Nix Knowledge Patch

Nix Knowledge Patch

Use this skill when changing Nix expressions, Nix CLI integrations, flakes, stores and caches, NixOS modules, Nixpkgs packages, or Home Manager configurations. Check the relevant reference before preserving an old option, output format, package name, service default, or builder interface.

Reference index

Start with migrations that can break evaluation

Use current path-literal lint settings

Replace the old URL-literal feature and Boolean short-path warning with the tri-state settings. Each accepts ignore, warn, or fatal.

lint-url-literals = fatal
lint-short-path-literals = warn
lint-absolute-path-literals = warn

Write relative paths as ./foo/bar; do not use relative paths in file: tarball references.

Treat integers as signed 64-bit values

Arithmetic overflow is an evaluation error. builtins.fromJSON rejects larger integers, and negative numeric values are invalid for flake nixConfig options.

Build structured derivations explicitly

Do not serialize JSON into the __json environment variable. Set __structuredAttrs = true on builtins.derivation; it is also required when debug-info splitting is combined with derivation reference checks.

builtins.derivation (attrs // { __structuredAttrs = true; })

Select current CLI and JSON interfaces

• Use nix profile add; nix profile install is only a compatibility alias.
• Pass --json-format to nix path-info --json; format 2 changes store-path keys and content-address representation, and format 3 structures signatures.
• Produce and consume derivation JSON version 4; nix derivation add rejects older versions.
• Do not parse human-readable sizes as fixed MiB values.
• Force --pretty or --no-pretty when JSON formatting must not depend on whether stdout is a terminal.

Update C and C++ integrations

Include C++ headers as nix/<component>/..., rely on pkg-config's include directory, and use public NIX_ configuration macros. Configure flakes on each evaluator-state builder; the global flake initializer is gone.

Indexed C accessors accept mutable values. Prefer lazy list and attribute accessors when a sub-value must remain unevaluated. Plugins may resolve the Nix executable's exported C symbols dynamically. Primop errors are sticky unless returned as NIX_ERR_RECOVERABLE.

Flake and source quick reference

Declare repository features on inputs

Relative path: inputs can point to flakes in the same repository, but their locks are incompatible with older clients. A Git-backed flake can declare inputs.self.submodules = true and inputs.self.lfs = true. Git URLs can also request LFS with lfs=1.

Lock generation ignores user and system registries for indirect inputs. Pin URLs explicitly, use nix registry resolve to inspect registry resolution, and remember that input updates preserve nested locks from the updated input.

Prefer explicit source operations

• nix flake prefetch-inputs fetches all inputs concurrently.
• nix flake prefetch --out-link protects or exposes a prefetched source.
• nix flake archive --no-check-sigs can copy directly to a remote store.
• nix formatter build builds the configured formatter and prints its executable path.
• nix fmt with no arguments does not imply ..
• builtins.getFlake accepts path values, although paths outside the store remain unsupported.

Store, cache, and build quick reference

Account for changed build locations and concurrency

Temporary builds live below the Nix state directory and their directory names are opaque. build-cores = 0 performs automatic CPU detection. Post-build hooks can overlap up to max-jobs; build hooks receive SIGTERM on shutdown.

Configure durable and safe garbage collection

Enable fsync-store-paths when new paths must reach durable storage before registration. Runtime roots can be served by nix store roots-daemon for a daemon without /proc tracing capability. Recursive deletion can keep live paths with nix store delete --recursive --skip-alive.

Keep cache automation format-aware

HTTP caches can compress narinfo, listing, and log metadata. Configure cache metadata lifetime with narinfo-cache-meta-ttl, refresh with nix store info --refresh, and supply mTLS credentials in HTTPS store URLs. Authentication failures are reported distinctly from missing objects.

S3 stores support STS, web-identity and container credentials, object version IDs, storage classes, multipart controls, and explicit addressing styles. Configure every overlay cache alongside its underlying cache to avoid local rebuilds of incomplete substituted closures.

NixOS migration quick reference

Review boot and switching assumptions

The initrd uses systemd by default. Replace /dev/root with a stable device, name LUKS mapper paths explicitly, and retain scripted stage 1 only as a temporary compatibility measure. nixos-rebuild-ng and the Rust system switch are the supported paths; remove obsolete switch implementation toggles.

/etc/nixos/system.nix can be a channel-free system entry point. Use nixos-rebuild --attr for an attribute set and --file for another entry.

State dependencies explicitly

Services needing connectivity must both want and follow network-online.target. Depend on postgresql.target when the database must be writable and initialized. Depend on the ACME certificate service when a syntactically valid certificate is required.

Migrate structured settings and secrets

Move legacy extraConfig and flat option values into their RFC 42-style settings attribute sets. Prefer secret-file and systemd-credential options; never place private keys or application secrets in the Nix store.

Treat state versions as migration switches

Use valid "YY.MM" NixOS state versions and change them only with the corresponding data and default migrations. Database and application defaults, private data directories, and several Home Manager paths are state-gated.

Nixpkgs packaging quick reference

Use current builders and dependency hooks

• Use buildGoModule, with env.CGO_ENABLED where required.
• Use cargoHash and rustPlatform.fetchCargoVendor for Rust vendoring.
• Set an explicit modern Python format, normally pyproject = true with a declared build-system.
• Use top-level fetchPnpmDeps and pnpmConfigHook, regenerating version-3 pnpm dependency hashes.
• Use Yarn build, config, and install hooks for remaining Yarn 1 packages.

Expect stricter derivation validation

env must be an attribute set, nested input lists are deprecated, dangling and build-directory symlinks fail checks, and meta.mainProgram can affect build output. Use structured attributes with buildEnv and put custom derivation arguments in derivationArgs.

Replace removed package and library names

Prefer top-level desktop packages, pkgs.nixfmt, current lib.cli rendering functions, and current lib.attrsets, lib.filesystem, and string-type helpers. Consult the packaging reference for exact one-to-one replacements.

Home Manager quick reference

Let the command own profile updates

Use home-manager switch --rollback or --specialisation NAME for safe activation. Direct activation-script callers must update the profile themselves. Legacy shadow-profile management is only a temporary escape hatch.

Review activation and login behavior

User services restart during activation by default. Use home-manager.startAsUserService when the home is unavailable until login. Minimal mode imports only essential modules; explicitly import everything else.

Migrate program configuration

Use structured SSH settings, per-profile Firefox extensions and Anki sync, services.syncthing.tray.enable, guiCredentials, dedicated editor-fork modules, and the renamed Neovim, man-viewer, and wallpaper-service options. State-version changes also affect XDG paths, Hyprland and Neovim configuration formats, application copying on Darwin, and automation defaults.

Verification checklist

Before shipping a migration:

1. Identify whether the change belongs to Nix, NixOS, Nixpkgs, or Home Manager.
2. Check state-version gates separately from package or executable versions.
3. Validate option types and renamed paths with evaluation, not string search.
4. Exercise machine-readable output with an explicit format version.
5. Test store and hook changes with concurrent jobs and garbage collection.
6. Read the topic reference for defaults, escape hatches, and exact replacements.