MCP Mode

Repl can expose your entire command graph as an MCP server, making every command available as an MCP tool to any compatible AI agent — Claude, Cursor, Copilot, and others.

Enable MCP

Add the package
Terminal window
```
dotnet add package Repl.Mcp
```

Register the MCP server

using Repl;
using Repl.Mcp;

var app = ReplApp.Create().UseDefaultInteractive();

// ... your commands ...

app.UseMcpServer();  // one line

return app.Run(args);

Configure your agent

{
  "mcpServers": {
    "myapp": {
      "command": "myapp",
      "args": ["mcp", "serve"]
    }
  }
}

That’s it — every Map(...) call is now an MCP tool.

How mapping works

Repl translates your command graph to MCP concepts automatically:

Repl concept	MCP concept
`app.Map("list", handler)`	Tool `list`
Return value	Tool result
`[Description]` attribute	Tool description
`Results.Error(...)`	Tool error
`.AsResource()`	Resource
`.AsPrompt()`	Prompt

MCP Apps

MCP Apps are host-rendered UI surfaces for capable clients (e.g., Claude). Register an HTML-returning command as an App resource:

app.Map("dashboard", (IContactStore contacts) => BuildHtml(contacts))
    .WithDescription("Open the contacts dashboard")
    .AsMcpAppResource();

The client renders the HTML in a webview panel — your command logic, surfaced as interactive UI.

Behavioral annotations

Control how the agent sees your commands:

app.Map("delete {id:int}", (int id) => Delete(id))
    .Destructive()
    .WithDescription("Delete a record permanently.");

Automation visibility

Some commands should be available to humans in REPL mode but hidden from agents:

app.Map("debug reset", Reset)
    .AutomationHidden();

Next steps

MCP Server cookbook — tools, resources, prompts, MCP Apps in depth
Testing — test your MCP surface with Repl.Testing