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Hovik Aghajanyan
Hovik Aghajanyan

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Turn the ASP.NET Core Web API you already have into an MCP server

#aspnetcore #mcp #dotnet #ai

Nabu.NET — turn your ASP.NET Core Web API into an MCP server with one attribute

· Repo: https://github.com/hovik-aghajanyan/nabu.net

· Docs: https://nabu.aghajanyan.me/

· NuGet: https://www.nuget.org/packages/Nabu.Mcp.AspNetCore

Nabu.Mcp.AspNetCore publishes existing ASP.NET Core
controller actions and Minimal API route handlers as Model Context Protocol
tools — without rewriting the API. Every tool call is replayed as a real HTTP request through your
application's own pipeline, so authentication, authorization, validation, filters and middleware keep
working exactly as they do over HTTP.

The entire integration 

[HttpGet("{id:guid}")]
[McpTool]                                   // <- that's it
public ActionResult<TodoItem> GetById(Guid id) => ...
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app.MapGet("/customers/{id}", (int id) => ...)
   .McpTool();                              // <- same thing for Minimal APIs
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Setup 

var builder = WebApplication.CreateBuilder(args);

builder.Services.AddControllers();
builder.Services.AddNabuMcp(options =>
{
    options.ServerName = "my-api";
    options.RequireAuthorization = true;                    // protect the MCP endpoint itself
    options.ToolVisibility = McpToolVisibility.Authorized;  // advertise only what the caller may invoke
});

var app = builder.Build();
app.UseAuthentication();
app.UseAuthorization();
app.UseNabuMcp();          // serves MCP at /mcp
app.MapControllers();
app.Run();
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Talk to it 

curl -X POST http://localhost:5000/mcp \
  -H "Authorization: Bearer $TOKEN" \
  -H 'Content-Type: application/json' \
  -d '{"jsonrpc":"2.0","id":1,"method":"tools/list"}'
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Tool descriptions come from your XML docs; input schemas are generated from your CLR types, data
annotations and nullability. [Authorize], policies and roles are enforced per call — the same action
that returns 403 over HTTP returns an isError tool result over MCP for the same caller, and
tools/list can show each caller only the tools it is actually allowed to invoke.

Why not just reflect over controllers?

Direct method invocation quietly drops everything that makes an action safe: [Authorize] is enforced
by middleware and filters, ModelState is populated by model binding, rate limiting and exception
handling live outside the method. Nabu instead captures the app's RequestDelegate at startup and pushes
a synthetic HttpContext — carrying the caller's identity and forwarded credentials — through the full
pipeline for every tool call.

Highlights

  • One action, several tools — apply [McpTool] repeatedly with different names, parameter subsets and pinned constants.
  • Per-caller tool visibility — tools/list evaluated with your own authorization policies.
  • Protected headers — a model-supplied argument can never override Authorization, Cookie or proxy headers.
  • Official SDK adapter — Nabu.Mcp.ModelContextProtocol serves the same tools through the official MCP C# SDK.
  • Broad targets — netstandard2.0 (ASP.NET Core 2.1+), net8.0, net10.0. MIT licensed.

📚 Full documentation: https://nabu.aghajanyan.me/

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