Azure.AI.OpenAI` NuGet

One Watch-Out on Latency

The Realtime API is designed for low-latency interactions, but it is not a magic wand. Most users of the Realtime API need to deliver and receive audio from an end-user in real time. The API is not designed to connect directly to end-user devices — it relies on client integrations to terminate end-user audio streams. Plan your server-side relay carefully if you’re building browser or mobile-first voice experiences: an extra network hop to your ASP.NET Core backend before hitting the WebSocket endpoint will cost you milliseconds that users will notice.