Nix Setup Skill

Reference for setting up development environments that assume Nix flakes (flakes + nix-command). Per-language templates are collected under assets/ so you can drop them in with a single cp.

When to use

• "Create a nix dev shell" / "Write a flake.nix"
• "Add nix to a new project"
• "Make nix work on claude code web / in a container"
• General questions about flake.nix / flake.lock / .envrc
• Troubleshooting nix develop / nix build

What's in assets/

assets/
├── setup_nix.sh              # Installer that sets up single-user Nix with sandbox disabled (for container / CCW)
├── apm.nix                   # Nix derivation for microsoft/apm (Agent Package Manager)
├── moonbit/{flake.nix,.envrc}        # moonbit-overlay + moon
├── rust/{flake.nix,.envrc}           # rust-overlay + stable pinned + cargo-nextest/watch
├── typescript/{flake.nix,.envrc}     # nodejs_24 + pnpm (top-level)
├── python-uv/{flake.nix,.envrc}      # python3 + uv
├── haskell/{flake.nix,.envrc}        # GHC + cabal + HLS + hlint + ormolu
├── ocaml/{flake.nix,.envrc,.gitignore}  # OCaml 5 + dune + merlin + ocaml-lsp + utop
├── oxcaml/{flake.nix,.envrc,.gitignore} # Jane Street OxCaml (opam source build)
└── home-manager/             # multi-host home-manager flake (macos + ccweb)
    ├── flake.nix
    ├── common.nix
    ├── macos.nix             # aarch64-darwin: full desktop
    ├── ccweb.nix             # x86_64-linux: minimal sandbox
    └── .gitignore            # Excludes private.nix / *.secret.nix

Every template ships with just, ast-grep, and apm (shared operational tools).

The templates assume you place flake.nix + apm.nix side-by-side at the project root. If you want to swap out the language side, apm.nix is reusable as-is.

apm.nix is optional: you can delete it for projects that don't use the APM skill. In that case, remove both apm = import ./apm.nix ... and the apm entry inside packages from flake.nix. If you only want to keep just / ast-grep, leave it alone.

Quick install

macOS / Linux (recommended)

Use the Determinate Systems installer. experimental-features = nix-command flakes is enabled out of the box, and uninstalling is easy.

curl --proto '=https' --tlsv1.2 -sSf -L https://install.determinate.systems/nix | sh -s -- install

Claude Code web / other sandbox environments

For environments that lack systemd, have only root, or forbid nested namespaces, use assets/setup_nix.sh.

• build-users-group = (empty) for single-user mode
• sandbox = false so chroot is not required
• Writes experimental-features = nix-command flakes to both /etc/nix/nix.conf and $HOME/.config/nix/nix.conf
• Drops /etc/profile.d/nix.sh so PATH works in subsequent shells

cp ~/.claude/skills/nix-setup/assets/setup_nix.sh .
bash setup_nix.sh
. "$HOME/.nix-profile/etc/profile.d/nix.sh"  # apply to the current shell too
nix --version

Minimal flake skeleton

{
  description = "...";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixpkgs-unstable";
    flake-utils.url = "github:numtide/flake-utils";
  };

  outputs = { self, nixpkgs, flake-utils }:
    flake-utils.lib.eachDefaultSystem (system:
      let pkgs = import nixpkgs { inherit system; };
      in {
        devShells.default = pkgs.mkShell {
          packages = [ /* ... */ ];
          shellHook = ''...'';
        };
      });
}

nix develop drops you into devShells.default. With direnv, write use flake to .envrc and run direnv allow.

