Microsoft released .NET 11 Release Candidate 1 on September 8, 2026, the first release candidate on the road to general availability on November 10, 2026. RC1 comes with a go-live support license, makes C# 15 the default language version with union types and closed hierarchies, and ships an experimental Microsoft.AspNetCore.Components.AI package for building agentic user interfaces in Blazor. For .NET developers building AI features, the notable change is framework-level UI components designed for streaming chat, tool calls and human approvals.

Key Facts#

  • Release: .NET 11.0.0-rc.1 shipped on September 8, 2026, the same day as the .NET 10.0.12 servicing update.
  • Go-live: Microsoft says RC1 carries a go-live support license, so it can be used in production.
  • Schedule: general availability is planned for November 10, 2026. .NET 11 is a Standard Term Support release, supported until November 9, 2028.
  • Road to RC: seven monthly previews preceded RC1, from Preview 1 on February 10, 2026, to Preview 7 on August 11, 2026.
  • C# 15: now the default for .NET 11 projects, with union types, collection expression arguments, closed hierarchies, extension indexers and labeled break and continue no longer requiring preview mode.
  • AI components: Microsoft.AspNetCore.Components.AI is experimental and stays prerelease throughout .NET 11.
  • Tooling: Microsoft says RC1 is supported in the latest Visual Studio 2026 Insiders release and in Visual Studio Code with the C# Dev Kit.

What Happened#

As with most release candidates, RC1 mixes final polish with a handful of new capabilities. In the libraries, Process.Signal() and a new ProcessExitStatus type make it possible to send POSIX signals and tell different kinds of process termination apart. An experimental set of caller-driven TLS session types, including TlsContext and TlsSession, gives advanced networking code control over TLS state, and DNS resolution on Linux now covers SRV, MX, TXT, CNAME, PTR and NS records. System.Text.Json gained built-in converters for BFloat16 and the new IEEE decimal types, plus support for closed-type polymorphism and union classification. Options validation can now run asynchronously through IAsyncValidateOptions<TOptions>.

The runtime notes highlight in-process crash reporting on Linux and macOS and the JIT's use of hardware FP16 instructions for System.Half arithmetic and conversions on supported processors. The SDK improves dotnet test for mobile targets, adds run-level controls such as timeouts and failure limits, produces reproducible container images, and lets file-based programs reuse Native AOT builds for faster launches. In ASP.NET Core, the SignalR authentication refresh APIs are finalized, OpenAPI documents now mark obsolete endpoints as deprecated, and an experimental package adds server-side Device Bound Session Credentials.

The most interesting addition for AI developers is the experimental Blazor AI components package. According to the release notes, the components work with any IChatClient from Microsoft.Extensions.AI, and an AGUI.Client package connects them to remote agents over the Agent User Interaction (AG-UI) protocol, which Microsoft Agent Framework can expose from ASP.NET Core endpoints. A ChatPage component provides a complete chat surface, while a UIAgent type turns streaming responses into observable content blocks for rich text, function invocations, UI actions, tool approvals and application-defined activities. Developers can customize rendering per block type, register frontend tools, render typed tool results, share typed state between the agent and the UI, and persist conversations through an IConversationThread abstraction.

Background#

.NET 11 is an odd-numbered release, so it follows .NET 10, the current Long Term Support version released in November 2025 (see our story on .NET 10 and C# 14). The overview documentation for .NET 11 lists broader themes from the preview cycle, including runtime-native async for cleaner stack traces and lower overhead, new Arm SVE2 intrinsics, Zstandard compression, IEEE 754 decimal floating-point types, new LINQ join operators and OpenTelemetry metrics for MemoryCache.

The Blazor AI components arrive after a year in which Microsoft stabilized the lower layers of its AI stack. Microsoft.Extensions.AI has been generally available since May 2025, and Microsoft Agent Framework reached 1.0 in April 2026. What was missing was a standard way to render agent behavior in a web UI: streamed tokens, tool calls in progress, approval prompts and shared state. Without shared components, teams had to build those pieces themselves.

Why It Matters for Developers#

The AI components address a real gap. Chat UIs look simple, but production agent interfaces need to show partial output, render tool calls, pause for human approval and keep client and server state in sync. A framework-level set of building blocks that speaks IChatClient and AG-UI means Blazor teams can focus on domain-specific rendering instead of plumbing. Because the package is explicitly experimental and will remain prerelease throughout .NET 11, use it for prototypes and internal tools first, and isolate it behind your own components if you ship it. Our Blazor guide covers the render modes and component architecture these building blocks plug into.

C# 15 union types are useful well beyond UI code. Agent code constantly deals with values that can be one of several shapes, such as a tool result, an error or a request for approval, and unions express that directly:

C#
public record class ToolSucceeded(string ToolName, string Output);
public record class ToolFailed(string ToolName, string Error);
public record class ApprovalRequired(string ToolName, string Reason);

public union ToolOutcome(ToolSucceeded, ToolFailed, ApprovalRequired);

public static class ToolOutcomeFormatter
{
    public static string Describe(ToolOutcome outcome) => outcome switch
    {
        ToolSucceeded s => $"{s.ToolName} returned {s.Output}",
        ToolFailed f => $"{f.ToolName} failed: {f.Error}",
        ApprovalRequired a => $"{a.ToolName} needs approval: {a.Reason}",
    };
}

Combined with the new JSON union classification support, this pattern can make it easier to model and validate model-generated output, a topic covered in our guide to structured outputs in C#. The numeric work matters too: hardware FP16 support for Half and JSON converters for BFloat16 help code that stores or exchanges reduced-precision vectors, such as embeddings.

Finally, the go-live license changes the testing calculus. Teams that plan to adopt .NET 11 can validate RC1 in staging now and report issues while they can still be fixed before GA. Teams that prefer long support windows can stay on .NET 10 until November 2028, a trade-off explained in our .NET release cadence guide.

What's Next#

General availability is scheduled for November 10, 2026. The Blazor AI components will stay prerelease for the entire .NET 11 lifecycle, and Microsoft is asking for feedback, so expect the API names and block types to change as the design settles. For developers wiring these components to agents, our Microsoft Agent Framework guide and Microsoft.Extensions.AI guide cover the server side.

Sources#