README.md

@@ -55,17 +55,17 @@ module reaches a host: **baseModules** (every NixOS host, via `flake.nix`),
 **host table** (listed explicitly per host in `flake.nix`), or **transitively**
 (pulled in by another module's `imports`).
 
-| Module             | Imported by                       | What it does / when to use it                                                                                                                                                                             |
+| Module             | Imported by                       | What it does / when to use it                                                                                                                                                                                                                                                                                                 |
-| ------------------ | --------------------------------- | --------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
+| ------------------ | --------------------------------- | ----------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------------- |
-| `common-nixos.nix` | baseModules (all NixOS)           | Timezone/locale, store hygiene (auto-optimise, big download buffer, **no** auto-GC), the nix-community binary cache, `nix-ld`, base CLI (`git`, `fastfetch`), and the fleet-wide font stack.              |
+| `common-nixos.nix` | baseModules (all NixOS)           | Timezone/locale, store hygiene (auto-optimise, big download buffer, **no** auto-GC), the nix-community binary cache, `nix-ld`, base CLI (`git`, `fastfetch`), and the fleet-wide font stack.                                                                                                                                  |
-| `users.nix`        | baseModules (all NixOS)           | Builds `users.users` from the registry for the host's `hostUsers`; enables zsh; enables Firefox + Thunderbird **only** when `features.swayDesktop.enable` is on. Applies per-user `linger`.               |
+| `users.nix`        | baseModules (all NixOS)           | Builds `users.users` from the registry for the host's `hostUsers`; enables zsh; enables Firefox + Thunderbird **only** when `features.swayDesktop.enable` is on. Applies per-user `linger`.                                                                                                                                   |
-| `features.nix`     | baseModules (all NixOS)           | Declares feature-flag options (currently `features.swayDesktop.enable`) so any host can read/set them without importing the heavy implementation module.                                                  |
+| `features.nix`     | baseModules (all NixOS)           | Declares the feature-flag options (`features.swayDesktop.enable`, `features.claudeCode.enable`) so any host can read/set them without importing the heavy implementation module, plus the CPU capability fact they derive from (`features.cpu.microarchLevel`) and the assertion that guards it. See "CPU capability gating". |
-| `workstation.nix`  | transitively (via laptop/desktop) | Form-factor-agnostic base for physical graphical hosts: turns on `swayDesktop`, Dvorak console, PipeWire, firewall (default-deny), fstrim, earlyoom, fwupd, thermald (x86), redistributable fw.           |
+| `workstation.nix`  | transitively (via laptop/desktop) | Form-factor-agnostic base for physical graphical hosts: turns on `swayDesktop`, Dvorak console, PipeWire, firewall (default-deny), fstrim, earlyoom, fwupd, thermald (x86), redistributable fw.                                                                                                                               |
-| `laptop.nix`       | host table (MBP, T400)            | `imports` workstation.nix, then adds the portable bits: iwd Wi-Fi, lid suspend/lock, Bluetooth + blueman.                                                                                                 |
+| `laptop.nix`       | host table (MBP, T400)            | `imports` workstation.nix, then adds the portable bits: iwd Wi-Fi, lid suspend/lock, Bluetooth + blueman.                                                                                                                                                                                                                     |
-| `desktop.nix`      | host table (Mac Pro)              | `imports` workstation.nix, then swaps Wi-Fi for wired NetworkManager. Pair with `portable = false` in the host table.                                                                                     |
+| `desktop.nix`      | host table (Mac Pro)              | `imports` workstation.nix, then swaps Wi-Fi for wired NetworkManager. Pair with `portable = false` in the host table.                                                                                                                                                                                                         |
-| `sway.nix`         | host table (graphical hosts)      | Implementation of `features.swayDesktop`: the system Sway package, the greetd/ReGreet (cage) greeter forced to Dvorak, xdg-portal, Wayland utility packages. Home-side Sway config is in `home/sway.nix`. |
+| `sway.nix`         | host table (graphical hosts)      | Implementation of `features.swayDesktop`: the system Sway package, the greetd/ReGreet (cage) greeter forced to Dvorak, xdg-portal, Wayland utility packages. Home-side Sway config is in `home/sway.nix`.                                                                                                                     |
-| `ssh.nix`          | host table (T400, Mac Pro, RPi5)  | Enables sshd, opens port 22, enforces a key-only policy (no password / keyboard-interactive, no root). Authorized keys come from the registry via `users.nix`.                                            |
+| `ssh.nix`          | host table (T400, Mac Pro, RPi5)  | Enables sshd, opens port 22, enforces a key-only policy (no password / keyboard-interactive, no root). Authorized keys come from the registry via `users.nix`.                                                                                                                                                                |
-| `firmware/`        | referenced by MBP host config     | Committed Apple peripheral firmware blobs for the Asahi MBP (see "MacBook (Asahi) firmware").                                                                                                             |
+| `firmware/`        | referenced by MBP host config     | Committed Apple peripheral firmware blobs for the Asahi MBP (see "MacBook (Asahi) firmware").                                                                                                                                                                                                                                 |
 
