Nuvyn CLI 1.2.0: Spec-Driven .NET MAUI, From a New App or One You Already Have

A coding agent will happily build a .NET MAUI app from a chat thread. The next session often builds a different one.

Nuvyn is the CLI that keeps that work on one stack and one written spec. Version 1.2.0 is on nuget.org as the global tool NuvyntraLabs.Nuvyn.Cli. The command is nuvyn.

This post is the walkthrough: what the tool is, how to install it, how to start a new app, how to attach it to an app you already have, and the slash commands the agent runs after that.

What Nuvyn is

Nuvyn is a standalone PackAsTool CLI for .NET 10. It depends on System.CommandLine and Spectre.Console. It is MIT licensed.

It does two jobs:

1. nuvyn init creates a new four-platform MAUI host (Android, iOS, Mac Catalyst, Windows) and locks plan and implement to the Nuvyntra stack.
2. nuvyn adopt attaches the same slash chain to an existing MAUI app. It writes workflow files only. The host you already have stays as it is.

The product domain is yours: clinic, field, bank, civic, retail, or anything else. The skills stay domain-agnostic. On a new host they lock only the MAUI and Lumina stack.

Individual plugins and NuvyntraLabs.UIKit stay installable on their own. You do not need Nuvyn to use one package.

Install it as a global dotnet tool. Do not add NuvyntraLabs.Nuvyn.Cli as a PackageReference in an app.

Why a spec, when the agent can just code

Spec-driven development treats a checked-in specification as the source of truth. Code follows that spec. A chat thread is a prompt log: it disappears, it contradicts itself, and a teammate cannot review it.

Nuvyn writes the artifacts into the repo:

That matters more once an agent writes the code. The same prompt can pick a different MVVM library on a different day. Long chats drop acceptance rules that were never written down. Agents add a login page, a second UI kit, or persistence nobody asked for, because a finished-looking app is an easy completion.

Nuvyn narrows that search space. Constitution locks the stack. Specify writes the product. Plan names packages and screens. Analysis fails the loop before implement invents a second architecture.

GitHub Spec Kit (specify init) remains the usual choice when the stack is open. Nuvyn is the door when the host should be Nuvyntra, or when an existing MAUI app should grow through the same slash chain.

What you need

Nuvyn targets those four platforms. It is the wrong tool for Tizen, Flutter, React Native, WPF, WinUI, Avalonia, or Uno.

Install

dotnet tool install -g NuvyntraLabs.Nuvyn.Cli --source https://api.nuget.org/v3/index.json
nuvyn version

nuvyn version prints NuvyntraLabs.Nuvyn.Cli 1.2.0.

Already installed? Update the tool only. Existing apps keep the packages they already reference:

dotnet tool update -g NuvyntraLabs.Nuvyn.Cli --source https://api.nuget.org/v3/index.json
nuvyn version

On an interactive terminal, nuvyn asks every four hours whether to update from nuget.org ([y/N], default no). Skip that prompt with --no-update-check or NUVYNTRA_NO_UPDATE_CHECK=1. The cache file is ~/.nuvyntra/cli-updates.json, shared with maui-dev and maui-perf. The CLIs do not phone home.

Create a new app

nuvyn init <folder_name> always creates a new folder. If that name already exists, init prints an error and exits 1. The existing tree is left untouched.

nuvyn init HarborDesk --agent cursor

Omit --agent and the CLI shows a searchable picker (cursor (Cursor), agy (Antigravity), and the rest of the Spec Kit set). Non-interactive runs default to Cursor.

What init does:

1. Copies an embedded three-project host (MVVMExpress + Lumina UIKit).
2. Adds the default Nuvyntra packages from nuget.org, unpinned, at the latest stable versions.
3. Writes .nuvyn/ (constitution, templates, reference, init-options.json).
4. Installs slash commands for the agent you picked.
5. Writes a project README with the slash chain.

The tree looks like this:

HarborDesk/
├── HarborDesk.sln
├── HarborDesk/                      # MAUI app
│   ├── MauiProgram.cs
│   ├── Pages/MainPage.xaml
│   └── Resources/Images/nuvyntra.png
├── HarborDesk.Core/                 # ViewModels
├── HarborDesk.Tests/
├── .nuvyn/
│   ├── constitution.md
│   ├── init-options.json
│   ├── templates/
│   └── reference/
├── specs/                           # empty until /nuvyn.specify
├── .cursor/skills/nuvyn-*/          # when --agent cursor
└── README.md

MainPage is a Lumina screen: the Nuvyntra mark, an NVHeading counter, and Increase / Decrease NVButton controls. There is no stock Entry / Button / Label starter, and no seeded Login / Items / Edit pages. Screens come from the spec.

Build it the usual way:

cd HarborDesk
dotnet restore
dotnet build
dotnet build HarborDesk/HarborDesk.csproj -f net10.0-android

Default packages on a new host

init adds these, and nothing else from the catalog, until the spec asks:

The MAUI app gets MVVMExpress plus Dialogs and Navigation, UIKit, HttpForge, FormValidation, and KeyboardManager. Core gets Plugin.Maui.MVVMExpress.Core and the source generators. Tests get Plugin.Maui.MVVMExpress.Testing. Versions are never pinned: dotnet add package runs without --version.

