Add MCP Apps UI support to Repl.Mcp

README.md

@@ -9,7 +9,7 @@
 **A .NET framework for building composable command surfaces.**
 
 - Define your commands once — run them as a CLI, explore them in an interactive REPL,
-- host them in session-based terminals, expose them as MCP servers for AI agents,
+- host them in session-based terminals, expose them as MCP servers and MCP Apps for AI agents,
 - or drive them from automation scripts.
 
 > **New here?** The [DeepWiki](https://deepwiki.com/yllibed/repl) has full architecture docs, diagrams, and an AI assistant you can ask questions about the toolkit.
@@ -83,6 +83,14 @@ app.UseMcpServer();  // add one line
 { "command": "myapp", "args": ["mcp", "serve"] }
 ```
 
+**MCP Apps** (same server, host-rendered UI for capable clients):
+
+```csharp
+app.Map("contacts dashboard", (IContactStore contacts) => BuildHtml(contacts))
+    .WithDescription("Open the contacts dashboard")
+    .AsMcpAppResource();
+```
+
 One command graph. CLI, REPL, remote sessions, and AI agents — all from the same code.
 
 ## What's included
@@ -93,7 +101,7 @@ One command graph. CLI, REPL, remote sessions, and AI agents — all from the sa
 | Interactive REPL — scopes, history, autocomplete | [![Repl.Defaults](https://img.shields.io/nuget/vpre/Repl.Defaults?logo=nuget&label=Repl.Defaults)](https://www.nuget.org/packages/Repl.Defaults) | <ul><li>[Interactive loop](docs/interactive-loop.md)</li><li>[Configuration](docs/configuration-reference.md)</li></ul> |
 | Parameters & options — typed binding, options groups, response files | [![Repl.Core](https://img.shields.io/nuget/vpre/Repl.Core?logo=nuget&label=Repl.Core)](https://www.nuget.org/packages/Repl.Core) | <ul><li>[Parameter system](docs/parameter-system.md)</li><li>[Route system](docs/route-system.md)</li></ul> |
 | Multiple output formats — JSON, XML, YAML, Markdown | [![Repl.Core](https://img.shields.io/nuget/vpre/Repl.Core?logo=nuget&label=Repl.Core)](https://www.nuget.org/packages/Repl.Core) | <ul><li>[Output system](docs/output-system.md)</li></ul> |
-| MCP server — expose commands as AI agent tools | [![Repl.Mcp](https://img.shields.io/nuget/vpre/Repl.Mcp?logo=nuget&label=Repl.Mcp)](https://www.nuget.org/packages/Repl.Mcp) | <ul><li>[MCP server](docs/mcp-server.md)</li><li>[MCP advanced](docs/mcp-advanced.md)</li></ul> |
+| MCP server + MCP Apps — expose commands as agent tools, resources, prompts, and UI | [![Repl.Mcp](https://img.shields.io/nuget/vpre/Repl.Mcp?logo=nuget&label=Repl.Mcp)](https://www.nuget.org/packages/Repl.Mcp) | <ul><li>[MCP server](docs/mcp-server.md)</li><li>[MCP advanced](docs/mcp-advanced.md)</li><li>[MCP sample](samples/08-mcp-server/)</li></ul> |
 | Typed results & interactions — prompts, progress, cancellation | [![Repl.Core](https://img.shields.io/nuget/vpre/Repl.Core?logo=nuget&label=Repl.Core)](https://www.nuget.org/packages/Repl.Core) | <ul><li>[Interaction channel](docs/interaction.md)</li></ul> |
 | Session hosting — WebSocket, Telnet, remote terminals | [![Repl.WebSocket](https://img.shields.io/nuget/vpre/Repl.WebSocket?logo=nuget&label=Repl.WebSocket)](https://www.nuget.org/packages/Repl.WebSocket) [![Repl.Telnet](https://img.shields.io/nuget/vpre/Repl.Telnet?logo=nuget&label=Repl.Telnet)](https://www.nuget.org/packages/Repl.Telnet) | <ul><li>[Runtime channels](docs/runtime-channels.md)</li><li>[Terminal metadata](docs/terminal-metadata.md)</li></ul> |
 | Shell completion — Bash, PowerShell, Zsh, Fish, Nushell | [![Repl.Core](https://img.shields.io/nuget/vpre/Repl.Core?logo=nuget&label=Repl.Core)](https://www.nuget.org/packages/Repl.Core) | <ul><li>[Shell completion](docs/shell-completion.md)</li></ul> |
@@ -115,7 +123,7 @@ Progressive learning path — each sample builds on the previous:
 5. **[Hosting Remote](samples/05-hosting-remote/)** — WebSocket / Telnet session hosting
 6. **[Testing](samples/06-testing/)** — multi-session typed assertions
 7. **[Spectre](samples/07-spectre/)** — Spectre.Console renderables, visualizations, rich prompts
-8. **[MCP Server](samples/08-mcp-server/)** — expose commands as MCP tools for AI agents
+8. **[MCP Server](samples/08-mcp-server/)** — expose commands as MCP tools, resources, prompts, and a minimal MCP Apps UI
 
 ## More documentation
 

docs/architecture.md

@@ -20,7 +20,7 @@
 - `Repl.Spectre`
  	- Spectre.Console integration: `SpectreInteractionHandler` for rich prompts, `IAnsiConsole` DI injection, `"spectre"` output transformer for auto-rendered tables, `SpectreConsoleOptions` for capability configuration.