 Form-factor decision: a **laptop** imports `laptop.nix` (default
 `portable = true`); a **wired desktop** imports `desktop.nix` and sets
@@ -74,6 +74,29 @@ Form-factor decision: a **laptop** imports `laptop.nix` (default
 serves. `portable` is threaded through to `home/sway.nix`, which drops the
 battery block and brightness keys on desktops.
 
+## CPU capability gating
+
+Not every host can run everything the fleet installs. Nix cannot probe the CPU
+(evaluation is pure, and a host may be built elsewhere), so each machine
+declares what it is and the shared modules derive from that:
+
+- `features.cpu.microarchLevel` — the x86-64 psABI level the CPU implements
+  (1 = baseline, 2 = SSE4.2/POPCNT, 3 = AVX2, 4 = AVX-512). Defaults to **2**;
+  only a host older than that sets it (the Mac Pro 3,1's 2008 Harpertown Xeons
+  are level 1). Ignored on non-x86_64 hosts.
+- `features.claudeCode.enable` — derived: on unless the host is below
+  x86-64-v2, because Claude Code's Node runtime needs SSE4.2/POPCNT.
+  [`home/claude.nix`](./home/claude.nix) reads it through home-manager's
+  `osConfig` and installs nothing (CLI, `CLAUDE.md`, output style, memory
+  symlink) when it is off. Hosts with no such option — the Darwin host and the
+  standalone `homeConfigurations` — fall back to enabled.
+- An assertion in `features.nix` fails evaluation if a host force-enables a
+  flag its declared CPU level cannot support, so the mistake surfaces in
+  `nix flake check`/CI rather than as an illegal-instruction crash on the box.
+
+Adding another CPU-sensitive tool means deriving one more flag there, not
+editing every host.
+
 ## Users
 
 Identity is data, kept separate from the reusable modules:

flake.nix

@@ -111,8 +111,14 @@
         ];
 
         # Unfree packages permitted to be built (replaces blanket allowUnfree).
+        # The NVIDIA entries are for the Mac Pro's Quadro P400 (hosts/MacPro31/
+        # nvidia.nix); unfree packages are not in the binary cache, so the
+        # kernel module is compiled on the host.
         unfreePackages = [
           "claude-code"
+          "nvidia-x11"
+          "nvidia-kernel-modules"
+          "nvidia-settings"
         ];
 
         # Per-user identity, keyed by username. See README "Users".

home/README.md

@@ -175,8 +175,16 @@ forced off there) but still runs the agent.
 
 ## Claude Code
 
-Managed declaratively by [`claude.nix`](./claude.nix) on every host (the CLI is
+Managed declaratively by [`claude.nix`](./claude.nix) on every host whose CPU
-`pkgs.claude-code`, tracked to unstable via the flake overlay).
+can run it (the CLI is `pkgs.claude-code`, tracked to unstable via the flake
+overlay).
+
+**Capability gate.** The module installs nothing — CLI or files — when
+`osConfig.features.claudeCode.enable` is off. That flag is derived fleet-wide
+from the host's declared CPU level (see "CPU capability gating" in the root
+README): the Node runtime needs SSE4.2/POPCNT, so anything below x86-64-v2 (the
+Mac Pro 3,1) is excluded. Hosts that do not define the option — the Darwin host
+and the standalone `homeConfigurations` — keep it enabled.
 