Shared tools (included in every template)

Tool	Purpose	Why it's always included
just	Task runner	justfile is the project standard
ast-grep	Structural search / lint	Enforces statically the rules you can't write with grep
apm	Agent dependency manager	Distributes skills/prompts reproducibly via apm.yml

Using the per-language templates

Installing a template is just a matter of placing flake.nix + apm.nix side-by-side at the project root:

# Example: MoonBit project
cp ~/.claude/skills/nix-setup/assets/apm.nix .
cp ~/.claude/skills/nix-setup/assets/moonbit/flake.nix .

nix develop           # builds on the first run
# If you're a direnv user:
echo 'use flake' > .envrc && direnv allow

MoonBit

• Uses moonbit-overlay.packages.${system}.moon-patched_latest
• The overlay is pinned to a known-working revision (50118f5c3c0298b5cb17cc6f1c346165801014c8). The latest HEAD sometimes depends on packages marked as broken on nixpkgs-unstable (e.g. tcc), so evaluation can fail for stretches. Verify before updating.
• Runs moon update automatically when moon.mod.json is present (first time only)
• Reference: vibe-lang's flake.nix

Rust

• Toolchain management via rust-overlay
• Default is stable.latest.default + rust-analyzer / clippy / rustfmt
• Pin to a specific version: rust-bin.stable."1.91.0".default
• rust-toolchain.toml integration: rust-bin.fromRustupToolchainFile ./rust-toolchain.toml
• WASM targets: targets = [ "wasm32-wasip1" "wasm32-unknown-unknown" ]

TypeScript

• nodejs_24 + pkgs.pnpm (top-level)
• nodePackages.* was removed from nixpkgs in 2025 — nodePackages.pnpm no longer works
• Point PNPM_HOME at a project-local path ($PWD/.pnpm) to avoid polluting $HOME
• To switch to npm / yarn, drop pkgs.pnpm and use the npm bundled with pkgs.nodejs_24, or add pkgs.yarn

Python + uv

• pkgs.python3 + pkgs.uv
• uv manages Python versions itself, so the nix side is just a fallback
• UV_PROJECT_ENVIRONMENT=$PWD/.venv, UV_CACHE_DIR=$PWD/.uv-cache keep $HOME clean

Haskell

• pkgs.haskellPackages.{ghc,cabal-install,haskell-language-server,hlint,ormolu,ghcid}
• Uses the nixpkgs default (haskellPackages). To pin the GHC version, swap in pkgs.haskell.packages.ghc98 or similar.
• If you need stricter reproducibility, consider haskell.nix (IOHK), though the learning curve and complexity jump considerably.
• To use stack, add pkgs.stack and switch to pinning GHC via stack.yaml's resolver.
• On macOS, stdenv.cc.cc.lib is not on PATH, which trips up some FFI cases → append things like buildInputs = [ pkgs.zlib ].

OCaml

• pkgs.ocamlPackages.{ocaml,dune_3,findlib,merlin,ocaml-lsp,ocamlformat,utop} + pkgs.opam
• nixpkgs default OCaml 5.x series. Pin with pkgs.ocaml-ng.ocamlPackages_5_2, etc.
• opam management: shellHook sets OPAMROOT=$PWD/.opam to confine state to the project, running opam init + opam switch create default --empty automatically on the first run. Subsequent runs only eval opam env.
• Using an empty switch lets the nix-provided ocaml be used directly, avoiding dual compiler management.
• .gitignore includes .opam/ so state can't be accidentally committed.
• Additional libraries land project-local via opam install <pkg>.
• merlin is a dependency of ocaml-lsp. Installing both gives complete editor integration.

OxCaml (Jane Street's fork)

• Not in nixpkgs. Follows the officially recommended "opam + source build" flow; the nix side only provides opam and the build toolchain (autoconf, automake, m4, pkg-config, gmp, libffi).
• On the first run of shellHook:
  1. Create $OPAMROOT/.opam
  2. Create the oxcaml-dev switch as empty
  3. Pin the upstream repo with opam pin add -ny git+https://github.com/oxcaml/oxcaml
  4. Set the invariant with opam switch set-invariant --packages oxcaml-dev
• The actual compiler build is not run automatically (takes 5-20 min / ~1 GB). Users run this explicitly:

  opam install oxcaml-dev
  opam install dune merlin ocaml-lsp-server ocamlformat utop
  

• Supported: arm64 macOS / arm64 Linux / x86_64 Linux. x86_64 macOS works but is unofficially supported.
• The ocaml template uses "the same opam infrastructure but a different switch", so you can flip between the two templates.

