The compiler and all cross-compilers come from the haskell.nix input pinned in flake.lock. The ghc914-sh compiler is the stable-haskell fork, not the stock GHC 9.14 release from GHCup. Use the same source checkout and lock file for every platform; Android ARMv7 does not require a separate branch or tag.

compiler-selection.nix and compiler-cache-overlay.nix match Hydra's compiler build context, including auxiliary iserv projects, so the pinned compiler can use its existing cache. Application dependencies keep the project's current packages.

Run these commands from the repository root:

make help
make nix-list                  # list this host's existing Hydra outputs
make nix-native                # native library and CLI
make nix-android               # ARM64 + ARMv7 libraries, on x86_64-linux
make nix-windows               # Windows library and CLI, on x86_64-linux
make nix-linux-static          # x86_64, i686, and ARM64 static Linux CLIs
make nix-build                 # all existing outputs for this build host

Each command builds this checkout, without cloning another branch. Output links are placed under result-nix/<build-system>/<package>. ZIP filenames inside the outputs retain the existing Hydra names. For example, the ARMv7 client library is result-nix/x86_64-linux/armv7a-android:lib:simplex-chat/pkg-armv7a-android-libsimplex.zip. Nix includes tracked working-tree changes; add new source files to Git before building so they are included.

For a remote builder, select its build system explicitly. Nix must already have that builder configured:

make nix-ios NIX_SYSTEM=aarch64-darwin
make nix-darwin-libs NIX_SYSTEM=x86_64-darwin
make nix-build NIX_SYSTEM=aarch64-linux

nix-ios produces the Haskell static-library ZIP. It does not run Xcode, sign an iOS application, or upload to TestFlight. ARM64 device libraries and Intel simulator libraries retain their separate outputs. The existing iOS preparation scripts retag ARM64 archives with mac2ios -s when an Apple Silicon simulator is needed.

The Cabal v2 builder stores ordinary package archives in its staged store. link-mobile-library.sh relinks the selected package and its dependencies before that store is pruned. iOS ZIPs contain both the plain libHSsimplex-chat-*.a and the rolled libHSsimplex-chat-*-ghc*.a used by update-pbxproj.sh. Android and Windows clients link the threaded RTS into libsimplex.so or libsimplex.dll; Android's separate libsupport.so resolves RTS symbols from the client library. ARM64 Android libraries retain the 16 KiB linker alignment. These steps run in the derivation's build/install phases because v2 component overrides do not run the older postBuild and postInstall hooks.

Mobile builds keep the RTS's small-address-space setting. On Darwin, the final link uses ABI-checked replacement RTS registrations, plus a replacement text on ARM64. These boot libraries and all ARM64 mobile app C/C++ code target the Apple A7 instruction baseline before mac2ios retags the archives. The cached native compiler itself stays unchanged. Android applies the RTS source patch when building its boot libraries.

NIX_ARGS passes additional options to nix build, and NIX_OUT_DIR changes the output-link directory. Each derivation has a two-hour timeout by default; set NIX_BUILD_TIMEOUT in seconds to change it. For a wall-clock limit on a local batch, use timeout from GNU coreutils to include environment setup in the limit:

timeout 6h nix develop -c make nix-build
make nix-native NIX_ARGS=--dry-run

.github/workflows/nix.yml runs these same outputs on Linux ARM64/x86_64 and Darwin ARM64/x86_64, with a six-hour job limit. It uploads the existing library ZIPs and CLIs under an artifact named for the source SHA. Each native CLI also runs the encrypted-store and profile-password smoke test, which can be repeated with python3 scripts/nix/smoke-cli.py result-nix/BUILD_SYSTEM/exe:simplex-chat/bin/simplex-chat. Hydra continues to consume the same hydraJobs attributes. The older application release workflow still uses its existing GHCup/Docker compiler settings; migrating that packaging and release path is separate from this Nix build lane.

Hydra needs builders advertising aarch64-darwin and x86_64-darwin to run the corresponding Mac/iOS jobs. Check its builder pool when jobs remain queued. For cross builds, the pinned haskell.nix also uses recursive-nix as a scheduling marker for slices that require a real x86_64 Linux host. Advertise that feature on a native x86_64 Linux builder; keep it off x86_64 Linux slots running through Rosetta on Apple Silicon. The GitHub workflow sets it only for its native x86_64 Linux runner.

To build both Android APK ABIs from the current checkout, preserving the same compiler and source, use the existing builder's source/skip-checkout flags:

scripts/android/build-android.sh -sg VERSION_CODE

This command also needs the Android SDK, Gradle, and its other existing tools. The APK reproduction script now checks out the supplied tag for both ABIs. Historical releases that used a separate -armv7a tag must be reproduced with their historical build scripts.

Migration validation

On 2026-10-15, all 21 existing Hydra derivations evaluated successfully at c72da15836d9b154532db29eb5013c3b8fb37a14: 11 on x86_64 Linux, two on ARM64 Linux, and four on each Darwin architecture. This includes both Android ABIs, Windows, and all three static Linux targets.

The native ARM64 Darwin library and CLI compiled with the pinned compiler using Cabal. Encrypted-store/profile-password, BBS, and native mobile C API smoke checks passed. The replacement RTS and text also built with Nix and passed ABI, instruction-baseline, UTF-8, and GC checks. These checks do not establish that all 21 final Nix artifacts build: that artifact matrix is still unverified. Android device tests and an Xcode application build remain unverified too.

Updating package hashes

Install nix, gawk and jq.
Start nix-shell from repo root:

nix-shell -p nix-prefetch-git

Run in nix shell:

gawk -f ./scripts/nix/update-sha256.awk cabal.project > ./scripts/nix/sha256map.nix