 - `Repl.Mcp`
- 	- MCP (Model Context Protocol) integration: `UseMcpServer()`, `BuildMcpServerOptions()`, tool/resource/prompt mapping, client roots, transport factory.
+  - MCP (Model Context Protocol) integration: `UseMcpServer()`, `BuildMcpServerOptions()`, tool/resource/prompt mapping, MCP Apps UI resources, client roots, transport factory.
 - `Repl.Testing`
  	- In-memory multi-session testing toolkit (`ReplTestHost`, `ReplSessionHandle`, typed execution results/events).
 - `Repl.Tests`
@@ -30,7 +30,7 @@
 - `Repl.ProtocolTests`
  	- Contract tests for machine-readable help/error payloads.
 - `Repl.McpTests`
- 	- Tests for MCP server options, tool mapping, and transport integration.
+  - Tests for MCP server options, tool/resource/prompt/app mapping, and transport integration.
 - `Repl.SpectreTests`
  	- Tests for Spectre.Console integration.
 - `Repl.ShellCompletionTestHost`

docs/best-practices.md

@@ -229,6 +229,16 @@ app.Map("clear", static async (IReplInteractionChannel ch, CancellationToken ct)
     .AutomationHidden();  // not exposed to agents
 ```
 
+For MCP Apps, mark the HTML-producing command as an app resource:
+
+```csharp
+app.Map("contacts dashboard", static (IContactStore contacts) => BuildHtml(contacts))
+    .WithDescription("Open the contacts dashboard")
+    .AsMcpAppResource();
+```
+
+This lets capable hosts render the UI while keeping raw HTML out of the model-facing transcript. The handler is still a normal Repl mapping, so it can use DI, cancellation tokens, and the usual command pipeline.
+
 Declare answer slots for interactive prompts so agents and `--answer:` flags can provide values:
 
 ```csharp

docs/comparison.md

@@ -88,6 +88,8 @@ Repl Toolkit is a command-surface framework — not just a CLI parser. It builds
 | Structured help output | ❌ Text only | ❌ Text only | ✅ JSON / XML / YAML |
 | Documentation export | ❌ | ❌ | ✅ `doc export` command |
 | Protocol passthrough (MCP, LSP...) | ❌ | ❌ | ✅ `AsProtocolPassthrough()` |
+| MCP server tools/resources/prompts | ❌ | ❌ | ✅ `Repl.Mcp` |
+| MCP Apps UI resources | ❌ | ❌ | ✅ `AsMcpAppResource()` |
 | Shell completion | ⚠️ Tab completion API | ❌ | ✅ Bash, PS, Zsh, Fish, Nu |
 
 ## When to Use What
@@ -113,7 +115,7 @@ Repl Toolkit is a command-surface framework — not just a CLI parser. It builds
 - Commands involve multi-step guided workflows (prompts, progress, confirmations)
 - Remote terminal hosting is planned (WebSocket, Telnet)
 - The command model must be testable in both one-shot and interactive contexts
-- AI/LLM agent readiness matters (structured help, protocol passthrough, pre-answered prompts)
+- AI/LLM agent readiness matters (structured help, MCP tools/resources/prompts, MCP Apps UI, protocol passthrough, pre-answered prompts)
 
 ## Migration from System.CommandLine
 

docs/glossary.md

@@ -66,6 +66,10 @@ Static text in a route template matched exactly.