How apm.nix works

microsoft/apm is distributed not as an npm package but as a PyInstaller-bundled native binary.

• _internal/ ships a Python 3.12 runtime + dependent libraries
• The apm binary alone references _internal as a sibling directory, so you can't extract it and use it standalone
• On Linux, autoPatchelfHook rewrites the links to glibc / libstdc++ / zlib so they point into the nix store
• Everything is placed under $out/libexec/apm/, and $out/bin/apm is a makeWrapper wrapper pointing there
• dontStrip = true + dontPatchELF = true are required — PyInstaller appends a PKG archive to the tail of the Mach-O / ELF. stdenv's default strip / patchelf cut off the tail, so without disabling those the binary fails at runtime with Could not load PyInstaller's embedded PKG archive.

Version update procedure:

# 1. Fetch the SHA256s for the new release
for a in darwin-arm64 darwin-x86_64 linux-arm64 linux-x86_64; do
  curl -sSL "https://github.com/microsoft/apm/releases/download/vX.Y.Z/apm-${a}.tar.gz.sha256"
done

# 2. Replace version and sources.*.sha256 in assets/apm.nix

Handling npm tools with Nix (as of 2026)

The right path after nodePackages.* was removed:

1. pkgs.pnpm / top-level packages — try this first. Use it if it's in nixpkgs.
2. buildNpmPackage + importNpmLock — build an official package with a lockfile yourself. Fully pinned.
3. NPM_CONFIG_PREFIX="$PWD/.npm-global" in shellHook — impure but project-scoped. An escape hatch when chasing hashes is too expensive.
4. npx / pnpm dlx — for CLIs you rarely use.

Avoid: writing npm install -g directly in shellHook (pollutes user $HOME), referencing nodePackages.* (removed), adopting dream2nix / node2nix for new projects (maintenance is slowing).

buildNpmPackage skeleton

my-cli = pkgs.buildNpmPackage {
  pname = "my-cli";
  version = "1.2.3";
  src = pkgs.fetchFromGitHub {
    owner = "owner"; repo = "my-cli"; rev = "v1.2.3";
    hash = "sha256-...";
  };
  # To switch to a lockfile-based build:
  # npmDeps = pkgs.importNpmLock { npmRoot = ./.; };
  npmDepsHash = "sha256-...";
  dontNpmBuild = true;   # no build needed for a pure CLI
};

home-manager (optional)

When you want to manage your whole user environment (zsh / git / starship / CLI packages) declaratively in Nix, use assets/home-manager/. When combining with chezmoi, chezmoi forget things like dot_zshrc first to transfer ownership to home-manager.

File layout

• flake.nix — two outputs: a macOS desktop profile (macos) and one for Claude Code web / ephemeral Linux (ccweb)
• common.nix — shared across hosts (zsh, git, starship, direnv, fzf, basic CLI)
• macos.nix — aarch64-darwin specific (heavy tools like helix, neovim)
• ccweb.nix — minimal x86_64-linux config (kept thin, aiming for cold start < 2 min)

Usage

# 1. Copy
cp -r ~/.claude/skills/nix-setup/assets/home-manager ~/.config/home-manager
cd ~/.config/home-manager

# 2. Edit `username` / `email` at the top of flake.nix
$EDITOR flake.nix

# 3. Apply
home-manager switch --flake .#macos     # macOS
home-manager switch --flake .#ccweb     # Linux for Claude Code web, etc.

Notes when managing in a public repo

This template contains zero secrets (username / email are flake.nix variables; no hostnames, tokens, or known_hosts). Safe to put on public GitHub.

