AG-UI gets a .NET SDK, and Microsoft's framework now sits on it
Serguey Asael Shinder2 min readDaniel Roth announced on 25 September that the .NET team has contributed an AG-UI SDK, built with CopilotKit and published on NuGet under the MIT licence. Two details in that sentence carry most of the weight: the SDK lives in the AG-UI repository alongside the existing TypeScript and Python SDKs rather than in a Microsoft repo, and the Microsoft Agent Framework's own AG-UI support for .NET is now based on this SDK instead of being a parallel implementation.
It ships as five packages - protocol types, wire formatters, protobuf support, an HTTP client, and a framework-agnostic server adapter built on Microsoft.Extensions.AI. AGUI.Server turns an agent into an AG-UI endpoint; AGUI.Client lets a .NET application consume one. The seam in both directions is IChatClient: your agent is an IChatClient, and AGUIChatClient is also an IChatClient, so it drops into any code that already accepts one - with the agent on the far end written in Python or any other supported language. Agent Framework wraps the same primitives as AddAGUIServer() and MapAGUIServer() in Microsoft.Agents.AI.Hosting.AGUI.AspNetCore, currently prerelease.
The SDK also takes responsibility for the fiddly part of the protocol rather than leaving it to callers: it closes open text and reasoning blocks before switching to a different message or tool call, and collapses multiple interrupts into a single terminal RUN_FINISHED. The web server stays yours.
What it means
The line worth noticing is the one about Agent Framework being rebased onto the community SDK. It is the difference between "Microsoft supports the protocol" and "Microsoft depends on the protocol's own implementation", and only the second creates pressure to keep the shared artefact correct - a bug in AGUI.Abstractions is now a bug in Microsoft's product. Vendors more often go the other way, shipping a private implementation that tracks a spec loosely; having the first-party framework consume the same packages third parties do is the stronger signal about the protocol, more so than the SDK's existence.
The IChatClient choice is what makes this cheap to try. Because both ends present as the same abstraction from Microsoft.Extensions.AI, adopting AG-UI is not a rewrite of your agent but a change of what you register - and the reverse is equally true, which is the part to keep in mind. An abstraction that makes a protocol easy to adopt makes it equally easy to swap out, so the lock-in here is in the event semantics (interrupts, reasoning blocks, RUN_FINISHED) rather than in the packages. That is where to look if you are deciding whether to build on this now or wait for the prerelease hosting package to settle.