Nix Dev Shell Auto-Activation

Overview

This skill automatically detects when a Nix flake is present and ensures development commands run inside the appropriate Nix dev shell environment, providing access to all declared dependencies.

Core Workflow

1. Flake Detection

Before running any development command, check for a flake:

# Check if flake.nix exists in current directory or parent directories
if [ -f flake.nix ]; then
    echo "Flake detected - will use nix dev shell"
fi

Use the Glob tool to detect flakes:

pattern: **/flake.nix

2. Dev Shell Activation

When a flake is detected, wrap commands using one of these methods:

Method 1: nix develop -c (recommended for flakes)

nix develop -c <command> <args>

Method 2: nix shell (alternative syntax)

nix shell --command <command> <args>

Method 3: Multiple commands

nix develop --command bash -c "command1 && command2"

Method 4: Legacy nix-shell (for shell.nix)

nix-shell --run "<command> <args>"

3. Commands That Should Use Dev Shell

Always run these in dev shell when flake.nix exists:

• Build commands: cargo build, npm run build, make, go build
• Test commands: cargo test, npm test, pytest, go test
• Development servers: npm run dev, cargo run
• Package managers: npm install, cargo add, pip install
• Formatters/linters: rustfmt, eslint, black, gofmt
• Custom project scripts defined in flake.nix

4. Commands That Should NOT Use Dev Shell

These Nix commands should run directly (not in dev shell):

• nix build
• nix flake check
• nix fmt
• nix develop (obviously)
• nh os switch
• nh os build

Usage Patterns

Pattern 1: Build/Test Commands

# User asks: "run the tests"
nix develop -c cargo test

# User asks: "build the project"
nix develop -c npm run build

Pattern 2: Multiple Commands

# User asks: "install dependencies and run tests"
nix develop --command bash -c "npm install && npm test"

Pattern 3: Simple Approach

# Alternative using nix shell
nix shell --command make test

# Or for legacy shell.nix
nix-shell --run "cargo build"

Decision Tree

Is flake.nix present?
├─ YES
│  ├─ Is it a Nix command (build, flake check, fmt)?
│  │  ├─ YES → Run directly (nix build, nix fmt, etc.)
│  │  └─ NO → Is it a dev/build/test command?
│  │     ├─ YES → Run in dev shell (nix develop -c <cmd>)
│  │     └─ NO → Ask user if command needs dev shell
└─ NO → Run command directly

Examples

Example 1: Rust Project

# User: "run cargo test"
# Skill behavior:

# Check for flake
[ -f flake.nix ] && {
    # Run in dev shell
    nix develop -c cargo test
}

Example 2: Node.js Project

# User: "install packages and start dev server"
# Skill behavior:

nix develop --command bash -c "npm install && npm run dev"

Example 3: NixOS Configuration

# User: "check the flake configuration"
# Skill behavior:

# This is a Nix command - run directly
nix flake check

Example 4: Format Code

# User: "format the nix files"
# Skill behavior:

# nix fmt runs directly (not in dev shell)
nix fmt

Important Notes

1. Always detect flake first: Use Glob or check for file existence before deciding
2. Preserve user intent: If user explicitly says "in dev shell" or "without dev shell", respect it
3. Background processes: For long-running commands (dev servers), consider using run_in_background: true
4. Error handling: If nix develop fails, suggest checking flake.nix or running nix flake check
5. Project context: Check CLAUDE.md for project-specific Nix commands and patterns

When to Activate This Skill

• User mentions "build", "test", "run", "start", or similar development commands
• User asks to execute package manager commands (npm, cargo, pip, etc.)
• Working in a project directory with flake.nix
• User explicitly mentions "dev shell" or "nix develop"

Integration with Project

This NixOS dotfiles repo has specific Nix commands documented in CLAUDE.md:

• nh os switch --hostname=<name> - Deploy NixOS config (run directly)
• nix flake check - Validate flake (run directly)
• nix fmt - Format Nix files (run directly)
• nix build .#<output> - Build specific output (run directly)

These are management commands and should NOT use dev shell.

Troubleshooting

Dev shell fails to start:

# Check flake validity first
nix flake check

# Try with verbose output
nix develop --verbose

Command not found in dev shell:

• Check if the command is defined in flake.nix devShells
• Verify flake.lock is up to date: nix flake update

Slow dev shell activation:

• Consider using nix develop --offline if dependencies are already cached
• Use direnv integration for automatic shell activation (if available)