.NET: Add MCP Apps support to AG-UI (.NET)

[halllo]

Motivation and Context

MCP Apps is a new MCP extension that allows attaching interactive HTML UI components to MCP tools. When a tool is invoked, the AG-UI host passes a ui:// resource URI alongside the tool result so a capable client can render the HTML app in-place. This PR extends the framework's AG-UI implementation to support MCP Apps, and adds a full end-to-end sample and conformance tests for the AsAGUIEventStreamAsync protocol to ensure correctness of the SSE event stream.

Description

Framework extensions (Microsoft.Extensions.AI.Hosting.AGUI.AspNetCore):

• Added ACTIVITY_SNAPSHOT event type to AGUIEventTypes, AGUIJsonSerializerContext, and a new ActivitySnapshotEvent model.
• Updated ChatResponseUpdateAGUIExtensions and AGUIEndpointRouteBuilderExtensions to emit ACTIVITY_SNAPSHOT events when a proxied tool result includes a UI resource URI.
• Added EmitToolMetadataMiddleware to attach tool metadata to the event stream.

New sample: dotnet/samples/02-agents/AGUI/Step06_McpApps/

A three-project sample demonstrating MCP Apps over AG-UI end-to-end:

• McpServer/ — ASP.NET Core MCP server exposing a get-time tool with a bundled HTML app (TypeScript/Vite, compiled to a single HTML file). The tool is annotated with [McpMeta("ui", ...)] to attach the ui:// resource URI.
• Server/ — AG-UI server that bridges the MCP server to AG-UI clients using MapAGUI with a ProxiedResultResolver overload to proxy resources/read and tools/call requests.
• Client/ — Console client using AGUIChatClient and RunStreamingAsync to stream responses and display ACTIVITY_SNAPSHOT events (which carry the ui:// URI and tool result for browser-based clients to render).

Tests:

• ChatResponseUpdateAGUIExtensionsTests.cs and AGUIEndpointRouteBuilderExtensionsTests.cs — unit tests for the new/updated extension methods.
• ConformanceTests.cs — end-to-end tests for the sample, exercising the AsAGUIEventStreamAsync event stream.

Contribution Checklist

• The code builds clean without any errors or warnings
• The PR follows the Contribution Guidelines
• All unit tests pass, and I have added new tests where possible
• Is this a breaking change? No — additive framework extensions and new sample only.

[Copilot]

Pull request overview

Adds MCP Apps support to the .NET AG-UI hosting + shared protocol layers by emitting ACTIVITY_SNAPSHOT events for tool results that include a ui:// resource URI, along with a new end-to-end sample and accompanying unit/conformance tests.

Changes:

• Introduces ACTIVITY_SNAPSHOT to the AG-UI event model/serializer and emits it after tool results when MCP tool metadata includes a UI resource URI.
• Extends MapAGUI with an optional ProxiedResultResolver to short-circuit proxied MCP resources/read and tools/call requests.
• Adds the Step06_McpApps sample (MCP server + AG-UI server + client) and protocol/conformance tests.

Reviewed changes

Copilot reviewed 32 out of 33 changed files in this pull request and generated 11 comments.

