Flox Environments

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 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

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

# .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

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

Version Pinning

[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

[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):

[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:

[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

[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

[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

[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

[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++

[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].

[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.

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

Anti-Patterns

Absolute Paths

# 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

# 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

# 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

# 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

# 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:

[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:

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:

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

Compose environments with [include]:

[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:

# 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

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 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