okay so

$ diff /etc/nix/nix.conf < /etc/nix/nix.conf.bak
13c13
< substituters = https://nix-community.cachix.org/ https://cache.nixos.org/
---
> substituters = https://nix-community.cachix.org/ https://cache.nixos-cuda.org/ https://cache.nixos.org/
15c15
< trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs=
---
> trusted-public-keys = cache.nixos.org-1:6NCHdD59X431o0gWypbMrAURkbJ16ZPMQFGspcDShjY= nix-community.cachix.org-1:mB9FSh9qf2dCimDSUo8Zy7bkq5CX+/rkCWyvRCYg3Fs= cache.nixos-cuda.org:74DUi4Ye579gUqzH4ziL9IyiJBlDpMRn9MBN8oNan9M=

$ nh os switch /home/devin/nix-config
error:
       … while calling the 'head' builtin
         at «github:nixos/nixpkgs/15f4ee454b1dce334612fa6843b3e05cf546efab?narHash=sha256-Ad49moKWeXtKBJNy2ebiTQUEgdLyvGmTeykAQ9xM%2BZ4%3D»/lib/attrsets.nix:1712:13:
         1711|           if length values == 1 || pred here (elemAt values 1) (head values) then
         1712|             head values
             |             ^
         1713|           else

       … while evaluating the attribute 'value'
         at «github:nixos/nixpkgs/15f4ee454b1dce334612fa6843b3e05cf546efab?narHash=sha256-Ad49moKWeXtKBJNy2ebiTQUEgdLyvGmTeykAQ9xM%2BZ4%3D»/lib/modules.nix:1166:7:
         1165|     // {
         1166|       value = addErrorContext "while evaluating the option `${showOption loc}':" value;
             |       ^
         1167|       inherit (res.defsFinal') highestPrio;

       … while evaluating the option `system.build.toplevel':

       … while evaluating definitions from `/nix/store/wjfxdzblindbl9sp2hbwhi4iyh5jh348-source/nixos/modules/system/activation/top-level.nix':

       … while evaluating the option `home-manager.users.devin.home.file':

       … while evaluating definitions from `/nix/store/54pwggrv42vxx9bj6rlccq2dk6cazrj5-source/modules/hm':

       … while evaluating the option `home-manager.users.devin.programs.nixcord.config.themes':

       … while evaluating definitions from `/nix/store/vvhsnhz2c15zdcdj1vchhcwixd4pxspy-source/global/graphical/discord.nix':

       (stack trace truncated; use '--show-trace' to show the full, detailed trace)

       error: unable to download 'https://cache.nixos-cuda.org/64p5qz6fkdvvyd5j21g8fvs822jiw0ih.narinfo': HTTP error 502

Also noticed nix isnt just auto falling back to another substituter. Removing it helped at least.

Devin do you have nixConfig.extra-substituters defined in flake.nix by chance?

hexa (UTC+1) it looks like harmonia doesn't like the socket path:

May 03 00:11:25 hydra harmonia[5383]: [2026-05-02T22:11:25Z INFO  harmonia] listening on /run/harmonia/socket
May 03 00:11:25 hydra harmonia[5383]: Error: Custom { kind: Other, error: invalid socket address }

Am I missing something?