Keep your own secrets (SSH config, internal hosts, API tokens) via one of:

1. Create ./private.nix and load it via imports. Exclude it in .gitignore (already included in the template).
2. Commit it encrypted with sops-nix / agenix.

Cold start estimates on Claude Code web

Step	With cache.nixos.org	Bare
setup_nix.sh	20-40 s	same
flake evaluation + substitute	30-90 s	2-8 min
activation (symlink expansion)	few seconds	same
Total	1-2 min	4-12 min

The more heavy packages you add to ccweb.nix, the longer cold start gets. Keep shared bits in common.nix and only write additional packages in ccweb.nix.

Adding to an existing repo

Unlike new projects (cp assets/<lang>/flake.nix .), existing repos have the following traps.

1. Save existing files before cp

The template cp will clobber existing .envrc / flake.nix / .gitignore. Follow save → merge:

# Save the existing one
[ -f .envrc ] && cp .envrc .envrc.pre-nix

# Drop the template in
cp ~/.claude/skills/nix-setup/assets/typescript/flake.nix .
cp ~/.claude/skills/nix-setup/assets/typescript/.envrc .

# Restore existing exports, etc.
# Merge by hand with .envrc.pre-nix as reference

2. .envrc merge policy

If the existing .envrc has things like export DATABASE_URL=..., put use flake first and leave existing exports after it (the devShell env is the base; repo-specific values override it).

# Correct order
use flake                              # start the devShell first
dotenv_if_exists .env.local            # secrets
export DATABASE_URL="postgres://..."   # preserve existing exports

Conversely, writing export first risks having it overwritten by use flake.

3. Lockfile migrations (npm → pnpm, etc.)

Mixing package-lock.json and pnpm-lock.yaml causes undefined behavior. If you migrate, do it in a separate PR from the Nix work:

git switch -c chore/pnpm-migration
rm package-lock.json && rm -rf node_modules
corepack enable && corepack prepare pnpm@10 --activate
pnpm install                          # re-resolve
pnpm why <critical-dep>               # check for major drift
pnpm build && pnpm test               # verify
git add pnpm-lock.yaml package.json && git rm package-lock.json
git commit -m "chore: migrate npm -> pnpm lockfile"

If pnpm-lock.yaml conflicts during rebase, don't fix it by hand — regenerate with pnpm install → git add.

4. Replacing actions/setup-node in CI

Rewrite existing ci.yml Node-related steps with the Nix-ification diff:

-      - uses: actions/setup-node@v4
-        with:
-          node-version: 24
-          cache: pnpm
-      - run: pnpm install --frozen-lockfile
-      - run: pnpm test
+      - uses: DeterminateSystems/nix-installer-action@main
+      - uses: DeterminateSystems/magic-nix-cache-action@main
+      - run: nix develop --command just ci

Define pnpm install --frozen-lockfile && pnpm build && pnpm test inside just ci (move it into the justfile). The trick to avoiding cold builds is to preserve the pnpm store separately via actions/cache.

5. Monorepo handling

For monorepos using pnpm-workspace.yaml / turborepo / Nx, placing a single flake.nix at the root shares nix develop across all workspaces. Keep per-package dev tools in package.json devDependencies and limit nix to the language runtime + cross-cutting tools (just / ast-grep).

6. Merging with existing .gitignore

Copying assets/ocaml/.gitignore or assets/oxcaml/.gitignore will overwrite an existing .gitignore. Merge is required.

• Always add result / result-* (Nix build artifacts)
• Add .direnv/ (nix-direnv cache)

direnv integration

Each language template ships with an .envrc. Copy it together with flake.nix and the devShell will be applied automatically on cd.

cp ~/.claude/skills/nix-setup/assets/apm.nix .
cp ~/.claude/skills/nix-setup/assets/rust/flake.nix .
cp ~/.claude/skills/nix-setup/assets/rust/.envrc .
direnv allow                             # approve once at the start

nix-direnv is required — bare direnv re-evaluates the flake on every cd, costing 10-60 s. nix-direnv caches the result + creates GC roots, bringing it down to < 100 ms. In assets/home-manager/common.nix it's already enabled via programs.direnv.nix-direnv.enable = true.

