Codex CLI for .NET 10 and C# 14: Aspire Integration, MCP Servers, and the dotnet/skills Ecosystem

codex-clidotnetcsharpaspiremcpskillsdotnet-10csharp-14agent-workflows

Codex CLI for .NET 10 and C# 14: Aspire Integration, MCP Servers, and the dotnet/skills Ecosystem

---

The .NET ecosystem has quietly assembled one of the richest AI-agent integration stories of any platform. Between the official dotnet/skills repository maintained by the .NET platform team, the community-driven managedcode/dotnet-skills catalogue with 166 installable skills, the stable C# MCP SDK at v1.3.0, and Aspire 13.2’s agent-native CLI, a well-configured .NET 10 project gives Codex CLI more domain-specific context than almost any other stack ¹²³.

This article covers the practical integration points: installing .NET skills, building and consuming C# MCP servers, leveraging Aspire’s detached mode for agent-driven development, and configuring AGENTS.md for C# 14’s new language features.

The .NET 10 and C# 14 Landscape

.NET 10, released November 2025 as an LTS release supported until November 2028, ships with C# 14 and brings several features that directly affect how Codex CLI generates code ⁴.

C# 14 Features That Matter for Agent Workflows

Field-backed properties eliminate boilerplate. Where previously you needed an explicit backing field:

private string _name;
public string Name
{
    get => _name;
    set => _name = value ?? throw new ArgumentNullException(nameof(value));
}

C# 14 allows the field keyword directly in accessors ⁵:

public string Name
{
    get;
    set => field = value ?? throw new ArgumentNullException(nameof(value));
}

Extension members generalise the old extension method pattern to include properties and static members ⁵:

public static class Enumerable
{
    extension<TSource>(IEnumerable<TSource> source)
    {
        public bool IsEmpty => !source.Any();
    }
}

Null-conditional assignment and unbound generic nameof round out the release. These features require AGENTS.md guidance to ensure Codex CLI generates idiomatic C# 14 rather than falling back to older patterns.

AGENTS.md Configuration for C# 14

## C# Style

- Target C# 14 / .NET 10. Use `field` keyword in property accessors
  instead of explicit backing fields where validation is needed.
- Prefer the new `extension` block syntax over static extension methods.
- Use `nameof(List<>)` for unbound generics — do not supply a dummy
  type argument.
- All projects use nullable reference types. Never suppress with `!`
  without a comment explaining why.

The dotnet/skills Ecosystem

Official Skills (dotnet/skills)

The .NET platform team maintains 13 curated plugins in the dotnet/skills repository, released under MIT licence ¹. Each skill follows the agentskills.io open standard, making them portable across Codex CLI, Claude Code, GitHub Copilot, and Visual Studio 2026 ⁶.

Plugin	Purpose
dotnet	Core .NET coding tasks
dotnet-data	Entity Framework and data access
dotnet-diag	Performance investigation and debugging
dotnet-msbuild	Build diagnostics and optimisation
dotnet-nuget	Package management and dependency modernisation
dotnet-upgrade	Framework migration and compatibility
dotnet-maui	Mobile app development
dotnet-ai	LLM integration, RAG pipelines, MCP, ML.NET
dotnet-template-engine	Project scaffolding
dotnet-test	Test execution and MSTest workflows
dotnet-aspnet	ASP.NET Core middleware and API patterns
dotnet-blazor	Component authoring and interactive web
dotnet11	.NET 11 preview APIs and features

Installation in Codex CLI uses the standard skill installation flow. Clone the repository and point your project’s skill configuration at the relevant plugin directories, or reference them via the marketplace ¹.

Community Skills (managedcode/dotnet-skills)

The managedcode/dotnet-skills project extends coverage to 166 skills spanning ASP.NET Core, Blazor, gRPC, SignalR, Aspire, Orleans, WPF, WinUI, MAUI, and Semantic Kernel ². It ships its own dotnet tool:

dotnet tool install -g dotnet-skills
dotnet skills recommend          # Analyses project, suggests skills
dotnet skills install dotnet-aspnet-core
dotnet skills install dotnet-aspire

The recommend subcommand inspects your .csproj files and NuGet references to suggest relevant skills — a useful bootstrap for new repositories.

