OneModelContext: a .NET SDK for the Model Context Protocol®

Status: 1.0. The public API is stable and checked by package validation. Tasks, Skills and Server Cards follow experimental MCP extensions and are marked [Experimental] until those extensions are stable.

OneModelContext is a clean-room .NET 10 library for MCP, the Model Context Protocol, on both sides: client and server. The core package (RecursiveAI.OneModelContext) depends on the .NET base class library only, and is trim- and NativeAOT-safe.

A server in ten lines

using RecursiveAI.OneModelContext;
 
await ModelContext.ServeAsync(new IndexTools(index));          // stdio
// await ModelContext.ServeHttpAsync(new IndexTools(index));   // Streamable HTTP on 127.0.0.1:3001
 
public partial class IndexTools(SearchIndex index)
{
    /// <summary>Searches the index.</summary>
    /// <param name="query">What to look for.</param>
    /// <param name="limit">Most results to return.</param>
    [Tool(ReadOnly = true)]
    public Task<string[]> Search(string query, int limit = 10) => index.SearchAsync(query, limit);
}

await using var client = await ModelContext.ConnectAsync("npx -y @modelcontextprotocol/server-filesystem .");
var tools = await client.ListToolsAsync();
var result = await client.CallToolAsync("read_file", new { path = "README.md" });
Console.WriteLine(result.Text);

The object-argument call uses reflection and is not trim- or NativeAOT-safe. For AOT, pass source-generated serializer metadata instead:

await using var client = await ModelContext.ConnectAsync("my-server");
var result = await client.CallToolAsync("read_file", new ReadFileArguments("README.md"), argumentsJsonTypeInfo);
Console.WriteLine(result.Text);

Here ReadFileArguments is your arguments record and argumentsJsonTypeInfo is its JsonTypeInfo<ReadFileArguments> from a source-generated JsonSerializerContext (with JSON property names matching the tool schema).

The server example assumes your application supplies SearchIndex and index; see the complete indexed search sample.

The generator writes each tool's JSON Schema 2020-12 from the signature and its doc comments, and an AOT-safe invoker. More in samples/.

What it does

• Both protocol eras, one API. The stateless 2026-07-28 revision first, and one legacy codec for 2024-11-05 through 2025-11-25. A client negotiates the era per server; a server answers both on one stdio stream or endpoint. Your code is written against the modern semantics and the era is a codec detail.
• Transports. stdio and any duplex Stream; Streamable HTTP as a client, as a host-agnostic server core, on a small built-in HTTP/1.1 listener (loopback by default) and on ASP.NET Core with MapModelContext; the deprecated HTTP+SSE transport as a client fallback.
• Tools, resources and prompts, declared with attributes or registered while the server runs, with RequestContext for cancellation, progress, user confirmation and elicitation. A handler that asks for input behaves the same on both eras (multi-round-trip).
• Authorization. A resource server (protected resource metadata, challenges, step-up, audience checks; JWT/JWKS, introspection, static-token and delegate validators), a full OAuth 2.1 client, Client Credentials and Enterprise-Managed Authorization, and an embedded AuthorizationServer whose login and consent stay with the host. See resource-server, oauth-client and authorization-server.
• Extensions. A public extension mechanism, MCP Apps, and Tasks, Skills and Server Cards, which are [Experimental] until their specifications are stable. See extensions.
• Sampling, Roots and Logging are implemented and marked [Obsolete] (diagnostic IDs OMC0001-OMC0003), following the revision that deprecates them.
• Observability. An ActivitySource and Meter named RecursiveAI.OneModelContext, a diagnostics event stream and a per-call audit hook.
• Safe by default. Every size, depth and time limit is a documented default on a public option (limits).
• A command line tool (onemodelcontext inspect | call | read | prompt | login | logout | proxy) and dotnet new templates.

Coming from the official C# SDK? See the task map.

Packages

All packages share one version.

Third-party licenses: THIRD-PARTY-NOTICES.md.

Repository layout

Build

dotnet build -c Release
dotnet test -c Release
tools/aot-check.sh        # NativeAOT publishes; needs a native toolchain (clang or gcc)

Build output goes to artifacts/ (ignored). Set ArtifactsPath to put it elsewhere. See CHANGELOG.md.

License

MIT OR Apache-2.0, Copyright (c) 2026 Recursive AI, LLC. See LICENSE-MIT and LICENSE-APACHE.

Model Context Protocol® is a registered trademark of LF Projects, LLC. OneModelContext is an independent project and is not affiliated with or endorsed by LF Projects, LLC or the Model Context Protocol project.