 | Managed (static, from Nix)                          | Left mutable (runtime state)                           |
 | --------------------------------------------------- | ------------------------------------------------------ |

home/claude.nix

@@ -1,4 +1,5 @@
-# Claude Code, configured declaratively via home-manager. Wanted on every host.
+# Claude Code, configured declaratively via home-manager. Wanted on every host
+# whose CPU can run it -- see the gate below.
 #
 # The STATIC config is managed here: the global CLAUDE.md (persona/context), the
 # custom output style, and the auto-memory directory. settings.json is
@@ -10,18 +11,33 @@
 # read-only into ~/.claude/memory, so the runtime "save a memory" path no longer
 # writes there -- recall still works, but new/changed memories must be added to
 # this repo and rebuilt. CLAUDE.md instructs Claude to do exactly that.
-{ ... }:
+{
+  lib,
+  # Set by the NixOS/Darwin home-manager module; absent for the standalone
+  # homeConfigurations, hence the default.
+  osConfig ? { },
+  ...
+}:
+let
+  # Capability gate, declared once for the whole fleet in modules/features.nix
+  # (default: on; off on CPUs below x86-64-v2, which cannot run the Node
+  # runtime Claude Code ships on). Hosts without that option -- the Darwin host
+  # and the portable standalone profile -- fall back to enabled.
+  enable = osConfig.features.claudeCode.enable or true;
+in
 {
   programs.claude-code = {
-    enable = true;
+    inherit enable;
     # package defaults to pkgs.claude-code (tracked to unstable via the flake
-    # overlay); installs the CLI on every host.
+    # overlay).
 
     # ~/.claude/CLAUDE.md -- global instructions / persona / memory workflow.
     context = ./claude/CLAUDE.md;
   };
 
-  home.file = {
+  # Nothing to place when the CLI is not installed: a ~/.claude/memory symlink
+  # with no Claude Code to read it is just dead state.
+  home.file = lib.mkIf enable {
     # Custom output style. The module has no option for output-styles/, so place
     # it directly; selection (settings.json `outputStyle`) stays mutable.
     ".claude/output-styles/soviet-engineer.md".source = ./claude/output-styles/soviet-engineer.md;

hosts/MacPro31/README.md

@@ -1,7 +1,7 @@
 # Mac Pro 3,1 (Early 2008) — install notes
 
 Flake host: `lyrathorpe-macpro31`. Desktop (`portable = false`, imports
-`../../modules/desktop.nix`). Files: `configuration.nix`,
+`../../modules/desktop.nix`). Files: `configuration.nix`, `nvidia.nix`,
 `hardware-configuration.nix`.
 
 ## Hardware configuration
@@ -27,18 +27,95 @@ either
 
 Partition the disk GPT with an ESP (vfat).
 
-## Graphics
+## Graphics — NVIDIA Quadro P400
 
-The stock card varies between units — **ATI Radeon HD 2600 XT** or **NVIDIA
+The stock card (**ATI Radeon HD 2600 XT** or **NVIDIA GeForce 8800 GT**,
-GeForce 8800 GT**. No proprietary driver is hardcoded; Sway relies on in-tree KMS:
+depending on the unit) has been replaced with an **NVIDIA Quadro P400** (Pascal,
+GP108). Everything driver-related lives in [`nvidia.nix`](./nvidia.nix):
 
-- ATI Radeon HD 2600 XT → `radeon` (or `amdgpu`) KMS
+- **Driver branch 580** (`nvidiaPackages.legacy_580`), _not_ the nixpkgs default
-- NVIDIA GeForce 8800 GT → `nouveau` KMS
+  (`production`, currently 595.x). 580 is the last branch that supports
+  Maxwell/Pascal/Volta and is maintained as an LTS branch until Aug 2028; a
+  newer branch does not drive this card at all.
+- `modesetting.enable = true` — mandatory for Wayland (sets
+  `nvidia-drm.modeset=1`); without it wlroots gets no GBM device and both Sway
+  and the greeter fail to start.
+- `open = false` — the open kernel modules require Turing or later.
+- Sway runs with `--unsupported-gpu` (`programs.sway.extraOptions`); wlroots
+  refuses the proprietary driver otherwise. `cage`/ReGreet needs no such flag.
+- nouveau and `nvidiafb` are blacklisted automatically by the NVIDIA module.
 
-These come up automatically. If a card needs forcing, set
+The driver is unfree, so it is **not in the binary cache**: the kernel module is
-`services.xserver.videoDrivers` and/or add the module to
+compiled on the machine, which on these 2008 Xeons is slow — budget for a long
-`boot.initrd.kernelModules` for early KMS (see the comment in
+first rebuild and again after every kernel bump. The package names are
-`configuration.nix`).
+allowlisted in `unfreePackages` in [`flake.nix`](../../flake.nix).
+
+Note the Mac Pro shows no EFI boot screen with a stock PC card (no Apple EFI
+ROM): the machine boots blind until KMS brings the display up. That is expected,
+not a fault.
+
+Verify after a rebuild:
+
+```sh
+nvidia-smi
+```
+
+## Docker with CUDA
+
+`nvidia.nix` also enables Docker and gives containers GPU access via **CDI**
+(`hardware.nvidia-container-toolkit.enable`), which generates device specs from
+the host driver at boot (regenerated by a udev rule when the `nvidia` device
+appears) and turns on the daemon's CDI feature:
+
+```sh
+docker run --rm --device=nvidia.com/gpu=all nvidia/cuda:12.9.1-base-ubuntu24.04 nvidia-smi
+```
+
+- Use the `--device=nvidia.com/gpu=all` form. `--gpus all` is the legacy
+  runtime-wrapper path (`virtualisation.docker.enableNvidia`), which is
+  deprecated upstream and deliberately not enabled here.
+- **CUDA version matters.** The P400 is compute capability 6.1 (`sm_61`); CUDA
+  13 dropped Maxwell/Pascal/Volta, so container images must ship a **CUDA 12.x
+  or older** runtime. The 580 driver itself is happy with either.
+- 2 GB of VRAM, 256 CUDA cores — fine for encode/decode and small models, not
+  for training anything serious.
+- Docker socket is local-only (no TCP listener, unlike the Pi). Users need the
+  `docker` group; the registry already grants it.
+
+### "Driver Not Loaded" from the CDI generator
+
+`nvidia-container-toolkit-cdi-generator.service` fails with
+`failed to initialize NVML: Driver Not Loaded` whenever the `nvidia` kernel
+module is not loaded in the **running** kernel. After a kernel bump that is
+unavoidable — the rebuilt module cannot load until reboot — so the unit is
+guarded with `ConditionPathExists=/proc/driver/nvidia/version` and skips
+instead of failing. Without that guard it also takes `docker.service`
+(`requiredBy`) with it and makes `nixos-rebuild switch` exit non-zero.
+
+**Reboot after a rebuild that touches the driver or the kernel.** The toolkit's
+udev rule restarts the generator when the GPU device appears, so the CDI specs
+are written on the next boot. To check the state:
+
+```sh
+lsmod | grep nvidia            # nvidia, nvidia_modeset, nvidia_drm, nvidia_uvm
+cat /proc/driver/nvidia/version
+nvidia-smi
+systemctl status nvidia-container-toolkit-cdi-generator.service
+ls /var/run/cdi                # the generated spec
+```
+
+If the module is genuinely absent after a reboot, check `dmesg | grep -i
+nvidia` (build/version mismatch, or nouveau still bound — the module blacklists
+it, so that should not happen).
+
+## Claude Code — not installed here
+
+The dual Harpertown Xeons are **x86-64-v1** (SSE4.1, but no SSE4.2/POPCNT) and
+the Node runtime Claude Code ships on requires x86-64-v2. `configuration.nix`
+declares `features.cpu.microarchLevel = 1`, which switches the tool off through
+the fleet-wide gate in [`../../modules/features.nix`](../../modules/features.nix)
+— see the root README. Forcing `features.claudeCode.enable` on here is an
+evaluation error, not a broken install.
 
 ## Networking
 
@@ -52,7 +129,7 @@ Graphical login via a Wayland greeter — `greetd` running ReGreet inside the
 every Sway host (gated on `features.swayDesktop.enable`). The greeter is forced
 to the Dvorak layout to match the console and Sway session. Set the user
 password (`passwd lyrathorpe`) after install, or the greeter cannot
-authenticate. Requires working KMS (radeon/nouveau — see Graphics).
+authenticate. Requires working KMS (NVIDIA modesetting — see Graphics).
 
 ## Apply
 

hosts/MacPro31/configuration.nix

@@ -7,8 +7,14 @@
 {
   imports = [
     ./hardware-configuration.nix
+    ./nvidia.nix
   ];
 
+  # Dual quad-core Xeon (Harpertown/Penryn): SSE4.1 but no SSE4.2 or POPCNT,
+  # i.e. x86-64-v1. Declaring it here switches off the fleet flags that need a
+  # newer CPU -- currently features.claudeCode (see ../../modules/features.nix).
+  features.cpu.microarchLevel = 1;
+
   # The Mac Pro 3,1 has 64-bit EFI (confirmed by the owner), so boot via
   # systemd-boot like the MBP -- no GRUB/BIOS shim needed.
   boot.loader.systemd-boot.enable = true;
@@ -33,17 +39,9 @@
   # enabled in workstation.nix.
   hardware.cpu.intel.updateMicrocode = true;
 
-  # GPU note: the stock card varies between units -- ATI Radeon HD 2600 XT or
+  # GPU: the stock card (ATI Radeon HD 2600 XT / NVIDIA GeForce 8800 GT) has
-  # NVIDIA GeForce 8800 GT. Sway needs a working KMS/modesetting driver; do NOT
+  # been replaced with an NVIDIA Quadro P400. Driver, Wayland quirks and
-  # install a proprietary blob here. Depending on the installed card, rely on
+  # GPU-enabled Docker live in ./nvidia.nix.
-  # the open kernel driver:
-  #   - ATI Radeon HD 2600 XT -> "radeon" (older) or "amdgpu" KMS
-  #   - NVIDIA GeForce 8800 GT -> "nouveau" KMS
-  # These come up automatically via the in-tree drivers + KMS, and the graphics
-  # stack itself is enabled by modules/sway.nix. If a card needs to be forced, add it
-  # here, e.g. `services.xserver.videoDrivers = [ "radeon" ];` (or "nouveau"),
-  # and/or `boot.initrd.kernelModules = [ "radeon" ];` in
-  # hardware-configuration.nix for early KMS.
 
   # See `man configuration.nix` / the stateVersion docs before changing.
   system.stateVersion = "26.05";

hosts/MacPro31/nvidia.nix

@@ -0,0 +1,66 @@
+# NVIDIA Quadro P400 (Pascal, GP108) on the Mac Pro 3,1: proprietary driver for
+# the Sway desktop, plus Docker with GPU/CUDA access for containers.
+#
+# Driver branch: 580 (nvidiaPackages.legacy_580), NOT the nixpkgs default
+# (`production`, currently 595.x). 580 is the last branch that supports
+# Maxwell/Pascal/Volta -- NVIDIA keeps it as an LTS branch to Aug 2028 -- and a
+# newer branch simply will not drive this card.
+#
+# The driver is unfree, so it is not in the binary cache: the kernel module is
+# compiled locally. On this machine's 2008 Xeons expect the first rebuild after
+# a kernel bump to take a long while.
+{ config, ... }:
+
+{
+  # Selects the proprietary driver; the module blacklists nouveau/nvidiafb and
+  # loads nvidia-uvm (needed by CUDA) via modprobe softdep. Naming is historical
+  # -- this option drives the kernel/driver choice on Wayland hosts too, which
+  # is why it is set on a machine that runs no X server.
+  services.xserver.videoDrivers = [ "nvidia" ];
+
+  hardware.nvidia = {
+    package = config.boot.kernelPackages.nvidiaPackages.legacy_580;
+    # Required for Wayland: sets nvidia-drm.modeset=1 (and fbdev=1), without
+    # which wlroots gets no GBM device and Sway/cage fail to start.
+    modesetting.enable = true;
+    # The open kernel modules need Turing or later; Pascal must use the closed
+    # ones. Explicit because the option has no default on driver >= 560.
+    open = false;
+  };
+
+  # The NVIDIA module only puts these in boot.kernelModules when
+  # services.xserver.enable is true, which is false on this Wayland-only host --
+  # so load them explicitly rather than relying on udev modalias autoloading.
+  # nvidia_uvm (needed by CUDA) is deliberately absent: the module's modprobe
+  # softdep pulls it in after the GPU device exists, which is the supported
+  # ordering.
+  boot.kernelModules = [
+    "nvidia"
+    "nvidia_modeset"
+    "nvidia_drm"
+  ];
+
+  # wlroots refuses the proprietary NVIDIA driver unless told to proceed. The
+  # greeter's compositor (cage) has no such check; only Sway needs the flag,
+  # which the module bakes into the wrapper the session's .desktop file runs.
+  programs.sway.extraOptions = [ "--unsupported-gpu" ];
+
+  virtualisation.docker.enable = true;
+
+  # CDI-based GPU access for containers: generates /var/run/cdi specs from the
+  # host driver at boot and turns on Docker's CDI feature. Run GPU workloads
+  # with `docker run --device=nvidia.com/gpu=all ...`. The deprecated
+  # virtualisation.docker.enableNvidia runtime wrapper is deliberately not used.
+  hardware.nvidia-container-toolkit.enable = true;
+
+  # The generator needs a loaded kernel module: without one it aborts with
+  # "failed to initialize NVML: Driver Not Loaded". That is guaranteed after a
+  # kernel bump, where the rebuilt module cannot load until reboot -- and since
+  # the unit is requiredBy docker.service and wantedBy multi-user.target, the
+  # failure takes Docker down and makes `nixos-rebuild switch` exit non-zero.
+  # Skip the run instead when no driver is loaded; the toolkit's udev rule
+  # restarts the unit as soon as the nvidia device appears, so the CDI specs are
+  # still generated on the next boot.
+  systemd.services.nvidia-container-toolkit-cdi-generator.unitConfig.ConditionPathExists =
+    "/proc/driver/nvidia/version";
+}

