Devshell on Simon Shine

Agentic Playwright testing with Nix

Sat, 15 Aug 2026 21:31:07 +0200

I don’t do much front-end web development these days, but I enable people who do.

I use a lot of coding agents for my daily work with back-end APIs and service meshes.

I’ve known for a long time that agentic workflows instrumenting Playwright to run headless browser tests based on prompts has advanced quite a lot. I just haven’t got around to try it out.

tl;dr ¶

Playwright, its browsers and its MCP server are all in nixpkgs. No package.json, no npm install, no npx playwright install.
nixpkgs’ playwright-test hardcodes the full browser set into its wrapper, so the naive setup drags 2.2 GB of Chromium, Firefox and WebKit into your devshell whether you want them or not. I managed to push that down to 634 MB.
Headed Chromium and interactive debugging lives behind a flake-parts partition.
The planner/generator/healer agents run on an existing Claude Code session.
I practice TDD workflows for writing end-to-end tests.

The application under test ¶

The thing I’m practising on is a generic client area dashboard for a software service, written in Rust on topcoat with toasty. So I’m doing three things here: Testing topcoat, toying around with client dashboards, and getting my agentic playwright testing workflow up.

Git config-based hooks with hk-nix

Sat, 01 Aug 2026 00:00:00 +0100

hk is a fast, modern git hook manager and project linter configured in Pkl. hk-nix is to hk what lefthook-nix is to lefthook: you install hk with Nixpkgs, declare your hooks in Nix, pin every tool with Nix, and the hooks install themselves whenever you enter the devshell. It is the same stack I previously built with lefthook, with a nicer configuration language and one fewer workaround.

Synergies ¶

Nix + direnv: You only ever need to install these two manually: Nix being the package manager, and direnv being the thing that activates the Nix devshell. All other tooling is derived, pinned and installed for you.

A dendritic sops-nix flake template

Sun, 05 Jul 2026 02:35:37 +0200

This tutorial sets up a flake using flake-parts, import-tree, devshell and sops-nix.

You can see the template here: https://github.com/sshine/nix-sops-flake-template

Unlike agenix/ragenix, the sops-nix NixOS module does not require importing a Nix file describing recipients, so it composes cleanly inside flake-parts modules without triggering IFDs.

1. A dendritic flake ¶

This flake auto-imports all .nix files in lib/.

You may want for that directory to be named something else, depending on your project.

Using lefthook and treefmt with Nix

Sat, 07 Mar 2026 21:47:14 +0100

In my recent attempts to modernize my CI pipelines, I’ve experimented with and subsequently let go of pre-commit. The reason why I didn’t stick with pre-commit is that pre-commit drags in several software runtimes and toolchains when building it from scratch. When Nix fails to build something from nixpkgs, it’s an omen of too many chefs.

Alternatives I’m aware of:

lefthook: A generic tool written in Go; defines hooks in a lefthook.yml file.
husky: A generic tool written in JavaScript; enough said.
cargo-husky: A tool specialized to Rust/Cargo; very nice, but not generic enough.

I initially disregarded lefthook because of its special lefthook.yml.