 
 Model Context Protocol. Allows AI agents to discover and invoke commands.
 
+### MCP App
+
+MCP UI extension that lets a command open a `ui://` HTML resource. Repl maps this with `.AsMcpAppResource()` on the HTML-producing command and returns launcher text for normal tool calls.
+
 ### Middleware
 
 Pipeline function registered via `app.Use()` that wraps handler execution.

docs/help-system.md

@@ -56,7 +56,7 @@ Each `ReplDocCommand` includes:
 This model powers:
 
 - `--help` text rendering
-- MCP tool/resource/prompt schema generation
+- MCP tool/resource/prompt schema generation and MCP Apps metadata
 - Shell completion candidate generation
 
 See also: [Commands](commands.md) | [MCP Server](mcp-server.md) | [Parameter System](parameter-system.md)

docs/mcp-advanced.md

@@ -1,11 +1,12 @@
-# MCP Advanced: Dynamic Tools, Roots, and Session-Aware Patterns
+# MCP Advanced: Dynamic Tools, Roots, MCP Apps, and Session-Aware Patterns
 
 This guide covers advanced MCP usage patterns for Repl apps:
 
 - Tool visibility that changes per session
 - Native MCP client roots
 - Soft roots for clients that don't support roots
 - Compatibility shims for clients that don't refresh dynamic tool lists well
+- Advanced MCP Apps patterns
 
 > **Prerequisite**: read [mcp-server.md](mcp-server.md) first for the basic setup.
 >
@@ -23,6 +24,7 @@ Use the techniques in this page when:
 - The agent needs to know which directories it is allowed to work in
 - Your MCP client does not support native roots
 - Your MCP client does not seem to refresh its tool list after `list_changed`
+- Your MCP App should render HTML without exposing that HTML as the model-facing tool result
 
 If your tool list is static, stay with the default setup from [mcp-server.md](mcp-server.md).
 
@@ -248,8 +250,142 @@ Avoid it when:
 | Client supports tools but misses dynamic refreshes | Enable `DiscoverAndCallShim` |
 | Client has both issues | Use soft roots and, if needed, the dynamic tool shim |
 
+## MCP Apps advanced patterns
+
+For the basic MCP Apps setup, start with [mcp-server.md](mcp-server.md#mcp-apps). This section covers the patterns that matter once the UI is more than a trivial inline HTML card.
+
+MCP Apps support is experimental in this version. Resource handlers should return generated HTML as `string`, `Task<string>`, or `ValueTask<string>`; the API is expected to become more flexible as host support and Repl's asset story evolve.
+
+### One mapping, two MCP surfaces
+
+`AsMcpAppResource()` keeps the Repl authoring model simple: one mapping produces both the launcher tool metadata and the UI resource.
+
+```csharp
+app.Map("contacts dashboard", (IContactDb contacts) => BuildHtml(contacts))
+    .WithDescription("Open the contacts dashboard")
+    .AsMcpAppResource();
+```
+
+The handler return value is used for `resources/read` and is returned as `text/html;profile=mcp-app`. When a client calls the MCP tool, Repl returns launcher text instead of raw HTML, using `WithMcpAppLauncherText(...)`, `WithDescription(...)`, or a generated fallback.
+
+Use `WithMcpAppLauncherText(...)` when the description is not the text you want in the chat transcript:
+
+```csharp
+app.Map("contacts dashboard", (IContactDb contacts) => BuildHtml(contacts))
+    .WithDescription("Open the contacts dashboard")
+    .AsMcpAppResource()
+    .WithMcpAppLauncherText("Opening the contacts dashboard.");
+```
+
+`WithMcpApp("ui://...")` remains available for advanced cases where a normal tool should point at a separately registered UI resource, but it is not the default pattern.
+
+```csharp
+app.Map("status dashboard", (IStatusStore store) => store.GetSummary())
+    .ReadOnly()
+    .WithMcpApp("ui://status/dashboard");
+```
+
+### Generated UI resource URIs
+
+`AsMcpAppResource()` generates a `ui://` URI from the route path, matching how `AsResource()` generates `repl://` URIs:
+
+```csharp
+app.Map("contact {id:int} panel", (int id, IContactDb contacts) => BuildHtml(contacts.Get(id)))
+    .AsMcpAppResource();
+```
+
+This produces a resource template like `ui://contact/{id}/panel`.