modules/features.nix

@@ -6,7 +6,72 @@
 # headless host (e.g. the Pi) must be able to leave it at its default without
 # pulling in modules/sway.nix. The implementation lives in modules/sway.nix,
 # gated on this flag.
-{ lib, ... }:
+#
+# The file also carries the host capability facts those flags derive from
+# (features.cpu.*). features.claudeCode.enable is such a derived flag: it is
+# computed from the declared CPU level here and read by home/claude.nix through
+# home-manager's osConfig, so a machine that cannot run the tool never installs
+# it, on any host, without per-host opt-outs.
 {
-  options.features.swayDesktop.enable = lib.mkEnableOption "the Sway desktop";
+  config,
+  lib,
+  pkgs,
+  ...
+}:
+let
+  cfg = config.features;
+
+  # Claude Code runs on Node, whose V8 build requires SSE4.2 and POPCNT -- the
+  # x86-64-v2 feature set. On an older x86_64 CPU it does not run (illegal
+  # instruction), so it must not be installed there.
+  claudeCodeMinLevel = 2;
+  claudeCodeSupported =
+    !pkgs.stdenv.hostPlatform.isx86_64 || cfg.cpu.microarchLevel >= claudeCodeMinLevel;
+in
+{
+  options.features = {
+    swayDesktop.enable = lib.mkEnableOption "the Sway desktop";
+
+    cpu.microarchLevel = lib.mkOption {
+      type = lib.types.ints.between 1 4;
+      default = 2;
+      example = 1;
+      description = ''
+        The x86-64 psABI microarchitecture level the host CPU implements:
+        1 = the original baseline, 2 = SSE4.2/POPCNT (Nehalem, 2008+),
+        3 = AVX2, 4 = AVX-512.
+
+        Nix cannot detect this (evaluation is pure and hosts are often built
+        elsewhere), so a machine older than the default declares its own level
+        and the flags below derive from it. Ignored on non-x86_64 hosts.
+      '';
+    };
+
+    claudeCode.enable = lib.mkOption {
+      type = lib.types.bool;
+      default = claudeCodeSupported;
+      defaultText = lib.literalMD ''
+        `true`, unless the host declares an x86-64 microarchitecture level
+        below ${toString claudeCodeMinLevel}
+      '';
+      description = ''
+        Whether to install Claude Code in this host's home-manager profiles
+        (implemented in home/claude.nix). Defaults off on CPUs below
+        x86-64-v${toString claudeCodeMinLevel}, which cannot run it; forcing it
+        on such a host is an evaluation error.
+      '';
+    };
+  };
+
+  config.assertions = [
+    {
+      assertion = cfg.claudeCode.enable -> claudeCodeSupported;
+      message = ''
+        features.claudeCode.enable is on, but this host declares
+        features.cpu.microarchLevel = ${toString cfg.cpu.microarchLevel}.
+        Claude Code needs x86-64-v${toString claudeCodeMinLevel}
+        (SSE4.2/POPCNT) and will not run on an older CPU.
+      '';
+    }
+  ];
 }