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Feature Flags Overview

Overview

Experimental features are Nix capabilities that are still being iterated on: they may change behavior, gain new requirements, or be removed entirely. Since Nix 2.4, each one is guarded by a named feature flag that must be enabled explicitly—nothing experimental runs unless you opt in via configuration or a CLI flag.

Before 2.4, unstable behavior often shipped without a clear switch, which made it hard to tell what was supported versus in flux. Flags exist so risky changes (language extensions, new CLI surfaces, store semantics) can land in releases while remaining off by default until the project is confident in the design.

This page explains how flags work, how to enable them, and how they relate to stabilization and RFCs. For individual flags and their status, see the sibling pages in this domain and Tracking Stabilization.

Version stamp (Phase 5.1 re-check, 2026-07-31): facts and the flag inventory below match the Nix stable manual — Experimental features for Nix 2.34.x (stable → /manual/nix/2.34/…; manual title 2.34.9; local nix (Nix) 2.34.8). Re-check that page and release notes after each Nix stable release or NixOS nix pin bump—flag names appear and disappear between releases. flakes and nix-command remain experimental in this series (still listed; not stabilized).

Details

Why flags exist. A flag guards a change when experience might still lead to revert or compatibility breaks: new Nix language constructs, CLI command shapes, or store/evaluation behavior that affects downstream tooling. Shipping behind a flag lets early adopters try real workflows without implying stability for everyone.

Typical lifecycle (from the manual):

  1. A change merges with its flag disabled by default.
  2. A Nix release ships; users who want the behavior enable the flag.
  3. Over time, the feature either stabilizes (flag removed, behavior always on) or is dropped (implementation and flag removed).

Stabilization is a judgment call, not a timer. Common criteria include evidence of real-world use, confidence in the API/design, understood interactions with other features, and acceptable maintenance burden. Tracking Stabilization summarizes what has landed versus what remains experimental in this wiki.

Relationship to RFCs. Feature flags and RFCs are orthogonal. An RFC documents and socializes a design; a flag controls whether an implementation is available. A feature can ship flagged without an RFC, an RFC can precede a flagged implementation, or both can run in parallel—the flag is about iteration safety, not community process.

Enabling flags.

Mechanism Where
experimental-features = … nix.conf — replaces the list for that setting (default: empty)
extra-experimental-features = … nix.conf — appends to experimental-features
--extra-experimental-features … One-shot on a single nix invocation (appends)
--experimental-features … / --option experimental-features … One-shot replace of the effective list
nix.settings.experimental-features NixOS module — writes the corresponding nix.conf settings

Flag values are space-separated names (e.g. nix-command flakes). Inspect the effective set with nix config show experimental-features (needs nix-command). Do not assume a flag from an older blog post still exists without checking the manual for your Nix version.

Flags available in Nix 2.34.x stable (21 names from the manual; not a full catalog of behavior):

Flag Wiki leaf (if any)
auto-allocate-uids auto-allocate-uids
blake3-hashes —
ca-derivations ca-derivations
cgroups cgroups
configurable-impure-env —
daemon-trust-override —
dynamic-derivations dynamic-derivations
external-builders —
fetch-closure —
fetch-tree fetch-tree-and-git
flakes flakes
git-hashing fetch-tree-and-git
impure-derivations impure-derivations
local-overlay-store —
mounted-ssh-store —
nix-command nix-command
parse-toml-timestamps —
pipe-operators pipe-operators-and-lang
read-only-local-store —
recursive-nix recursive-nix
verified-fetches fetch-tree-and-git

Common entry points among those: nix-command, flakes (concept: Flake; workflows: Flakes), and ca-derivations. Other flags and language experiments live under Experimental Features; the nine flags without dedicated leaves are listed in Experimental backlog.

Maintenance cadence. After each Nix stable / NixOS nix pin bump: re-diff this table to the manual, move rows to or from Experimental backlog as leaves are written, and re-stamp Tracking Stabilization. See Release cadence.

Examples

Persistent enable in nix.conf — typical for flakes and the new CLI:

experimental-features = nix-command flakes

Append without replacing an existing list:

extra-experimental-features = ca-derivations

One-shot on the command line (does not change nix.conf):

nix --extra-experimental-features 'nix-command flakes' flake show .

NixOS (declarative equivalent):

nix.settings.experimental-features = [ "nix-command" "flakes" ];

Inspect what is actually enabled:

nix config show experimental-features

References

See also