diff --git a/docs/guides/building.md b/docs/guides/building.md
index a76eb6c..f785026 100644
--- a/docs/guides/building.md
+++ b/docs/guides/building.md
@@ -22,6 +22,25 @@ make qemu     # build and boot it
 | `curl`, `tar` | fetching ports | Git Bash | present | present | present |
 | `xorriso` **or** `mkisofs` | `make iso` only | `scoop install cdrtools` | `xorriso` | `brew install xorriso` | `pkg install xorriso` |
 
+On NixOS, `nix-shell` supplies every row above — see below.
+
+## NixOS
+
+```sh
+nix-shell        # then: make, make qemu, make iso
+```
+
+`shell.nix` in the repository root declares the lot. Worth using rather than
+assembling it by hand, for one specific reason: **`make` falls back to the GNU
+toolchain for the kernel but the UEFI loader genuinely cannot be built without
+clang and `lld-link`.** A machine with only gcc therefore produces a kernel that
+builds and a loader that silently stays whatever it already was — which left a
+stale boot splash on this tree for half a day before anyone looked at the
+screen rather than the build output.
+
+PowerShell is the one thing the shell does not supply, and `tools/windows/`
+needs it. See the note at the end of this file.
+
 A host C compiler (`cc`) is also needed, for the tools that build the disk
 images. On Windows that must be **mingw-w64's gcc**, not clang: clang defaults
 to an MSVC target there, and `tools/mkfs.c` needs `<unistd.h>` and `<dirent.h>`,
diff --git a/docs/guides/layout.md b/docs/guides/layout.md
index a8dfbcc..d2f8e28 100644
--- a/docs/guides/layout.md
+++ b/docs/guides/layout.md
@@ -47,6 +47,7 @@ schos/
 │                 can be re-fetched sparsely. Copying from it is governed by
 │                 the per-file licence check in docs/licensing.md §3 --
 │                 NetBSD is not uniformly licensed.
+├─ shell.nix      NixOS development shell: the whole toolchain in one file
 ├─ docs/          architecture/ what it is   plan/    what's next
 │                 guides/      how to work   history/ why it's like this
 └─ build/         ALL generated output (see below)
diff --git a/shell.nix b/shell.nix
new file mode 100644
index 0000000..a51936c
--- /dev/null
+++ b/shell.nix
@@ -0,0 +1,48 @@
+# A development shell with everything docs/guides/building.md asks for.
+#
+#     nix-shell          # then: make, make qemu, make iso
+#
+# Why this exists: the UEFI loader is the one part that genuinely cannot be
+# built without clang and lld. `make` falls back to the GNU toolchain for the
+# kernel, so a machine with only gcc gets a kernel that builds and a loader that
+# silently stays whatever it was -- which is exactly the failure that left a
+# stale boot splash on this tree for half a day.
+#
+# Pinned to whatever nixpkgs the host is on rather than to a flake input, on
+# purpose: this is a convenience for one developer's machine, not a
+# reproducibility claim. If it ever needs to be the latter it wants a flake.
+{ pkgs ? import <nixpkgs> { } }:
+
+pkgs.mkShell {
+  name = "schos";
+
+  nativeBuildInputs = with pkgs; [
+    # The cross-compiler, and the reason this file exists. clang targets
+    # bare-metal ELF, 16-bit code and PE/COFF from one install; lld-link is what
+    # produces BOOTX64.EFI.
+    clang
+    lld
+    llvm                # llvm-objcopy, llvm-objdump
+
+    # The host compiler, for tools/ -- mkfs, mkschfs, the SchFS suites. Must be
+    # a real Unix compiler: those need <unistd.h> and <dirent.h>.
+    gcc
+
+    gnumake
+    perl                # generates the interrupt vector table
+    python3             # boot logo, crash-screen QR, mklogo
+
+    qemu                # make qemu, and every test harness
+    OVMF                # firmware for the UEFI path; the Makefile searches for it
+
+    xorriso             # make iso
+    mtools              # ESP staging
+    curl gnutar         # fetching ports
+  ];
+
+  shellHook = ''
+    echo "schos: clang $(clang --version | head -1 | grep -oE '[0-9]+\.[0-9]+\.[0-9]+')," \
+         "$(qemu-system-x86_64 --version | head -1 | grep -oE '[0-9]+\.[0-9]+\.[0-9]+')"
+    echo "       make  |  make qemu  |  make schfs-check  |  make iso"
+  '';
+}