+
+The generated URI uses the full route path, including contexts:
+
+```csharp
+app.Context("viewer", viewer =>
+{
+    viewer.Context("session {id:int}", session =>
+    {
+        session.Map("attach", (int id) => BuildHtml(id))
+            .AsMcpAppResource();
+    });
+});
+```
+
+This produces `ui://viewer/session/{id}/attach`. MCP URI templates keep the variable name but not the Repl route constraint, so `{id:int}` becomes `{id}` in the URI and is validated when Repl dispatches the resource read through the normal command pipeline.
+
+Pass an explicit URI only when you need a stable public URI that is decoupled from the route path:
+
+```csharp
+app.Map("contacts dashboard", (IContactDb contacts) => BuildHtml(contacts))
+    .AsMcpAppResource("ui://contacts/summary");
+```
+
+### Display preferences
+
+MCP Apps standard display modes are `inline`, `fullscreen`, and `pip`, but hosts decide what they support. Repl can express a preference:
+
+```csharp
+app.Map("contacts dashboard", (IContactDb contacts) => BuildHtml(contacts))
+    .AsMcpAppResource()
+    .WithMcpAppDisplayMode(McpAppDisplayModes.Fullscreen);
+```
+
+As of April 2026, VS Code renders MCP Apps inline only. Microsoft 365 Copilot declarative agents support fullscreen display requests for widgets. Other hosts vary; check [mcp-server.md](mcp-server.md#mcp-apps-host-compatibility) for the current compatibility notes.
+
+If the HTML uses the MCP Apps JavaScript bridge, it should still ask the host what is available before requesting a different display mode:
+
+```javascript
+const modes = app.getHostContext()?.availableDisplayModes ?? [];
+if (modes.includes("fullscreen")) {
+  await app.requestDisplayMode({ mode: "fullscreen" });
+}
+```
+
+For host-specific hints that are not yet modeled by Repl, use simple string metadata:
+
+```csharp
+app.Map("contacts dashboard", (IContactDb contacts) => BuildHtml(contacts))
+    .AsMcpAppResource()
+    .WithMcpAppUiMetadata("presentation", "flyout");
+```
+
+### HTML now, assets later
+
+The v1 Repl API expects the UI resource handler to return generated HTML. This is enough for small cards, forms, and proof-of-concept dashboards.
+
+For WebAssembly UIs such as Uno-Wasm, the likely shape is:
+
+1. Map a `ui://` app resource that returns a small HTML shell.
+2. Serve published static assets such as `embedded.js`, `_framework/*`, `.wasm`, fonts, and images from an HTTP endpoint.
+3. Inject the HTTP base URL into the generated shell.
+4. Set CSP metadata for asset and fetch domains.
+
+```csharp
+var assetBaseUri = new Uri("http://127.0.0.1:5123/");
+
+app.Map("contacts dashboard", () => BuildUnoShellHtml(assetBaseUri))
+    .AsMcpAppResource()
+    .WithMcpAppCsp(new McpAppCsp
+    {
+        ResourceDomains = [assetBaseUri.ToString()],
+        ConnectDomains = [assetBaseUri.ToString()],
+    });
+```
+
+Keep the shell and asset server host-aware: clients may preload or cache UI resources, and not every host supports every display mode or browser capability.
+
 ## Troubleshooting patterns
 
+### My MCP App shows HTML text in the chat
+
+Use `.AsMcpAppResource()` on the HTML-producing command instead of linking a normal tool to raw HTML manually. Repl will return launcher text for tool calls and reserve the HTML for `resources/read`.
+
+Also restart or reload the MCP server in the client. Some hosts cache tool lists and will not pick up MCP Apps metadata changes until the server is refreshed.
+
+### My MCP App does not open fullscreen
+
+Check whether the host supports fullscreen. VS Code currently renders MCP Apps inline only, even when Repl sets `preferredDisplayMode: McpAppDisplayModes.Fullscreen`.
+
+For hosts that support display mode changes, request fullscreen from inside the HTML app after checking host capabilities.
+
 ### The agent doesn't see tools that should appear later
 
 Check: