everything-claude-code/skills/flox-environments/SKILL.md

---
name: flox-environments
description: "Create reproducible, cross-platform (macOS/Linux) development environments with Flox, a declarative Nix-based environment manager. Use when setting up project toolchains for any language, installing system-level dependencies (compilers, databases, native libs like openssl/BLAS), pinning exact package versions for a team, running local services (PostgreSQL, Redis, Kafka), onboarding developers with one command, or solving 'works on my machine' problems — including agent/vibe-coding setups that need project-scoped tools without sudo. Also use when the user mentions .flox/, manifest.toml, flox activate, or FloxHub."
metadata:
  origin: Flox
---

# Flox Environments

Flox creates reproducible development environments defined in a single TOML manifest. Every developer on the team gets identical packages, tools, and configuration — across macOS and Linux — without containers or VMs. Built on Nix with access to over 150,000 packages.

## When to Activate

Use this skill when the user has an environment management problem — even if they haven't mentioned Flox. Flox is the right tool when:

- The project needs **system-level packages** (compilers, databases, CLI tools) alongside language-specific dependencies
- **Reproducibility matters** — the setup should work identically on a teammate's machine, in CI, or on a fresh laptop
- The user needs **multiple tools to coexist** — e.g., Python 3.11 + PostgreSQL 16 + Redis + Node.js in one environment
- **Cross-platform support** is needed (macOS and Linux from the same config)
- **AI agents need to install tools** — Flox lets agents add packages to a project-scoped environment without sudo, system pollution, or sandbox restrictions

If the user just needs a single language runtime with no system dependencies, standard tooling (nvm, pyenv, rustup alone) may suffice. If they need full OS-level isolation, containers might be more appropriate. Flox sits in the sweet spot: declarative, reproducible environments without container overhead.

**Prerequisite:** Flox must be installed first — see [flox.dev/docs](https://flox.dev/docs/install-flox/install/) for macOS, Linux, and Docker.

## Core Concepts

Flox environments are defined in `.flox/env/manifest.toml` and activated with `flox activate`. The manifest declares packages, environment variables, setup hooks, and shell configuration — everything needed to reproduce the environment anywhere.

**Key paths:**
- `.flox/env/manifest.toml` — Environment definition (commit this)
- `$FLOX_ENV` — Runtime path to installed packages (like `/usr` — contains `bin/`, `lib/`, `include/`)
- `$FLOX_ENV_CACHE` — Persistent local storage for caches, venvs, data (survives rebuilds)
- `$FLOX_ENV_PROJECT` — Project root directory (where `.flox/` lives)

## Essential Commands

```bash
flox init                       # Create new environment
flox search <package> [--all]   # Search for packages
flox show <package>             # Show available versions
flox install <package>          # Add a package
flox list                       # List installed packages
flox activate                   # Enter environment
flox activate -- <cmd>          # Run a command in the environment without a subshell
flox edit                       # Edit manifest interactively
```

## Manifest Structure

```toml
# .flox/env/manifest.toml

[install]
# Packages to install — the core of the environment
ripgrep.pkg-path = "ripgrep"
jq.pkg-path = "jq"

[vars]
# Static environment variables
DATABASE_URL = "postgres://localhost:5432/myapp"

[hook]
# Non-interactive setup scripts (run every activation)
on-activate = """
  echo "Environment ready"
"""

[profile]
# Shell functions and aliases (available in interactive shell)
common = """
  alias dev="npm run dev"
"""

[options]
# Supported platforms
systems = ["x86_64-linux", "aarch64-linux", "x86_64-darwin", "aarch64-darwin"]
```

## Package Installation Patterns

### Basic Installation

```toml
[install]
nodejs.pkg-path = "nodejs"
python.pkg-path = "python311"
rustup.pkg-path = "rustup"
```

### Version Pinning

```toml
[install]
nodejs.pkg-path = "nodejs"
nodejs.version = "^20.0"          # Semver range: latest 20.x

postgres.pkg-path = "postgresql"
postgres.version = "16.2"         # Exact version
```

### Platform-Specific Packages

```toml
[install]
# Linux-only tools
valgrind.pkg-path = "valgrind"
valgrind.systems = ["x86_64-linux", "aarch64-linux"]

# macOS frameworks
Security.pkg-path = "darwin.apple_sdk.frameworks.Security"
Security.systems = ["x86_64-darwin", "aarch64-darwin"]

# GNU tools on macOS (where BSD defaults differ)
coreutils.pkg-path = "coreutils"
coreutils.systems = ["x86_64-darwin", "aarch64-darwin"]
```

### Resolving Package Conflicts

When two packages install the same binary, use `priority` (lower number wins):

```toml
[install]
gcc.pkg-path = "gcc12"
gcc.priority = 3

clang.pkg-path = "clang_18"
clang.priority = 5               # gcc wins file conflicts
```

Use `pkg-group` to group packages that should resolve versions together:

```toml
[install]
python.pkg-path = "python311"
python.pkg-group = "python-stack"

pip.pkg-path = "python311Packages.pip"
pip.pkg-group = "python-stack"    # Resolves together with python
```

