Coding Agent VMs on NixOS with microvm.nix

2026-02-01T09:00:00+01:00

I have come to appreciate coding agents to be valuable tools for working with computer program code in any capacity, such as learning about any program’s architecture, diagnosing bugs or developing proofs of concept. Depending on the use-case, reviewing each command the agent wants to run can get tedious and time-consuming very quickly. To safely run a coding agent without review, I wanted a Virtual Machine (VM) solution where the agent has no access to my personal files and where it’s no big deal if the agent gets compromised by malware: I can just throw away the VM and start over.

Instead of setting up a stateful VM and re-installing it when needed (ugh!), I prefer the model of ephemeral VMs where nothing persists on disk, except for what is explicitly shared with the host.

The microvm.nix project makes it easy to create such VMs on NixOS, and this article shows you how I like to set up my VMs.

Setting up microvm.nix

Let’s jump right in! The next sections walk you through how I set up my config.

Step 1: network prep

First, I created a new microbr bridge which uses 192.168.33.1/24 as IP address range and NATs out of the eno1 network interface. All microvm* interfaces will be added to that bridge:

systemd.network.netdevs."20-microbr".netdevConfig = {
  Kind = "bridge";
  Name = "microbr";
};

systemd.network.networks."20-microbr" = {
  matchConfig.Name = "microbr";
  addresses = [ { Address = "192.168.83.1/24"; } ];
  networkConfig = {
    ConfigureWithoutCarrier = true;
  };
};

systemd.network.networks."21-microvm-tap" = {
  matchConfig.Name = "microvm*";
  networkConfig.Bridge = "microbr";
};

networking.nat = {
  enable = true;
  internalInterfaces = [ "microbr" ];
  externalInterface = "eno1";
};

Step 2: `flake.nix`

Then, I added the microvm module as a new input to my flake.nix (check out the microvm.nix documentation for details) and enabled the microvm.nixosModules.host module on the NixOS configuration for my PC (midna). I also created a new microvm.nix file, in which I declare all my VMs. Here’s what my flake.nix looks like:

{
  inputs = {
    nixpkgs = {
      url = "github:nixos/nixpkgs/nixos-25.11";
    };
    # For more recent claude-code
    nixpkgs-unstable = {
      url = "github:nixos/nixpkgs/nixos-unstable";
    };
    stapelbergnix = {
      url = "github:stapelberg/nix";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    zkjnastools = {
      url = "github:stapelberg/zkj-nas-tools";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    microvm = {
      url = "github:microvm-nix/microvm.nix";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    home-manager = {
      url = "github:nix-community/home-manager/release-25.11";
      inputs.nixpkgs.follows = "nixpkgs";
    };
    configfiles = {
      url = "github:stapelberg/configfiles";
      flake = false; # repo is not a flake
    };
  };

  outputs =
    {
      self,
      stapelbergnix,
      zkjnastools,
      nixpkgs,
      nixpkgs-unstable,
      microvm,
      home-manager,
      configfiles,
    }@inputs:
    let
      system = "x86_64-linux";
      pkgs = import nixpkgs {
        inherit system;
        config.allowUnfree = false;
      };
      pkgs-unstable = import nixpkgs-unstable {
        inherit system;
        config.allowUnfree = true;
      };
    in
    {
      nixosConfigurations = {
        midna = nixpkgs.lib.nixosSystem {
          system = "x86_64-linux";
          specialArgs = { inherit inputs; };
          modules = [
            (import ./configuration.nix)
            stapelbergnix.lib.userSettings
            # Use systemd for network configuration
            stapelbergnix.lib.systemdNetwork
            # Use systemd-boot as bootloader
            stapelbergnix.lib.systemdBoot
            # Run prometheus node exporter in tailnet
            stapelbergnix.lib.prometheusNode
            zkjnastools.nixosModules.zkjbackup
            microvm.nixosModules.host
            ./microvm.nix
          ];
        };
      };
    };
}

Step 3: `microvm.nix`

The following microvm.nix declares two microvms, one for Emacs (about which I wanted to learn more) and one for Go Protobuf, a code base I am familiar with and can use to understand Claude’s capabilities:

{
  config,
  lib,
  pkgs,
  inputs,
  ...
}:

let
  inherit (inputs)
    nixpkgs-unstable
    stapelbergnix
    microvm
    configfiles
    home-manager
    ;

  microvmBase = import ./microvm-base.nix;
in
{
  microvm.vms.emacsvm = {
    autostart = false;
    config = {
      imports = [
        stapelbergnix.lib.userSettings
        microvm.nixosModules.microvm
        (microvmBase {
          hostName = "emacsvm";
          ipAddress = "192.168.83.6";
          tapId = "microvm4";
          mac = "02:00:00:00:00:05";
          workspace = "/home/michael/microvm/emacs";
          inherit
            nixpkgs-unstable
            configfiles
            home-manager
            stapelbergnix
            ;
        })
        ./microvms/emacs.nix
      ];
    };
  };

  microvm.vms.goprotobufvm = {
    autostart = false;
    config = {
      imports = [
        stapelbergnix.lib.userSettings
        microvm.nixosModules.microvm
        (microvmBase {
          hostName = "goprotobufvm";
          ipAddress = "192.168.83.7";
          tapId = "microvm5";
          mac = "02:00:00:00:00:06";
          workspace = "/home/michael/microvm/goprotobuf";
          inherit
            nixpkgs-unstable
            configfiles
            home-manager
            stapelbergnix
            ;
          extraZshInit = ''
            export GOPATH=$HOME/go
            export PATH=$GOPATH/bin:$PATH
          '';
        })
        ./microvms/goprotobuf.nix
      ];
    };
  };
}

Step 4: `microvm-base.nix`

The microvm-base.nix module takes these parameters and declares:

Network settings: I like using systemd-networkd(8) and systemd-resolved(8) .
Shared directories for:
- the workspace directory, e.g. ~/microvm/emacs
- the host’s Nix store, so the VM can access software from cache (often)
- this VM’s SSH host keys
- ~/claude-microvm, which is a separate state directory, used only on the microvms.
an 8 GB disk overlay (var.img), stored in /var/lib/microvms/
cloud-hypervisor (QEMU also works well!) as the hypervisor, with 8 vCPUs and 4 GB RAM.
A workaround for systemd trying to unmount /nix/store (which causes a deadlock).

Expand full microvm-base.nix code

Michael Stapelbergs Website: posts tagged ai

Coding Agent VMs on NixOS with microvm.nix

See also

Setting up microvm.nix

Step 1: network prep

Step 2: `flake.nix`

Step 3: `microvm.nix`

Step 4: `microvm-base.nix`

Michael Stapelbergs Website: posts tagged ai

Coding Agent VMs on NixOS with microvm.nix

See also

Setting up microvm.nix

Step 1: network prep

Step 2: flake.nix

Step 3: microvm.nix

Step 4: microvm-base.nix

Step 2: `flake.nix`

Step 3: `microvm.nix`

Step 4: `microvm-base.nix`