git:20261007.124bcd8 to git:20261009.0e12440

---

name: nix-config-debug

description:

Use when something in this repo is broken, such as an eval or build error, a failed activation, a

crashed service, or an unreachable host or MicroVM guest.

---

# Debugging this repository

Follow the global `systematic-debugging` skill for the process (root cause before any fix). This

skill is only the map: which layer failed, and where to look. Rolling back, isolating a bad input

bump, and the commands that destroy rollback points are in the `nix-config-update` skill.

## Core rules

1. **Restore service first.** A down host or a dead session gets the previous generation before any

investigation. Rolling back the machine you are on needs `sudo`, so the user does it.

2. **Name the layer before touching code.** Eval, build, activation, and runtime failures have

different causes; a green `just test` says nothing about activation.

3. **Never get to green by weakening a check**: no disabled test, dropped assertion, or `mkForce`

over the failing value.

4. **Preserve existing user changes.** Do not discard or stash them without authorization. If a

clean baseline is needed, record the current diff and ask before isolating it.

## 1. Localize by layer

| Layer | Symptom | Look with |

| ------------- | ---------------------------------------------- | ------------------------------------------------------------------------------------------------------------------------------------------- |

| eval | `just test` fails, an eval error | `just eval-host <host>` (already passes `--show-trace`) |

| build | build error, hash mismatch, "marked as broken" | `just build-host <host>`, then `nix log <drv>` |

| activation | the deploy fails after building | the deploy output; `journalctl -u home-manager-$USER -b` for Home Manager |

| runtime | a unit is failed or restarting | `just list-failed`, `systemctl status <unit>`, `journalctl -u <unit> -b` |

| proxy/DNS | proxied sites time out, node and rules look ok | `dig +short <site>` must return a fake-ip (`198.18.x.x`); a real IP means the system resolver bypasses `dns-hijack` — mihomo README Gotchas |

| boot | errors at boot, wrong kernel | `journalctl -b -p err`; `just history` for what is booted |

| session | desktop or app misbehaves | `journalctl --user -b -p err`; the `nix-config-desktop` skill |

| remote host | anything on a Colmena host | `ssh root@<host> journalctl -b -p err`, same commands over SSH |

| MicroVM guest | guest down or unreachable | on the VM host: `systemctl status microvm@<guest>` and `microvm-tap-interfaces@<guest>`; then `br0` |

| secrets | missing or unreadable `/etc/agenix/*` | the `nix-config-secrets` skill |

## 2. Read the evaluated value

When `just test` fails, re-run the suite directly to get the trace (`nix eval` exits 0 even when the

suite returns `false`, so read its output):

```bash

nix eval .#evalTests --show-trace

```

The trace names the failing test under `outputs/<system>/tests/<name>/`. Read its `expr.nix` and

`expected.nix`, and evaluate the expression to see what actually came out before changing anything.

Most eval-layer questions are "what did this option actually end up as?". Ask the configuration

directly instead of reading modules:

```bash

nix eval .#nixosConfigurations.<host>.config.<option.path>

nix eval .#nixosConfigurations.<host>.config.systemd.services \

--apply 's: builtins.filter (n: builtins.match "microvm.*" n != null) (builtins.attrNames s)'

nix repl # then `:lf .` and inspect nixosConfigurations.<host>.config

```

Renamed or removed options are the most common eval break after an update; the error or warning

names the replacement. Treat deprecation warnings as failures waiting to happen.

## 3. Your change or the update?

`git status`, `git log --oneline -5 -- flake.lock`, and `git diff flake.lock` tell you which. If the

lock moved, isolate the input as described in the `nix-config-update` skill before reading code.

Preserve unrelated working-tree changes; do not use `git checkout`, `git stash`, or cleanup commands

to manufacture a clean tree without authorization.

## 4. Prove the fix

Re-run the exact command that failed, then `just test`, then `just build-host <host>` for every

affected host before anyone deploys. Report the evidence ("the unit is active, `just test` is

`true`"), not the intent.

## Why these rules exist

- `5ee8b728 fix(restic): use the correct systemd timer option names (#319)` - an option-name error

that only reading the evaluated value catches quickly.

- `9315055b fix(darwin): unblock home-manager activation` - an activation failure that eval and

build both passed.