devenv-ecosystem

Patterns for devenv configuration — language setup, services, tooling (git-hooks, scripts, processes), outputs, and profiles. For Nix language syntax, flake structure, or general NixOS module patterns, see nix-ecosystem instead; this skill covers only what is devenv-specific.

devenv 2.0

Released March 2026. Two changes affect existing configs directly:

The native process manager is now the default, replacing process-compose. A config that assumes process-compose behavior (health checks, depends_on, readiness probes) silently gets the native manager instead unless process.manager.implementation = "process-compose"; is set explicitly — there is no error, just different runtime behavior for devenv up. To keep process-compose:
```
process.manager.implementation = "process-compose";
```

processes.api = { exec = "cargo run"; process-compose.readiness_probe.http_get = { host = "localhost"; port = 8080; path = "/health"; }; };


- devenv 0.x is deprecated; support drops in devenv 3.

Other 2.0 changes are cosmetic to the config author: a live TUI on every command, background rebuilds in the
native shell (save a file, `Ctrl+Alt+R` applies the new environment without leaving the shell), a C FFI
backend replacing spawned `nix` subprocesses, an incremental per-attribute eval cache, and cross-repo output
references.

## devenv vs. flakes

Use devenv over pure flakes when you need `services.*` (simpler than hand-rolled service derivations),
git-hooks integration, process supervision, or `languages.*` version management. Use flakes directly for pure
derivations or NixOS system configuration — devenv adds overhead there.

## Multi-shell and custom outputs (2.0+)

`devenv.shells.<name>` defines multiple shell configurations in one file — each is a full devenv module, not
an overlay of the default:

```nix
{ pkgs, ... }:
{
devenv.shells.default = {
  languages.python.enable = true;
  packages = [ pkgs.git ];
};
devenv.shells.ci = {
  languages.python.enable = true;
  env.CI = "true";
};
}

devenv.outputs exposes arbitrary flake outputs (packages, checks) from within the module:

{ pkgs, config, ... }:
{
  devenv.outputs = {
    packages.my-app = config.languages.rust.import ./. {};
    checks.lint = pkgs.runCommand "lint" {} ''
      ${pkgs.clippy}/bin/cargo-clippy
      touch $out
    '';
  };
}

Language modules expose an import builder for packaging an app with the same toolchain the shell uses — config.languages.rust.import ./rust-app {} — instead of hand-writing a stdenv.mkDerivation.

Profiles (1.9+)

Three distinct mechanisms, easy to conflate:

Value override — profiles."python-3.12".config = { languages.python.version = "3.12"; };, selected with devenv shell --profile python-3.12.
Composable modules — profiles.backend.module = { ... }, profiles.frontend.module = { ... }, then profiles.fullstack.extends = [ "backend" "frontend" ]; to combine them.
Automatic selection by environment — profiles.hostname."ci-server".module and profiles.user."developer".module apply based on the machine's hostname or the invoking user, with no explicit --profile flag needed. This is a silent-selection mechanism: a shell picks up the profile because of who or where it's running, not because of anything in the invocation — worth checking for before debugging "works on my machine" config drift.

git-hooks

git-hooks.hooks wires pre-commit hooks (formatters: nixfmt-rfc-style, prettier, black, rustfmt, ormolu, gofmt; linters: eslint, clippy, shellcheck, yamllint, actionlint, hadolint, markdownlint; security/hygiene: detect-private-keys, check-merge-conflicts, check-case-conflicts, editorconfig-checker; treefmt as a universal formatter wrapper). Custom hooks take an entry shell command and a files regex:

git-hooks.hooks.my-custom-hook = {
  enable = true;
  entry = "${pkgs.bash}/bin/bash -c 'echo Running custom hook'";
  files = "\\.nix$";
  pass_filenames = true;
};

Scripts, tasks, processes

scripts.<name>.exec puts a command on $PATH inside the shell; tasks.<name> adds dependency ordering via before/after, including hooking into shell entry itself (before = [ "devenv:enterShell" ];) — a task can run automatically on every devenv shell invocation this way, which is easy to miss when reading a config that otherwise looks static.

processes.<name>.exec are managed by whichever process manager is active (native by default in 2.0, see above). Keep enterShell fast and offline-capable — it runs on every shell entry — and push network-dependent or slow setup into a task with an explicit trigger instead.

Environment and secrets

dotenv.enable = true; (with dotenv.filename to pick a non-default file) loads .env-style files that should stay out of version control. Don't put real secrets in the env attribute directly — it lands in the Nix store, which is world-readable on the local machine.

Context7

Library id /cachix/devenv (trust score 9.7, ~1354 snippets) — use for current languages.*/services.* option shapes, available git-hooks, and the devenv 2.0 migration guide, since option surfaces here change between minor versions faster than this file is updated.

nix-ecosystem — Nix language syntax, flake structure and inputs, general NixOS module patterns, lib.* functions; devenv builds on all of it but doesn't re-teach it.
serena-usage — symbol operations for navigating devenv configurations.
context7-usage — fetching current devenv documentation via Context7 MCP.

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