nix-direnv-perf - SKILL.md Agent Skill

name: nix-direnv-perf description: Diagnose and prevent slow/contending nix-direnv rebuilds in this dotfiles repo. Use when a `flake.lock` update triggers concurrent rebuilds, terminals hang on `direnv export`, or fish/Nix daemon troubleshooting. Keywords nix-direnv, flake.lock, direnv export, nix-daemon, concurrent rebuild, .envrc.

nix-direnv Performance & Troubleshooting

Repo-specific guidance for the ~/Development/dotfiles flake. Loaded when terminals hang on direnv, the Nix daemon misbehaves, or you're about to bump flake.lock.

Concurrent Rebuild Problem

When flake.lock changes, every open terminal with direnv tries to rebuild simultaneously:

Lock contention (Nix serializes operations)
Redundant downloads (each shell fetches the same closures)
3-10x slower rebuilds, high CPU/memory

Detect with:

ps aux | grep -E 'direnv export|nix.*print-dev-env'
# Multiple sessions = contention

Prevention

1. Close extra terminals before flake updates. Keep ONE terminal in dotfiles/ while running nix flake update or editing flake.lock. Reopen others after the rebuild completes.

2. Prime the cache manually:

cd ~/Development/dotfiles
nix build --no-link '.#devShells.aarch64-darwin.default'

3. Optional --impure for faster eval:

echo 'use flake --impure' > .envrc
direnv allow

Emergency Fix (rebuild already in progress)

pkill -f 'direnv export'
pkill -f 'nix.*print-dev-env'
ps aux | grep -E 'direnv|nix.*print-dev-env' | grep -v grep   # verify
# Close extra dotfiles terminals; the remaining one resumes

General Nix/direnv Troubleshooting

Source daemon: . /nix/var/nix/profiles/default/etc/profile.d/nix-daemon.sh
Fish not seeing Nix? exec fish -l and confirm config/fish/conf.d/10-nix.fish
Fedora: keep SELinux enforcing; the Determinate installer ships the policy
Direnv not loading? direnv status and ensure .envrc is allowed
macOS installer issues? System Settings → Privacy & Security for blocked components

Best Practices When Bumping Packages

nix flake update from ONE terminal with others closed
Pre-build with nix build before opening multiple shells
First post-update build takes 2-5 min; subsequent shells are instant