McpIt 1.0.1

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McpIt

You already have a Web API. Expose it to AI agents as MCP tools at build time: one [McpTool] attribute, zero reflection, zero proxy, zero hand-written server.

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McpIt is a build-time Roslyn source generator that turns your existing ASP.NET Core endpoints into Model Context Protocol tools. The official MCP C# SDK makes you hand-write [McpServerTool] classes for every operation you want an agent to use. McpIt generates those tool classes for you from the controller actions and minimal-API endpoints you already have: you mark an action with [McpTool], and at compile time McpIt emits the MCP tool on top of the official ModelContextProtocol.AspNetCore SDK. No reflection, no internal HTTP self-call, no separate server to write and keep in sync.

Install

dotnet add package McpIt

McpIt brings in the official MCP SDK transitively, so you do not need to add ModelContextProtocol.AspNetCore yourself. The [McpTool] and [McpToolOutput] attributes ship in the small McpIt.Abstractions package, which also comes in transitively.

Minimal setup in Program.cs:

using McpIt;

var builder = WebApplication.CreateBuilder(args);
builder.Services.AddControllers();

builder.Services.AddMcpServer()
    .WithHttpTransport(o => o.Stateless = true)
    .WithToolsFromAssembly();   // discovers the tools McpIt generated

builder.Services.AddMcpEndpoints();   // in-process invoker for the generated tools

var app = builder.Build();
app.MapControllers();
app.MapMcp("/mcp");   // MCP server at /mcp; your API stays where it is
app.Run();

Your REST API runs unchanged, and an MCP server is now served at /mcp.

Before / after

A normal ASP.NET Core controller action:

[ApiController]
[Route("orders")]
public class OrdersController : ControllerBase
{
    [HttpGet("{id}")]
    public string GetOrder(int id) => $"order-{id}";
}

The same action, exposed to AI agents. Add one attribute (and a <summary> for the description):

using McpIt;
using Microsoft.AspNetCore.Mvc;

[ApiController]
[Route("orders")]
public class OrdersController : ControllerBase
{
    /// <summary>Gets an order by its id.</summary>
    [HttpGet("{id}")]
    [McpTool(Name = "getOrder")]
    public string GetOrder(int id) => $"order-{id}";
}

At compile time McpIt generates an MCP tool class for getOrder. It reads the action's HTTP verb, route template, parameters, and description, and builds the tool's input schema and safety hints from them. The id route parameter becomes a typed tool argument, and the <summary> becomes the tool description. Nothing else changes in your project.

To an MCP client, a tools/list call now returns the tool:

{
  "tools": [
    {
      "name": "getOrder",
      "description": "Gets an order by its id.",
      "inputSchema": {
        "type": "object",
        "properties": { "id": { "type": "integer" } },
        "required": ["id"]
      },
      "annotations": { "readOnlyHint": true, "idempotentHint": true }
    }
  ]
}

When the agent calls getOrder, McpIt invokes your real GetOrder action in-process, so your routing, model binding, validation, and business logic all run exactly as they do for an HTTP caller. There is no second HTTP request and no reflection at runtime.

Why McpIt

Without McpIt you write and maintain a parallel [McpServerTool] class for every endpoint you want an agent to reach, keeping its parameters, schema, and description in sync with the controller by hand. McpIt removes that layer: the endpoints you already have become the tools.

The only comparable library, Api.ToMcp, performs an internal HTTP self-call at runtime and supports controllers only. McpIt differs on four points:

1. Direct in-process invocation. Tool calls run your action directly, with no internal HTTP self-call.
2. Controllers and minimal APIs. Both endpoint styles can be exposed with [McpTool].
3. AOT-safe, zero reflection. It is a source generator, so the tool code exists at build time. Nothing is discovered by reflection at runtime.
4. Polished and tested. 75 tests cover generation, invocation, output shaping, and the token report.

Features

• Controllers and minimal APIs. Mark a controller action or a minimal-API endpoint with [McpTool] to opt it in. Exposure is opt-in: only annotated endpoints become tools.

• Tool names. [McpTool] derives a camelCase name from the method, or set Name explicitly. Placed on a controller class, [McpTool] sets defaults (such as NamePrefix) for that class's annotated actions without exposing anything on its own.

• Output shaping with [McpToolOutput]. Keep responses lean. Fields projects the response down to the top-level JSON properties you list (per object, or per array element), and MaxLength truncates the result. Shaping is best-effort: malformed JSON passes through untouched.

  [HttpGet("{id}")]
  [McpTool]
  [McpToolOutput(Fields = new[] { "id", "status" }, MaxLength = 500)]
  public Order GetOrder(int id) { ... }
  

• Safety hints from HTTP verbs. MCP tool annotations are derived from the verb: GET and HEAD are read-only and idempotent; POST, PUT, PATCH, and DELETE are flagged destructive (PUT and DELETE also idempotent). Exposing a destructive operation raises a build warning until you acknowledge it with [McpTool(AllowDestructive = true)].

• MCPGEN diagnostics. Build-time warnings keep your tool surface honest: MCPGEN001 when a tool has no description, MCPGEN002 when a destructive operation is exposed without acknowledgement.

• Token-cost report. The mcp-token-report tool measures what your tool list costs the model and can fail a CI build over a budget (see below).

Token report tool

mcp-token-report is an offline analyzer that shows how many tokens your tools/list surface spends in the model's context. AI agents load every tool's name, description, and input schema before the user asks anything, so a large tool surface is a real, recurring context cost. The tool reads a running MCP server or a saved tools/list JSON file, reports per-tool and total token counts, and can gate a build with --budget. It is fully offline and deterministic, so it is safe in CI.

dotnet tool install -g McpIt.TokenReport.Tool

mcp-token-report http://localhost:5199/mcp            # or a saved tools-list.json
mcp-token-report http://localhost:5199/mcp --markdown # Markdown table for CI artifacts
mcp-token-report http://localhost:5199/mcp --budget 2000   # exit 1 if over budget

Token counts use an offline heuristic tokenizer (estimates, not exact billing): ideal for comparing tools and catching bloat.

Compatibility

• .NET 10. The library targets net10.0.
• Built on the official MCP SDK. McpIt layers on ModelContextProtocol.AspNetCore 1.4.0. It generates the tool classes; the official SDK serves them over the MCP transport you configure (AddMcpServer().WithHttpTransport(...)).
• AOT-friendly. Generation happens at compile time, with no runtime reflection.

Links

• NuGet: McpIt · McpIt.Abstractions · McpIt.TokenReport.Tool
• Repository: github.com/norequest/McpIt
• Official MCP C# SDK: ModelContextProtocol

License

MIT. Free for personal and commercial use, no warranty. Keep the copyright and license notice.