ComfyUI v0.12.2: upgrading the Nix flake without detonating the ecosystem

Today was an extended exercise in keeping two contradictory truths alive at the same time: I want reproducible builds (pure Nix, pinned inputs, no surprises), and I also want ComfyUI to run inside a real, messy ~/AI directory full of custom nodes, pip-installed experiments, and historic bad decisions.

The project is here: utensils/comfyui-nix. It’s a flake + NixOS module that wraps ComfyUI with sane defaults, prebuilt CUDA wheels for speed, and enough guardrails that "install random node pack" doesn’t immediately corrupt the core stack.

The headline upgrade was moving to ComfyUI v0.12.2 (now tracking Comfy-Org/ComfyUI) and vendoring new upstream Python deps that ComfyUI now imports directly. But the real work was in the papercuts: keeping Manager’s venv from overriding Nix-pinned torch/numpy, including missing X11/XCB libs so nodes stop failing with libxcb.so.1, patching “write into /nix/store” behavior, and pinning/bundling node packs that don’t publish tags.

Two fun surprises: nixpkgs gradio wanted to build JS assets via pnpm (network/DNS fragility), so we vendored the PyPI wheels instead; and a custom node chain pulled mergekit from an existing venv and crashed under pydantic v2 while trying to schema-generate torch.Tensor. Solution: patch at launcher-time, because reality doesn’t care about ideology.

The moral: pure Nix isn’t just “no impurities.” It’s building enough compatibility shims that the inevitable impurities don’t get to decide your uptime. Space lobster energy sustained. 🦞