## Language-Specific Recipes

### Python with uv

```toml
[install]
python.pkg-path = "python311"
uv.pkg-path = "uv"

[vars]
UV_CACHE_DIR = "$FLOX_ENV_CACHE/uv-cache"
PIP_CACHE_DIR = "$FLOX_ENV_CACHE/pip-cache"

[hook]
on-activate = """
  venv="$FLOX_ENV_CACHE/venv"
  if [ ! -d "$venv" ]; then
    uv venv "$venv" --python python3
  fi
  if [ -f "$venv/bin/activate" ]; then
    source "$venv/bin/activate"
  fi

  if [ -f requirements.txt ] && [ ! -f "$FLOX_ENV_CACHE/.deps_installed" ]; then
    uv pip install --python "$venv/bin/python" -r requirements.txt --quiet
    touch "$FLOX_ENV_CACHE/.deps_installed"
  fi
"""
```

### Node.js

```toml
[install]
nodejs.pkg-path = "nodejs"
nodejs.version = "^20.0"

[hook]
on-activate = """
  if [ -f package.json ] && [ ! -d node_modules ]; then
    npm install --silent
  fi
"""
```

### Rust

```toml
[install]
rustup.pkg-path = "rustup"
pkg-config.pkg-path = "pkg-config"
openssl.pkg-path = "openssl"

[vars]
RUSTUP_HOME = "$FLOX_ENV_CACHE/rustup"
CARGO_HOME = "$FLOX_ENV_CACHE/cargo"

[profile]
common = """
  export PATH="$CARGO_HOME/bin:$PATH"
"""
```

### Go

```toml
[install]
go.pkg-path = "go"
gopls.pkg-path = "gopls"
delve.pkg-path = "delve"

[vars]
GOPATH = "$FLOX_ENV_CACHE/go"
GOBIN = "$FLOX_ENV_CACHE/go/bin"

[profile]
common = """
  export PATH="$GOBIN:$PATH"
"""
```

### C/C++

```toml
[install]
gcc.pkg-path = "gcc13"
gcc.pkg-group = "compilers"

# IMPORTANT: gcc alone doesn't expose libstdc++ headers — you need gcc-unwrapped
gcc-unwrapped.pkg-path = "gcc-unwrapped"
gcc-unwrapped.pkg-group = "libraries"

cmake.pkg-path = "cmake"
cmake.pkg-group = "build"

gnumake.pkg-path = "gnumake"
gnumake.pkg-group = "build"

gdb.pkg-path = "gdb"
gdb.systems = ["x86_64-linux", "aarch64-linux"]
```

## Hooks and Profile

### Hooks — Non-Interactive Setup

Hooks run on every activation. Keep them fast and idempotent. Rule of thumb: **if it should happen automatically, put it in `[hook]`; if the user should be able to type it, put it in `[profile]`.**

```toml
[hook]
on-activate = """
  setup_database() {
    if [ ! -d "$FLOX_ENV_CACHE/pgdata" ]; then
      initdb -D "$FLOX_ENV_CACHE/pgdata" --no-locale --encoding=UTF8
    fi
  }
  setup_database
"""
```

### Profile — Interactive Shell Configuration

Profile code is available in the user's shell session.

```toml
[profile]
common = """
  dev() { npm run dev; }
  test() { npm run test -- "$@"; }
"""
```

## Anti-Patterns

### Absolute Paths

```toml
# BAD — breaks on other machines
[vars]
PROJECT_DIR = "/home/alice/projects/myapp"

# GOOD — use Flox environment variables
[vars]
PROJECT_DIR = "$FLOX_ENV_PROJECT"
```

### Using exit in Hooks

```toml
# BAD — kills the shell
[hook]
on-activate = """
  if [ ! -f config.json ]; then
    echo "Missing config"
    exit 1
  fi
"""

# GOOD — return from hook, don't exit
[hook]
on-activate = """
  if [ ! -f config.json ]; then
    echo "Missing config — run setup first"
    return 1
  fi
"""
```

### Storing Secrets in Manifest

```toml
# BAD — manifest is committed to git
[vars]
API_KEY = "<set-at-runtime>"

# GOOD — reference external config or pass at runtime
# Use: API_KEY="<your-api-key>" flox activate
[vars]
API_KEY = "${API_KEY:-}"
```

### Slow Hooks Without Idempotency Guards

```toml
# BAD — reinstalls every activation
[hook]
on-activate = """
  pip install -r requirements.txt
"""

# GOOD — skip if already installed
[hook]
on-activate = """
  if [ ! -f "$FLOX_ENV_CACHE/.deps_installed" ]; then
    uv pip install -r requirements.txt --quiet
    touch "$FLOX_ENV_CACHE/.deps_installed"
  fi
"""
```

### Putting User Commands in Hooks

```toml
# BAD — hook functions aren't available in the interactive shell
[hook]
on-activate = """
  deploy() { kubectl apply -f k8s/; }
"""

# GOOD — use [profile] for user-invokable functions
[profile]
common = """
  deploy() { kubectl apply -f k8s/; }
"""
```