The host wires up like this:

builder
    .UseMauiApp<App>()
    .UseMvvmExpress(o => o
        .UseNavigationPage((nav, _) => nav
            .Map<MainPageViewModel, MainPage>("main"))
        .UseDialogs())
    .UseNuvyntraUIKit()
    .UseHttpForge()
    .UseMauiFormValidation()
    .UseKeyboardManager();

builder.Services.AddTransient<MainPageViewModel>();
builder.Services.AddTransient<MainPage>();

Register both the page and the view-model in MauiProgram. [RegisterViewModel] is a map for the generator, and it is not the DI registration. Chrome is NavigationPage unless the spec names Shell.

LocalStore, NuvexaDB, AppLock, and the rest of the catalog wait until you ask. /nuvyn.plan then picks the smallest fit. An outside library needs a Catalog gap row in plan.md.

Adopt an existing MAUI app

1.2.0 adds nuvyn adopt. This is the door for a tree that already exists.

cd FieldApp
nuvyn adopt --agent cursor

Or point at another folder:

nuvyn adopt --path ../FieldApp --agent copilot

Adopt:

• Refuses the folder when it is not a MAUI app (UseMaui in a csproj), or when .nuvyn/ is already there.
• Scans MVVM, chrome, UI kit, and HTTP. The scan is read-only.
• Writes .nuvyn/, agent skills, an empty specs/ folder, and .nuvyn/adopt-report.md.
• Sets "mode": "adopt" in init-options.json.
• Runs maui-dev doctor when MauiDev is on PATH.

It does not add MVVMExpress, Lumina UIKit, or HttpForge. It does not edit MauiProgram, pages, or HttpClient call sites. New work keeps the stack the app already uses. Lumina NV* is allowed on new screens only when UIKit is already referenced. /nuvyn.plan and /nuvyn.implement read adopt-report.md first.

The slash chain

Open the project folder in the agent you selected. Run the commands in order. Extra text after a command is the prompt. An empty /nuvyn.specify asks you to describe the product.

/nuvyn.constitution → /nuvyn.specify → /nuvyn.clarify → /nuvyn.plan
    → /nuvyn.checklist → /nuvyn.task → /nuvyn.analysis
    → /nuvyn.implement → /nuvyn.converge

In Cursor the skill folders are named nuvyn-constitution (a folder name cannot contain .). Type /nuvyn-constitution in Cursor. Type /nuvyn.constitution in Copilot, Claude, Gemini, and the other command-file agents.

A specify prompt can carry the product in one line:

/nuvyn.specify Resident 311 desk: sign in, report a bin miss, see live bus times.
/nuvyn.implement Implement only Foundation

Standing law lives in .nuvyn/reference/constraints.md. Each slash command stays short and points at that file, so the agent spends the window on the domain instead of restating the catalog every turn.

UI on a new host is Lumina first. One recipe per screen. Bound fields go inside the recipe so they replace the demo seed. Prefer NVInputField, NVPasswordField, and NVButton when an NV* control exists. Control reference: UIKit(MAUI) docs.

Which agent gets which folder

nuvyn init and nuvyn adopt write into the folder that agent already reads. The set matches Spec Kit, including more than thirty additional agents.

Pass --agent cursor-agent when that Spec Kit key is the one you already use.

Refresh skills after a CLI update

Do not re-run nuvyn init on a tree that already exists. After the global tool updates, refresh slash files from inside the app:

cd HarborDesk
nuvyn update
nuvyn update --agent cursor

nuvyn update overwrites .nuvyn/templates/, .nuvyn/reference/, and the agent command files. It leaves host code, specs/, .nuvyn/constitution.md, and .nuvyn/adopt-report.md alone. It does not change PackageReference versions.

nuvyn check confirms dotnet and the payload. Inside a greenfield app it also proves the host still uses MVVMExpress, UIKit, HttpForge, FormValidation, and KeyboardManager. Inside an adopted app it prints the inventory and skips that proof.

When MauiDev is on PATH, init, adopt, and check run maui-dev doctor --path. A missing doctor is a warning. Nuvyn still succeeds. Install it when you want the environment report:

dotnet tool install -g Plugin.Maui.MauiDev.Cli --source https://api.nuget.org/v3/index.json
maui-dev doctor

When something looks wrong

Where Nuvyn sits next to the other tools

Try it

dotnet tool install -g NuvyntraLabs.Nuvyn.Cli --source https://api.nuget.org/v3/index.json
nuvyn init HarborDesk --agent cursor
cd HarborDesk

Open that folder in your agent and run /nuvyn.constitution, then /nuvyn.specify with the product in one sentence.

If the app already exists:

cd YourMauiApp
nuvyn adopt --agent cursor

Then read .nuvyn/adopt-report.md before /nuvyn.plan.

• NuGet: NuvyntraLabs.Nuvyn.Cli
• Repository: github.com/nuvyntralabs/Nuvyn
• User guide: nuvyntralabs.github.io/toolkits/nuvyn/guide/
• Technical docs: nuvyntralabs.github.io/toolkits/nuvyn/docs/

By Niladri