Building MCP Servers in C#

The official C# MCP SDK, maintained jointly by Anthropic and Microsoft, reached v1.3.0 stable on 8 May 2026 ³. It targets netstandard2.0, running on .NET 8 LTS, .NET 9, and .NET 10.

Package Structure

<!-- Core SDK — minimal client and low-level server APIs -->
<PackageReference Include="ModelContextProtocol.Core" Version="1.3.0" />

<!-- Full SDK — hosting, DI extensions -->
<PackageReference Include="ModelContextProtocol" Version="1.3.0" />

<!-- ASP.NET Core hosting for Streamable HTTP transport -->
<PackageReference Include="ModelContextProtocol.AspNetCore" Version="1.3.0" />

Minimal MCP Server Example

using ModelContextProtocol;
using Microsoft.Extensions.Hosting;

var builder = Host.CreateApplicationBuilder(args);
builder.Services.AddMcpServer()
    .WithStdioTransport()
    .WithTools<ProjectTools>();

await builder.Build().RunAsync();

[McpTool("analyse_solution", "Analyse .NET solution structure")]
public static class ProjectTools
{
    [McpTool]
    public static async Task<string> AnalyseSolution(
        [McpParameter("path", "Path to .sln file")] string path)
    {
        // Parse solution, return structure summary
        var content = await File.ReadAllTextAsync(path);
        var projects = content.Split('\n')
            .Where(l => l.StartsWith("Project("))
            .Select(l => l.Split('"')[3]);
        return string.Join("\n", projects);
    }
}

Registering with Codex CLI

Add the server to your project-level .codex/config.toml:

[mcp_servers.dotnet-tools]
command = "dotnet"
args = ["run", "--project", "./tools/McpServer/McpServer.csproj"]

For Streamable HTTP servers using ASP.NET Core and Kestrel, use the url form instead ⁷:

[mcp_servers.dotnet-api]
type = "url"
url = "http://localhost:5100/mcp"

The C# SDK’s advantages over Python or TypeScript implementations include Native AOT publishing for sub-second cold starts in stdio mode, and first-party ASP.NET Core hosting with Kestrel, OpenTelemetry, and JWT bearer auth out of the box ³.

The NuGet MCP Server

Microsoft ships an official NuGet MCP server that gives Codex CLI direct access to package search, version resolution, and dependency analysis ⁸. Configure it alongside your project tools:

[mcp_servers.nuget]
command = "dotnet"
args = ["tool", "run", "nuget-mcp-server"]

Aspire 13.2: The Agent-Native Application Platform

Aspire (formerly .NET Aspire) reached v13.2 in March 2026, with the current stable at v13.3.5 ⁹. The 13.2 release was purpose-built for AI coding agents, introducing detached mode, structured output, health-check blocking, and telemetry streaming ¹⁰.

Why Aspire Matters for Codex CLI

Aspire describes your entire distributed application — services, containers, databases, caches, and connections — in code. The CLI can start the whole stack in the background, letting Codex CLI interact with running services without blocking the terminal.

flowchart LR
    A[Codex CLI] -->|aspire start| B[Aspire CLI]
    B -->|Detached Mode| C[Background Process]
    C --> D[API Service]
    C --> E[PostgreSQL]
    C --> F[Redis Cache]
    A -->|HTTP requests| D
    A -->|aspire logs| C
    A -->|aspire stop| C

Agent Workflow with Aspire

# Start the application stack in detached mode
aspire start

# Wait for all resources to become healthy
aspire wait --all

# Codex can now interact with running services
codex exec "Run integration tests against the live API at localhost:5000"

# Inspect logs for failures
aspire logs --resource api-service --tail 50

# Shut down when done
aspire stop

The aspire docs command brings aspire.dev documentation directly into agent workflows, and aspire doctor validates the entire environment before an agent starts building ¹⁰.