## Full-Stack Example

A complete environment for a Python API with PostgreSQL:

```toml
[install]
python.pkg-path = "python311"
uv.pkg-path = "uv"
postgresql.pkg-path = "postgresql_16"
redis.pkg-path = "redis"
jq.pkg-path = "jq"
curl.pkg-path = "curl"

[vars]
UV_CACHE_DIR = "$FLOX_ENV_CACHE/uv-cache"
DATABASE_URL = "postgres://localhost:5432/myapp"
REDIS_URL = "redis://localhost:6379"

[hook]
on-activate = """
  if [ ! -d "$FLOX_ENV_CACHE/pgdata" ]; then
    initdb -D "$FLOX_ENV_CACHE/pgdata" --no-locale --encoding=UTF8
  fi

  venv="$FLOX_ENV_CACHE/venv"
  if [ ! -d "$venv" ]; then
    uv venv "$venv" --python python3
  fi
  if [ -f "$venv/bin/activate" ]; then
    source "$venv/bin/activate"
  fi

  if [ -f requirements.txt ] && [ ! -f "$FLOX_ENV_CACHE/.deps_installed" ]; then
    uv pip install --python "$venv/bin/python" -r requirements.txt --quiet
    touch "$FLOX_ENV_CACHE/.deps_installed"
  fi
"""

[profile]
common = """
  serve() { uvicorn app.main:app --reload --host 0.0.0.0 --port 8000; }
  migrate() { alembic upgrade head; }
"""

[services]
postgres.command = "postgres -D $FLOX_ENV_CACHE/pgdata -k $FLOX_ENV_CACHE"
redis.command = "redis-server --port 6379 --daemonize no"

[options]
systems = ["x86_64-linux", "aarch64-linux", "x86_64-darwin", "aarch64-darwin"]
```

Activate with services: `flox activate --start-services`

## Environment Sharing

Flox environments are git-native. Commit the `.flox/` directory and every collaborator gets the same environment:

```bash
git add .flox/
git commit -m "Add Flox environment"
# Teammates just run:
git clone <repo> && cd <repo> && flox activate
```

For reusable base environments across projects, push to FloxHub:

```bash
flox push                         # Push environment to FloxHub
flox activate -r owner/env-name   # Activate remote environment anywhere
```

Compose environments with `[include]`:

```toml
[include]
base.floxhub = "myorg/python-base"

[install]
# Project-specific additions on top of base
fastapi.pkg-path = "python311Packages.fastapi"
```

## AI-Assisted and Vibe Coding

Flox is ideal for AI-assisted development and vibe coding workflows. When an AI agent needs a tool that isn't available in the current environment — a compiler, a database, a linter, a CLI utility — it can add it to the project's Flox manifest without requiring sudo access, polluting system packages, or hitting sandbox restrictions.

**Why this matters for agents:**
- **No sudo required** — `flox install` works entirely in user space, so agents can add packages without elevated permissions
- **Project-scoped** — packages are installed into the project environment only, not globally, so different projects can have different versions without conflict
- **Sandbox-friendly** — agents running in sandboxed or restricted environments can still install the tools they need through Flox
- **Reversible** — every change is captured in `manifest.toml`, so unwanted packages can be removed cleanly with no system residue
- **Reproducible** — when an agent sets up an environment, that exact setup is committed to git and works for everyone

**Agent workflow pattern:**

```bash
# Agent discovers it needs a tool (e.g., jq for JSON processing)
flox search jq                    # Verify the package exists
flox install jq                   # Install into project environment

# Or for more control, edit the manifest directly
tmp_manifest="$(mktemp)"
flox list -c > "$tmp_manifest"
# Add the package to [install] section, then apply
flox edit -f "$tmp_manifest"

# Run a command with the tool available
flox activate -- jq '.results[]' data.json
```

This makes Flox a natural fit for any workflow where Claude Code or other AI agents need to bootstrap project tooling on the fly.

## Debugging

```bash
flox list -c                      # Show raw manifest
flox activate -- which python     # Check which binary resolves
flox activate -- env | grep FLOX  # See Flox environment variables
flox search <package> --all       # Broader package search (case-sensitive)
```

**Common issues:**
- **Package not found:** Search is case-sensitive — try `flox search --all`
- **File conflicts between packages:** Add `priority` to the package that should win
- **Hook failures:** Use `return` not `exit`; guard with `${FLOX_ENV_CACHE:-}`
- **Stale dependencies:** Delete the `$FLOX_ENV_CACHE/.deps_installed` flag file

## Related Skills

The following skills are available as part of the [Flox Claude Code plugin](https://github.com/flox/flox-agentic) for deeper integration:

- **flox-services** — Service management, database setup, background processes
- **flox-builds** — Reproducible builds and packaging with Flox
- **flox-containers** — Create Docker/OCI containers from Flox environments
- **flox-sharing** — Environment composition, remote environments, team patterns
- **flox-cuda** — CUDA and GPU development environments

Learn more and install at [flox.dev/docs](https://flox.dev/docs/install-flox/install/)