Patterns in the shipped .envrcs:

• moonbit: use flake only (minimal)
• rust: use flake + watch_file rust-toolchain.toml — auto-reload when the toolchain switches
• typescript / python-uv: use flake + dotenv_if_exists .env.local — load local API keys and the like

Editor integration

Installing the VS Code direnv.direnv extension, or the direnv plugins for Helix / Neovim, makes the LSP server inherit the devShell PATH. rust-analyzer picks up the rustToolchain from flake.nix, moon ide grabs the overlay version of moon, etc.

GC root cleanup

nix-direnv-prune     # detect and remove unused .direnv

About every six months. When /nix/store starts bloating.

GitHub Actions

jobs:
  test:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v4

      - uses: DeterminateSystems/nix-installer-action@main
      - uses: DeterminateSystems/magic-nix-cache-action@main

      - name: Run tests in dev shell
        run: nix develop --command just test

magic-nix-cache-action puts the Nix store on the free GitHub Actions cache. Plenty for personal projects. For large-scale cases, consider Cachix / attic.

Troubleshooting

error: experimental Nix feature 'nix-command' is disabled

mkdir -p ~/.config/nix
echo 'experimental-features = nix-command flakes' >> ~/.config/nix/nix.conf

error: cannot build on ... due to sandbox

In container / rootless environments the sandbox doesn't work. Add sandbox = false to /etc/nix/nix.conf (setup_nix.sh does this).

error: attribute 'pnpm' missing / nodePackages.pnpm not found

In nixpkgs >= 24.11, nodePackages.* has been removed. Use pkgs.pnpm (top-level).

Permission errors on /nix/store on macOS

The Determinate installer is recommended. Doing it manually on Apple Silicon requires APFS volume splitting.

autoPatchelfHook failure (libstdc++.so.6 not found)

Add stdenv.cc.cc.lib to buildInputs. The same fix used in apm.nix.

Updating flake inputs

nix flake update              # all inputs
nix flake update nixpkgs      # individual

direnv-2.37.x build hangs forever in the macOS Nix sandbox

./test/direnv-test.zsh blocks indefinitely when direnv 2.37.x is built locally under the Determinate Nix sandbox on macOS. Symptom: building '/nix/store/...-direnv-2.37.x.drv'... sits with ~0 CPU for tens of minutes; a zsh ./test/direnv-test.zsh child stays at 0:00.01 forever.

Fix: turn off doCheck for direnv globally via an overlay. Doing it on programs.direnv.package alone is not enough — nix-direnv carries the original direnv as a propagated dep, so the test phase still runs through that path.

# common.nix (home-manager) or any nixpkgs consumer
nixpkgs.overlays = [
  (_: prev: {
    direnv = prev.direnv.overrideAttrs (_: { doCheck = false; });
  })
];

Drop the override once cache.nixos.org ships a prebuilt direnv 2.37.x for your platform.

direnv 2.37.x ignores DIRENV_LOG_FORMAT / DIRENV_LOG_FILTER

Setting export DIRENV_LOG_FORMAT="" no longer silences direnv as of 2.37.x — the env-var path was reworked and direnv reads only direnv.toml. The noisy direnv: loading … / using devbox / export +AR +AS +CC … lines that fire on every shell start come through unaffected.

Fix: write ~/.config/direnv/direnv.toml instead. With home-manager:

programs.direnv.config.global.log_format = "";

Without home-manager:

# ~/.config/direnv/direnv.toml
[global]
log_format = ""

Errors and warnings still print; only the routine status lines are suppressed.

References

• Nix manual (flakes)
• nixpkgs JavaScript section
• DeterminateSystems/nix-installer
• oxalica/rust-overlay
• moonbit-community/moonbit-overlay
• microsoft/apm