Repl.Mcp 0.12.0-dev.123

Repl.Mcp

Website: repl.yllibed.org

MCP server integration for Repl Toolkit — expose your command graph as AI agent tools, resources, prompts, and MCP Apps UI via the Model Context Protocol.

Use Repl.Mcp when you already have, or want to build, a Repl command graph and make the same operations available to AI agents without writing a separate MCP server by hand.

Upgrading from a 1.x SDK build

This version builds on ModelContextProtocol 2.x. The changes a consumer meets first are below; the repository's MCP reference carries all eight.

• The SDK moves to 2.x. It is a transitively public dependency, so a consumer referencing it directly moves with this package. The 1.x and 2.x assemblies cannot coexist.
• IMcpFeedback.SendMessageAsync takes McpMessageLevel instead of the SDK's deprecated LoggingLevel. Same members, same values — the swap is mechanical.
• Tool results can carry extra content blocks. A message the client could not receive as a notification is appended after the command's payload. The payload stays the first block and StructuredContent is untouched, but a test asserting exactly one block will fail.
• An uncaught exception no longer reaches the client as text. A command that throws, or an application callback that fails while supplying a parameter, is surfaced as Command failed with exit code N. — the framework renders that message for an operator, and over MCP the reader is a remote client. Feedback the application reported itself still travels; return an error from the command when the client needs the reason.
• .LongRunning() no longer advertises task support on the protocol surface, because SDK 2.x removed the per-tool execution augmentation. The annotation still reaches help and documentation.
• Module presence no longer varies with the client on 2026-07-28, which requires the advertised set not to vary per connection. Discovery there runs every presence predicate against fixed answers — IsSupported, IsLoggingSupported and IsProgressSupported are true, HasSoftRoots is false, Current and GetAsync() are empty — and whatever the predicate returns is what every client is offered. Read it off the result, not off the member: one that comes out true is advertised to every client and stays callable by every client, so that command must now return a clear error instead of relying on being absent; one that comes out false, including a negated capability gate such as !roots.IsSupported, is advertised to none and disappears with no error, so map it unconditionally. Earlier revisions are unchanged.

Install

dotnet add package Repl.Mcp

One line to add

using Repl.Mcp;

app.UseMcpServer();

Your commands become MCP tools. Route constraints become JSON Schema. Annotations become safety hints.

myapp mcp serve   # AI agents connect here
myapp              # still a CLI / interactive REPL

IReplInteractionChannel user feedback maps to MCP-native transports:

• progress → progress notifications
• notice / warning / problem feedback → MCP message notifications, or the result itself

On 2026-07-28 a request that declared no log level must receive no message notifications, so that feedback is appended to the tool or prompt result instead — and to the surfaced error when the call fails. A resource read is the exception: its body has to match the advertised MIME type, so a read that succeeds keeps only that body and the feedback it reported is dropped on purpose, while a read that fails carries it in the surfaced error. Everywhere else it survives.

Keep operator logging on ILogger; do not rely on user-facing interaction as a logging sink.

Agent configuration

Most MCP clients use the same shape:

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

Use the executable that matches your app packaging. For local project samples, build once and use dotnet run --no-build --project ... -- mcp serve so host startup does not rebuild or write build output to stdout.

MCP Apps

Repl.Mcp can also expose MCP Apps UI resources:

This support is experimental in the current version. AsMcpAppResource() handlers should return generated HTML as string, Task<string>, or ValueTask<string>; richer return shapes and asset helpers may be added later.

app.Map("contacts dashboard", (ContactStore contacts) =>
        $"<!doctype html><html><body>{contacts.All.Count} contacts</body></html>")
    .WithDescription("Open the contacts dashboard")
    .AsMcpAppResource()
    .WithMcpAppBorder()
    .WithMcpAppDisplayMode(McpAppDisplayModes.Fullscreen);

Clients with MCP Apps support render the generated ui:// resource. Other MCP clients still receive the command's normal launcher text instead of raw HTML.

What agents see

Safety guidelines

Annotate every command that is visible to agents:

app.Map("contacts list", handler).ReadOnly();

app.Map("contacts import {file}", handler)
    .OpenWorld()
    .LongRunning();

app.Map("contacts delete {id:int}", handler)
    .Destructive();

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

Unannotated tools force agents to assume the worst. Use .ReadOnly() for safe queries, .Destructive() for important mutations, .OpenWorld() for external systems, .LongRunning() for slow operations (a documentation hint today — protocol-level MCP task advertisement returns once Repl integrates the SDK Tasks extension), and .AutomationHidden() for commands that should stay available to humans but invisible to MCP automation.

Prefer returning JSON-friendly objects instead of writing prose-only output. Structured results are easier for agents to inspect, retry, test, and summarize.

Works with

Claude Desktop, Claude Code, VS Code Copilot, Cursor, and any MCP-compatible agent.

MCP Apps host support varies. VS Code currently renders MCP Apps inline; hosts that support display mode requests can honor preferredDisplayMode.

Docs

• MCP Mode — quick start, annotations, mental model
• MCP In Depth — interaction degradation, client compatibility, agent configuration
• Agent-Native Development — designing commands for AI consumption
• For coding agents — decision rules and copyable instructions for coding agents
• Cookbook: MCP Server — resources, tools, prompts, annotations, and MCP Apps UI in action