/flox-environments

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

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 = """
  i