README.md

@@ -56,10 +56,10 @@ module reaches a host: **baseModules** (every NixOS host, via `flake.nix`),
 (pulled in by another module's `imports`).
 
 | 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.                                                                                                                                  |
 | `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.                                                                                                                               |
 | `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.                                                                                                                                                                                                         |
@@ -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:

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/nvidia.nix

@@ -0,0 +1,43 @@
+# 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;
+  };
+
+  # 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;
+}

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.
+      '';
+    }
+  ];
 }