Show a summary per file

File	Description
dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests.csproj	Adds TestHost dependency for new endpoint-level unit tests.
dotnet/tests/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore.UnitTests/AGUIEndpointRouteBuilderExtensionsTests.cs	Adds unit tests covering proxied-result short-circuiting behavior.
dotnet/tests/Microsoft.Agents.AI.AGUI.UnitTests/ChatResponseUpdateAGUIExtensionsTests.cs	Adds AsAGUIEventStreamAsync protocol conformance tests including ACTIVITY_SNAPSHOT.
dotnet/src/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore/EmitToolMetadataMiddleware.cs	New middleware to attach tool metadata onto tool results for downstream AG-UI emission.
dotnet/src/Microsoft.Agents.AI.Hosting.AGUI.AspNetCore/AGUIEndpointRouteBuilderExtensions.cs	Adds MapAGUI overload supporting proxied-result resolution and minimal SSE response path.
dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ChatResponseUpdateAGUIExtensions.cs	Emits ACTIVITY_SNAPSHOT after tool results when tool metadata includes MCP Apps UI URI; adds tool-result normalization helper.
dotnet/src/Microsoft.Agents.AI.AGUI/Shared/BaseEventJsonConverter.cs	Adds JSON (de)serialization support for ACTIVITY_SNAPSHOT.
dotnet/src/Microsoft.Agents.AI.AGUI/Shared/AGUIJsonSerializerContext.cs	Registers ActivitySnapshotEvent for source-generated JSON.
dotnet/src/Microsoft.Agents.AI.AGUI/Shared/AGUIEventTypes.cs	Adds ACTIVITY_SNAPSHOT discriminator constant.
dotnet/src/Microsoft.Agents.AI.AGUI/Shared/ActivitySnapshotEvent.cs	New event model for ACTIVITY_SNAPSHOT.
dotnet/samples/02-agents/AGUI/Step06_McpApps/Tests/Tests.csproj	New sample test project for MCP Apps AG-UI conformance tests.
dotnet/samples/02-agents/AGUI/Step06_McpApps/Tests/ConformanceTests.cs	Adds (skipped by default) integration/conformance tests for SSE + proxy flows.
dotnet/samples/02-agents/AGUI/Step06_McpApps/Server/Server.csproj	New AG-UI server project for the MCP Apps sample.
dotnet/samples/02-agents/AGUI/Step06_McpApps/Server/Properties/launchSettings.json	Launch settings for sample AG-UI server.
dotnet/samples/02-agents/AGUI/Step06_McpApps/Server/Program.cs	Implements AG-UI↔MCP bridge, emits tool metadata, and proxies MCP requests via ProxiedResultResolver.
dotnet/samples/02-agents/AGUI/Step06_McpApps/Server/appsettings.json	Sample server configuration.
dotnet/samples/02-agents/AGUI/Step06_McpApps/Server/appsettings.Development.json	Sample server dev configuration.
dotnet/samples/02-agents/AGUI/Step06_McpApps/README.md	Full guide documenting MCP Apps over AG-UI, proxy flows, and event stream shapes.
dotnet/samples/02-agents/AGUI/Step06_McpApps/McpServer/vite.config.ts	Vite config for bundling the MCP App to a single HTML file.
dotnet/samples/02-agents/AGUI/Step06_McpApps/McpServer/Properties/launchSettings.json	Launch settings for sample MCP server.
dotnet/samples/02-agents/AGUI/Step06_McpApps/McpServer/Program.cs	Sample MCP server exposing get-time tool and ui:// HTML resource.
dotnet/samples/02-agents/AGUI/Step06_McpApps/McpServer/package.json	Frontend MCP App build dependencies and script.
dotnet/samples/02-agents/AGUI/Step06_McpApps/McpServer/package-lock.json	Locked frontend dependency graph for the MCP App build.
dotnet/samples/02-agents/AGUI/Step06_McpApps/McpServer/McpServer.csproj	New MCP server project file.
dotnet/samples/02-agents/AGUI/Step06_McpApps/McpServer/get-time.ts	MCP App frontend logic using @modelcontextprotocol/ext-apps.
dotnet/samples/02-agents/AGUI/Step06_McpApps/McpServer/get-time.html	MCP App HTML entrypoint for the Vite build.
dotnet/samples/02-agents/AGUI/Step06_McpApps/McpServer/appsettings.json	Sample MCP server configuration.
dotnet/samples/02-agents/AGUI/Step06_McpApps/McpServer/appsettings.Development.json	Sample MCP server dev configuration.
dotnet/samples/02-agents/AGUI/Step06_McpApps/Client/Program.cs	Sample console client that streams AG-UI responses and prints ACTIVITY_SNAPSHOT + proxied reads.
dotnet/samples/02-agents/AGUI/Step06_McpApps/Client/Client.csproj	New sample client project file.
dotnet/samples/02-agents/AGUI/README.md	Links and run instructions for the new Step06 MCP Apps sample.
dotnet/Directory.Packages.props	Adds ModelContextProtocol.AspNetCore central package version.
dotnet/agent-framework-dotnet.slnx	Includes new Step06 sample projects in the solution.

Files not reviewed (1)

• dotnet/samples/02-agents/AGUI/Step06_McpApps/McpServer/package-lock.json: Language not supported