Aspire MCP and Skills

Running aspire new or aspire init automatically generates Aspire-specific skills and an MCP server configuration, so coding agents can discover and interact with application resources from the start ¹⁰. This means a fresh Aspire project is agent-ready out of the box — no manual MCP configuration required.

Putting It Together: A Complete .NET 10 Agent Configuration

A production-ready .codex/config.toml for a .NET 10 Aspire project:

model = "o3"
sandbox_mode = "workspace-write"

[mcp_servers.dotnet-tools]
command = "dotnet"
args = ["run", "--project", "./tools/McpServer/McpServer.csproj"]

[mcp_servers.nuget]
command = "dotnet"
args = ["tool", "run", "nuget-mcp-server"]

[sandbox_workspace_write]
writable_roots = ["./src", "./tests", "./tools"]

[shell_environment_policy]
inherit = "core"
set = { DOTNET_CLI_TELEMETRY_OPTOUT = "1", ASPNETCORE_ENVIRONMENT = "Development" }

Pair this with an AGENTS.md that references the installed skills and establishes .NET conventions:

## Project Structure

This is a .NET 10 Aspire solution. Use `aspire start` to bring up all
services in detached mode. Run `aspire logs` to check service health.

## Skills

The following dotnet/skills plugins are installed:
- dotnet-aspnet (ASP.NET Core patterns)
- dotnet-data (Entity Framework Core)
- dotnet-test (test execution)

## Build and Test

- `dotnet build` — builds the entire solution
- `dotnet test --no-build` — runs all tests
- Never commit without running `dotnet format --verify-no-changes`

## Review Checklist

- All public APIs must have XML doc comments
- Use field-backed properties (C# 14) for validation
- Prefer extension blocks over static extension methods
- Check NuGet vulnerability audit: `dotnet list package --vulnerable`

Configuration Precedence for Enterprise Teams

For enterprise deployments, use managed configuration to enforce .NET-specific policies across teams ¹¹:

# requirements.toml — deployed via MDM or cloud-managed config
[rules]
[[rules.entries]]
pattern = "rm -rf"
decision = "forbidden"

[[rules.entries]]
pattern = "dotnet publish"
decision = "prompt"

[[rules.entries]]
pattern = "dotnet nuget push"
decision = "forbidden"

This prevents agents from publishing packages or deleting directories whilst allowing builds and tests to run autonomously.

What Is Coming Next

The .NET 11 preview (expected November 2026) already has a dedicated dotnet11 skill in the official catalogue ¹. The MCP C# SDK roadmap includes improved streaming support and tighter Aspire integration. ⚠️ Exact .NET 11 features and MCP SDK v2.0 timelines are unconfirmed at time of writing.

The convergence of platform-team-maintained skills, a stable MCP SDK, and Aspire’s agent-native CLI means .NET is no longer playing catch-up in the AI-assisted development space — it is setting the pace for how platform teams should support coding agents.

Citations

1. dotnet/skills — Repository for skills to assist AI coding agents with .NET and C# ↩ ↩² ↩³ ↩⁴

2. managedcode/dotnet-skills — Installable .NET skill catalog and CLI ↩ ↩²

3. modelcontextprotocol/csharp-sdk — The official C# SDK for Model Context Protocol ↩ ↩² ↩³

4. What’s new in .NET 10 — Microsoft Learn ↩

5. What’s new in C# 14 — Microsoft Learn ↩ ↩²

6. Extend your coding agent with .NET Skills — .NET Blog ↩

7. Build a Model Context Protocol (MCP) server in C# — .NET Blog ↩

8. Using the NuGet Model Context Protocol (MCP) Server — Microsoft Learn ↩

9. Aspire releases — GitHub ↩

10. Announcing Aspire 13.2 — Aspire Blog ↩ ↩² ↩³

11. Managed configuration — Codex OpenAI Developers ↩