Nix Devshell Init

Nix DevShell Init

Scaffold a reproducible flake.nix and .envrc for a project based on detected languages and tooling needs.

What this skill does

1. Detects project language(s) from sentinel files in the working directory.
2. Selects the appropriate devShell packages for each language.
3. Generates a flake.nix with a devShells.default output.
4. Creates an .envrc with use flake for direnv integration.
5. Adds .direnv/ to .gitignore (direnv cache directory).
6. Verifies the generated flake evaluates cleanly.

Language detection

Scan the project root for these sentinel files. Multiple matches produce a combined multi-language devShell.

Sentinel file(s)           → Language    → Primary packages
─────────────────────────────────────────────────────────
go.mod                     → Go          → go, gopls, gotools
package.json               → Node.js     → nodejs, npm
pyproject.toml              → Python      → python3, uv
requirements.txt           → Python      → python3, uv
Cargo.toml                 → Rust        → rustc, cargo, rust-analyzer
Makefile (alone)           → Generic     → gnumake

If no sentinel files are found, ask the user which language(s) to support and proceed with that selection.

Version inference

When possible, infer the language version from project metadata:

• Go: parse go directive from go.mod (e.g., go 1.22 → go_1_22 if available in nixpkgs)
• Node: parse engines.node from package.json (e.g., >=18 → nodejs_18)
• Python: parse requires-python from pyproject.toml (e.g., >=3.11 → python311)
• Rust: if rust-toolchain.toml exists, note the channel but use the nixpkgs Rust (pinning Rust nightly requires an overlay — suggest oxalica/rust-overlay in a comment)

When the exact version is unavailable in nixpkgs, default to the latest stable and add a comment noting the requested version.

Flake generation

Generate flake.nix at the project root. The structure follows:

{
  description = "<project-name> development environment";

  inputs = {
    nixpkgs.url = "github:NixOS/nixpkgs/nixos-unstable";
  };

  outputs = { nixpkgs, ... }:
    let
      # Support both x86_64 and aarch64 for Linux and Darwin
      forAllSystems = nixpkgs.lib.genAttrs [
        "x86_64-linux" "aarch64-linux"
        "x86_64-darwin" "aarch64-darwin"
      ];
    in
    {
      devShells = forAllSystems (system:
        let pkgs = nixpkgs.legacyPackages.${system}; in
        {
          default = pkgs.mkShell {
            packages = with pkgs; [
              # <detected packages here>
            ];

            shellHook = ''
              # <language-specific shell hooks>
            '';
          };
        }
      );
    };
}

For per-language package lists and shellHook contents, see references/language-templates.md.

Multi-platform support

Always use forAllSystems (or flake-utils.lib.eachDefaultSystem) rather than hardcoding a single system string. This ensures the flake works on both Linux and macOS, x86_64 and aarch64. If the project is exclusively Linux, add a comment noting the restriction rather than removing other systems.

.envrc generation

Create .envrc at the project root:

use flake

If .envrc already exists, append use flake only if it is not already present.

.gitignore updates

Add these entries to .gitignore if not already present:

# Nix / direnv
.direnv/
result

Do not add flake.lock to .gitignore — the lock file pins package versions and must be committed for reproducibility.

Verification

After generating the flake, verify it evaluates:

nix flake check --no-build

If nix flake check fails, read the error and fix the flake.nix. Common issues:

• Missing attribute — a package name was wrong for nixpkgs. Search with nix search nixpkgs <name>.
• Infinite recursion — usually a let binding that references itself. Check for circular definitions.
• IFD (import-from-derivation) — avoid in devShells; use pkgs.mkShell with static package lists only.

If direnv is installed, run:

direnv allow

Existing flake handling

If flake.nix already exists in the project:

1. Read it and report what it currently provides.
2. Ask the user whether to replace, extend, or leave it unchanged.
3. Never overwrite an existing flake.nix without confirmation.

If flake.lock exists without flake.nix, the lock file is orphaned. Note this to the user and proceed with generation.

Error handling

• Nix not installed — stop and suggest the nix-install skill.
• Flakes not enabled — stop and suggest enabling flakes (see nix-install references for the config command).
• No sentinel files and no user input — create a minimal generic shell with gnumake, curl, jq and inform the user.
• flake check fails — do not leave a broken flake. Fix or remove it before finishing. Report what failed and why.

References

• references/language-templates.md — per-language package lists, shellHook contents, and common extras (LSPs, formatters, linters).
• references/flake-anatomy.md — annotated flake.nix structure explaining each section for users new to Nix.